In my previous article, I have written a sample example showing how to configure JPA for running it in a Java SE environment. I also showed a sample persistence.xml file which looks like below -

<persistence xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence

http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd"

  version="2.1">

  <persistence-unit name="jpa-example" transaction-type="RESOURCE_LOCAL">
    <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>

    <properties>
      <!-- Configuring JDBC properties -->
      <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost/jpa_example" />
      <property name="javax.persistence.jdbc.user" value="root" />
      <property name="javax.persistence.jdbc.password" value="my_root_password" />
      <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />

      <!-- Hibernate properties -->
      <property name="hibernate.show_sql" value="true" />
      <property name="hibernate.format_sql" value="true" />
      <property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5InnoDBDialect" />
      <property name="hibernate.hbm2ddl.auto" value="validate" />

      <!-- Configuring Connection Pool -->
      <property name="hibernate.c3p0.min_size" value="5" />
      <property name="hibernate.c3p0.max_size" value="20" />
      <property name="hibernate.c3p0.timeout" value="500" />
      <property name="hibernate.c3p0.max_statements" value="50" />
      <property name="hibernate.c3p0.idle_test_period" value="2000" />
    </properties>
  </persistence-unit>
</persistence>

Those who are only starting out on JPA, please let me explain some of the components of this configuration.

The first section is used for configuring JDBC connection that will be used by the persistence provider. Usually we specify the JDBC url, database username, password and fully qualified name of the Driver class in this section. The second section configures some property values for hibernate, and is explained below -

  1. The “hibernate.show_sql” property specifies whether or not hibernate will print the queries in the log file (provided that you have configured log4j properly). This is specially helpful if you want to view which queries are being executed for reading/writing/deleting some entities. In the production environment you can set to false if you want so that the queries will not be logged.
  2. The “hibernate.format_sql” property specifies whether or not the queries will be formatted to a more readable form before logging.
  3. The “hibernate.dialet” property specifies which type of dialects we intend to use. If you do not know what they are then please read this excellent answer on StackOverflow.
  4. The “hibernate.hbm2ddl.auto” property is very interesting one. By changing its value you can enable hibernate to create/drop your database tables for you, or validate an existing schema against your mapping. This has also been explained very well on StackOverflow.

The last section configures the connection pool that hibernate will use for the database. Hibernate usually provides a built-in connection pooling mechanism which is good enough for development and testing, but is not suitable for production environment. So to get the optimal connection pooling behavior in production you need to use something more mature. C3P0 is a popular production-grade connection pooling library which is very easy to use with Hibernate. All you need to do is just specify the property values like minimum/maximum number of connections in the pool, timeout values etc., and the rest will be taken care of by Hibernate.

There is also another important point that I have skipped in the last article. In order for the entities to be found by the persistence provider in a Java SE environment, they will have to be listed in the persistence.xml file as follows -

<persistence xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence

http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd"

  version="2.1">

  <class>fully.qualified.path.of.the.entity</class>

  <!-- rest of the contents -->
  
  </persistence-unit>
</persistence>

Doing this will ensure that your entities will be found by the persistence provider and will be ready for persistence. However, hibernate auto-scans the packages for classes marked with the “Entity” annotation and make them persistent, so we did not have to worry about that. Keep in mind that this is also the case when you use JPA in a Java EE environment (i.e., your application runs in a full-blown application server). In this case the application server scans the application during its deployment and finds the classes marked as entities. If you run your application in a Java SE environment using a provider which does not have this type of auto scanning ability, then you will be required to list the entities in the persistence.xml file like above.

That’s it for setting up JPA in a Java SE environment. Hopefully once you have this setup you will be easily able to persist your entities without much trouble.


JPA stands for Java Persistence API, which basically is a specification that describes a way to persist data into a persistent storage, usually a database. We can think of it as something similar to ORM tools like Hibernate, except that it is an official part of the Java EE specification (and it’s also supported on Java SE).

There are many reasons to learn an ORM tool like JPA. I will not go into the details of this because there are already many posts on the web which perfectly answer this question, like this one, or this one. However, we should also keep in mind that this is not a single magic bullet which will solve our every problem.

When I first started out with JPA, I had real difficulties to set it up because most of the articles on the web are written for Java EE environment only, whereas I was trying to use it in a Java SE environment. I hope that this article will be helpful for those who wish to do the same in the future.

In this example we will use Maven to set up our required dependencies. Since JPA is only a specification, we will also need an implementation. There are many good implementations of JPA available freely (like EclipseLink, Hibernate etc.). For this article I have chosen to use Hibernate. As for the database, I will use MySQL.

Let us first create a simple maven project. I have created mine using the quick start archetype from the command line. If you do not know how to do that, you can follow this tutorial.

OK, so let us get the dependencies for the JPA next. Include the following lines in your pom.xml -

<dependency>
  <groupId>javax.persistence</groupId>
  <artifactId>persistence-api</artifactId>
  <version>1.0.2</version>
</dependency>
<dependency>
  <groupId>org.hibernate</groupId>
  <artifactId>hibernate-entitymanager</artifactId>
  <version>4.3.6.Final</version>
  <exclusions>
    <exclusion>
      <groupId>org.hibernate.javax.persistence</groupId>
      <artifactId>hibernate-jpa-2.1-api</artifactId>
    </exclusion>
  </exclusions>
</dependency>

The first dependency specifies the standard JPA interface, and the second one specifies the implementation. Including JPA dependencies this way is desirable because it gives us the freedom to switch vendor-specific implementation in the future without much problem (see details here). However we will not be able to use the latest version of the API this way because the API version 1.0.2 is the last version that is released as an independent JAR. At the time of writing this article, the latest version of the JPA specification is 2.1 which is not available independently (there are lots of requests for it though). If we want to use that one now then our only options are to choose from either a vendor-specific JAR or use an application server which provides the API along with its implementation. I have decided to use the API specification provided by Hibernate. In that case including only the following dependency will suffice -

<dependency>
  <groupId>org.hibernate</groupId>
  <artifactId>hibernate-entitymanager</artifactId>
  <version>4.3.6.Final</version>
</dependency>

Next step is to include the dependency for MySQL. Include the following lines in your pom.xml -

<dependency>
  <groupId>mysql</groupId>
  <artifactId>mysql-connector-java</artifactId>
  <version>5.1.31</version>
</dependency>

After including the rest of the dependencies (i.e., jUnit, Hamcrest etc.) the full pom.xml looks like below -

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">

  <modelVersion>4.0.0</modelVersion>

  <groupId>com.keertimaan.javasamples</groupId>
  <artifactId>jpa-example</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <packaging>jar</packaging>

  <name>jpa-example</name>
  <url>http://sayemdb.wordpress.com</url>

  <properties>
    <java.version>1.8</java.version>
    <hibernate.version>4.3.6.Final</hibernate.version>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <dependencies>
    <!-- JPA -->
    <dependency>
      <groupId>org.hibernate</groupId>
      <artifactId>hibernate-entitymanager</artifactId>
      <version>${hibernate.version}</version>
    </dependency>

    <!-- For connection pooling -->
    <dependency>
      <groupId>org.hibernate</groupId>
      <artifactId>hibernate-c3p0</artifactId>
      <version>${hibernate.version}</version>
    </dependency>

    <!-- Database -->
    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>5.1.31</version>
    </dependency>

    <!-- Test -->
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.11</version>
      <scope>test</scope>
      <exclusions>
        <exclusion>
          <groupId>org.hamcrest</groupId>
          <artifactId>hamcrest-core</artifactId>
        </exclusion>
      </exclusions>
    </dependency>
    <dependency>
      <groupId>org.hamcrest</groupId>
      <artifactId>hamcrest-all</artifactId>
      <version>1.3</version>
      <scope>test</scope>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>2.5.1</version>
        <configuration>
          <source>${java.version}</source>
          <target>${java.version}</target>
          <compilerArgument>-Xlint:all</compilerArgument>
          <showWarnings>true</showWarnings>
          <showDeprecation>true</showDeprecation>
        </configuration>
      </plugin>
    </plugins>
  </build>
</project>

Now it’s time to configure our database. I will use the following schema in all of my future JPA examples which I found from this excellent online book -

Database Schema

Database Schema

Create an equivalent database following the above schema in your local MySQL installation.

Our next step is to create the persistence.xml file which will contain our database specific information for JPA to use. By default JPA expects this file to be in the class path under the META-INF folder. For our maven project, I have created this file under project_root/src/main/resources/META-INF folder -

<persistence xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence

http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd"

  version="2.1">

  <persistence-unit name="jpa-example" transaction-type="RESOURCE_LOCAL">
  <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>

  <properties>
    <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost/jpa_example" />
    <property name="javax.persistence.jdbc.user" value="root" />
    <property name="javax.persistence.jdbc.password" value="my_root_password" />
    <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />

    <property name="hibernate.show_sql" value="true" />
    <property name="hibernate.format_sql" value="true" />
    <property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5InnoDBDialect" />
    <property name="hibernate.hbm2ddl.auto" value="validate" />

    <!-- Configuring Connection Pool -->
    <property name="hibernate.c3p0.min_size" value="5" />
    <property name="hibernate.c3p0.max_size" value="20" />
    <property name="hibernate.c3p0.timeout" value="500" />
    <property name="hibernate.c3p0.max_statements" value="50" />
    <property name="hibernate.c3p0.idle_test_period" value="2000" />
    </properties>
  </persistence-unit>
</persistence>

The above file requires some explanation if you are an absolute begineer in JPA. In my next article I will try to explain it as much as possible, but for running this example you will only need to change the first three property values to match your environment (namely the database name, username and password). Also keep a note of the value of the name attribute of the persistence-unit element. This value will be used to instantiate our EntityManagerFactory instance later in the code.

Ok, let us now create an entity to test our configuration. Create a class called Address with the following contents -

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "address")
public class Address {
  @Id
  @GeneratedValue
  private Integer id;

  private String street;
  private String city;
  private String province;
  private String country;
  private String postcode;

  /**
   * @return the id
   */
  public Integer getId() {
    return id;
  }

  /**
   * @param id the id to set
   */
  public Address setId(Integer id) {
    this.id = id;
    return this;
  }

  /**
   * @return the street
   */
  public String getStreet() {
    return street;
  }

  /**
   * @param street the street to set
   */
  public Address setStreet(String street) {
    this.street = street;
    return this;
  }

  /**
   * @return the city
   */
  public String getCity() {
    return city;
  }

  /**
   * @param city the city to set
   */
  public Address setCity(String city) {
    this.city = city;
    return this;
  }

  /**
   * @return the province
   */
  public String getProvince() {
    return province;
  }

  /**
   * @param province the province to set
   */
  public Address setProvince(String province) {
    this.province = province;
    return this;
  }

  /**
   * @return the country
   */
  public String getCountry() {
    return country;
  }

  /**
   * @param country the country to set
   */
  public Address setCountry(String country) {
    this.country = country;
    return this;
  }

  /**
   * @return the postcode
   */
  public String getPostcode() {
    return postcode;
  }

  /**
   * @param postcode the postcode to set
   */
  public Address setPostcode(String postcode) {
    this.postcode = postcode;
    return this;
  }
}

This class has been properly mapped to the address table and its instances are fully ready to be persisted in the database. Now let us create a helper class called PersistenceManager with the following contents -

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public enum PersistenceManager {
  INSTANCE;

  private EntityManagerFactory emFactory;

  private PersistenceManager() {
    // "jpa-example" was the value of the name attribute of the
    // persistence-unit element.
    emFactory = Persistence.createEntityManagerFactory("jpa-example");
  }

  public EntityManager getEntityManager() {
    return emFactory.createEntityManager();
  }

  public void close() {
    emFactory.close();
  }
}

Now let us write some sample persistence code in our Main method to test everything out -

import javax.persistence.EntityManager;

public class Main {
  public static void main(String[] args) {
    Address address = new Address();
    address.setCity("Dhaka")
        .setCountry("Bangladesh")
        .setPostcode("1000")
        .setStreet("Poribagh");

    EntityManager em = PersistenceManager.INSTANCE.getEntityManager();
    em.getTransaction()
        .begin();
    em.persist(address);
    em.getTransaction()
        .commit();

    em.close();
    PersistenceManager.INSTANCE.close();
  }
}

If you check your database, you will see that a new record has been inserted in your address table.

This article explains how to set up JPA without using any other frameworks like Spring. However it is a very good idea to use Spring to set up JPA because in that case we do not need to worry about managing entity managers, transactions etc. Beside setting up JPA, spring is also very good for many other purposes too.

That’s it for today. In the next article I will try to explain the persistence.xml file and the corresponding configuration values as much as possible. Stay tuned!

The full code can be found at github.


This is a continuation of an introductory discussion on Generics, previous parts of which can be found here.

In the last article we were discussing about recursive bounds on type parameters. We saw how recursive bound helped us to reuse the vehicle comparison logic. At the end of that article, I suggested that a possible type mixing may occur when we are not careful enough. Today we will see an example of this.

The mixing can occur if someone mistakenly creates a subclass of Vehicle in the following way -

/**
 * Definition of Vehicle
 */
public abstract class Vehicle<E extends Vehicle<E>> implements Comparable<E> {
    // other methods and properties

    public int compareTo(E vehicle) {
        // method implementation
    }
}

/**
 * Definition of Bus
 */
public class Bus extends Vehicle<Bus> {}

/**
 * BiCycle, new subtype of Vehicle
 */
public class BiCycle extends Vehicle<Bus> {}

/**
 * Now this class’s compareTo method will take a Bus type
 * as its argument. As a result, you will not be able to compare
 * a BiCycle with another Bicycle, but with a Bus.
 */
cycle.compareTo(anotherCycle);  // This will generate a compile time error
cycle.compareTo(bus);    // but you will be able to do this without any error

This type of mix up does not occur with Enums because JVM takes care of subclassing and creating instances for enum types, but if we use this style in our code then we have to be careful.

Let’s talk about another interesting application of recursive bounds. Consider the following class -

public class MyClass {
  private String attrib1;
  private String attrib2;
  private String attrib3;
  private String attrib4;
  private String attrib5;

  public MyClass() {}

  public String getAttrib1() {
    return attrib1;
  }

  public void setAttrib1(String attrib1) {
    this.attrib1 = attrib1;
  }

  public String getAttrib2() {
    return attrib2;
  }

  public void setAttrib2(String attrib2) {
    this.attrib2 = attrib2;
  }

  public String getAttrib3() {
    return attrib3;
  }

  public void setAttrib3(String attrib3) {
    this.attrib3 = attrib3;
  }

  public String getAttrib4() {
    return attrib4;
  }

  public void setAttrib4(String attrib4) {
    this.attrib4 = attrib4;
  }

  public String getAttrib5() {
    return attrib5;
  }

  public void setAttrib5(String attrib5) {
    this.attrib5 = attrib5;
  }
}

If we want to create an instance of this class, then we can do this -

MyClass mc = new MyClass();
mc.setAttrib1("Attribute 1");
mc.setAttrib2("Attribute 2");

The above code creates an instance of the class and initializes the properties. If we could use Method Chaining here, then we could have written -

MyClass mc = new MyClass().setAttrib1("Attribute 1")
    .setAttrib2("Attribute 2");

which obviously looks much better than the first version. However, to enable this type of method chaining, we need to modify MyClass in the following way -

public class MyClass {
  private String attrib1;
  private String attrib2;
  private String attrib3;
  private String attrib4;
  private String attrib5;

  public MyClass() {}

  public String getAttrib1() {
    return attrib1;
  }

  public MyClass setAttrib1(String attrib1) {
    this.attrib1 = attrib1;
    return this;
  }

  public String getAttrib2() {
    return attrib2;
  }

  public MyClass setAttrib2(String attrib2) {
    this.attrib2 = attrib2;
    return this;
  }

  public String getAttrib3() {
    return attrib3;
  }

  public MyClass setAttrib3(String attrib3) {
    this.attrib3 = attrib3;
    return this;
  }

  public String getAttrib4() {
    return attrib4;
  }

  public MyClass setAttrib4(String attrib4) {
    this.attrib4 = attrib4;
    return this;
  }

  public String getAttrib5() {
    return attrib5;
  }

  public MyClass setAttrib5(String attrib5) {
    this.attrib5 = attrib5;
    return this;
  }
}

and then we will be able to use method chaining for instances of this class. However, if we want to use method chaining where inheritance is involved, things kind of get messy -

public abstract class Parent {
  private String attrib1;
  private String attrib2;
  private String attrib3;
  private String attrib4;
  private String attrib5;

  public Parent() {}

  public String getAttrib1() {
    return attrib1;
  }

  public Parent setAttrib1(String attrib1) {
    this.attrib1 = attrib1;
    return this;
  }

  public String getAttrib2() {
    return attrib2;
  }

  public Parent setAttrib2(String attrib2) {
    this.attrib2 = attrib2;
    return this;
  }

  public String getAttrib3() {
    return attrib3;
  }

  public Parent setAttrib3(String attrib3) {
    this.attrib3 = attrib3;
    return this;
  }

  public String getAttrib4() {
    return attrib4;
  }

  public Parent setAttrib4(String attrib4) {
    this.attrib4 = attrib4;
    return this;
  }

  public String getAttrib5() {
    return attrib5;
  }

  public Parent setAttrib5(String attrib5) {
    this.attrib5 = attrib5;
    return this;
  }
}

public class Child extends Parent {
  private String attrib6;
  private String attrib7;

  public Child() {}

  public String getAttrib6() {
    return attrib6;
  }

  public Child setAttrib6(String attrib6) {
    this.attrib6 = attrib6;
    return this;
  }

  public String getAttrib7() {
    return attrib7;
  }

  public Child setAttrib7(String attrib7) {
    this.attrib7 = attrib7;
    return this;
  }
}

/**
 * Now try using method chaining for instances of Child
 * in the following way, you will get compile time errors.
 */
Child c = new Child().setAttrib1("Attribute 1").setAttrib6("Attribute 6");

The reason for this is that even though Child inherits all the setters from its parent, the return type of all those setter methods are of type Parent, not Child. So the first setter will return reference of type Parent, calling setAttrib6 on which will result in compilation error,  because it does not have any such method.

We can resolve this problem by introducing a generic type parameter on Parent and defining a recursive bound on it. All of its children will pass themselves as type argument when they extend from it, ensuring that the setter methods will return references of its type -

public abstract class Parent<T extends Parent<T>> {
  private String attrib1;
  private String attrib2;
  private String attrib3;
  private String attrib4;
  private String attrib5;

  public Parent() {
  }

  public String getAttrib1() {
    return attrib1;
  }

  @SuppressWarnings("unchecked")
  public T setAttrib1(String attrib1) {
    this.attrib1 = attrib1;
    return (T) this;
  }

  public String getAttrib2() {
    return attrib2;
  }

  @SuppressWarnings("unchecked")
  public T setAttrib2(String attrib2) {
    this.attrib2 = attrib2;
    return (T) this;
  }

  public String getAttrib3() {
    return attrib3;
  }

  @SuppressWarnings("unchecked")
  public T setAttrib3(String attrib3) {
    this.attrib3 = attrib3;
    return (T) this;
  }

  public String getAttrib4() {
    return attrib4;
  }

  @SuppressWarnings("unchecked")
  public T setAttrib4(String attrib4) {
    this.attrib4 = attrib4;
    return (T) this;
  }

  public String getAttrib5() {
    return attrib5;
  }

  @SuppressWarnings("unchecked")
  public T setAttrib5(String attrib5) {
    this.attrib5 = attrib5;
    return (T) this;
  }
}

public class Child extends Parent<Child> {
  private String attrib6;
  private String attrib7;

  public String getAttrib6() {
    return attrib6;
  }

  public Child setAttrib6(String attrib6) {
    this.attrib6 = attrib6;
    return this;
  }

  public String getAttrib7() {
    return attrib7;
  }

  public Child setAttrib7(String attrib7) {
    this.attrib7 = attrib7;
    return this;
  }
}

Notice that we have to explicitly cast this to type T  because compiler does not know whether or not this conversion is possible, even though it is because T by definition is bounded by Parent<T>. Also since we are casting an object reference to T, an unchecked warning will be issued by the compiler. To suppress this we used @SuppressWarnings(“unchecked”) above the setters.

With the above modifications, it’s perfectly valid to do this -

Child c = new Child().setAttrib1("Attribute 1")
  .setAttrib6("Attribute 6");

When writing method setters this way, we should be careful as to not to use recursive bounds for any other purposes, like to access children’s states from parent, because that will expose parent to the internal details of its subclasses and will eventually break the encapsulation.

With this post I finish the basic introduction to Generics. There are so many things that I did not discuss in this series, because I believe they are beyond the introductory level.

Until next time.

A Big Change

Posted: June 10, 2014 in Uncategorized

I have been going through some very big changes in my life over the past year. I have changed my job, and moved with my whole family to Tokyo. I have been so busy with these stuff that I could not find enough time to write blogs.

I am still trying to adjust to my new life here at Tokyo. So far things have been a little rocky, but I am slowly settling down.

As for my job, I have joined one of the biggest e-commerce companies in the world. So far things have been going sort of OK here.

As soon as I get settled in, I will start writing again!

As for the regular readers (well, there might be at least one :D) of this blog: I deeply apologize for being absent for so long. Hopefully you will see me writing again!

Hope to see you all soon!


This is a continuation of an introductory discussion on generics, previous parts of which can be found here.

In this post I will focus on type parameter bounds and their usage.

Bounded Type Parameters

When a generic type is compiled, all occurrences of a type parameter are removed by the compiler and replaced by a concrete type. The compiler also generates appropriate casting needed for type safety by itself during this procedure. This concrete type is typically Object, but compiler can use other types as well. This process is called Type Erasure and will be explained in a future post. For the time being, all we need to understand is that the type information of generic types are lost once they are compiled. For this reason, if we want to access a method/property using a type parameter, we’ll typically be able to access those ones that are defined in the class Object (I am oversimplifying here as we’ll be able to access other methods/properties as well if we use a bound, which we will discuss here in this post). For example, take a look at the following code snippet –

public class MyGenericClass<E> {
    private E prop;

    public MyGenericClass(E prop) {
        this.prop = prop;
    }

    public E getProp() {
        return this.prop;
    }

    public void printProp() {

        //OK, because toString is defined within Object
        System.out.println(this.prop.toString());
    }

    public int getValue() {
        /**
         * NOT OK, because Object doesn’t have
         * compareTo method. Compile-time Error.
         */
        return this.prop.intValue();
    }
}

After compiling the above class, I get the following message –

MyGenericClass.java:37: error: cannot find symbol

return this.prop.intValue();

^

symbol:   method intValue(E)

location: variable prop of type E

where E is a type-variable:

E extends Object declared in class MyGenericClass

1 error

Although the message seems cryptic, the reason behind this is the one that I’ve stated above – when this type is compiled the type parameter E here will be replaced by Object by the compiler, and it doesn’t have intValue method.

So the problem occurs here because the compiler is using Object to replace the type parameters. If I could somehow tell the compiler to use other types during this erasure which has an intValue (Number, for example) method, then this error would have been resolved.

This is exactly what parameter bounds do. By using a type as a bound on a type parameter, I can instruct compiler to use that type during the erasure in place of Object, and then I can easily access the methods/properties defined in that type.

The general syntax for specifying a type parameter bound is as follows –

public class MyGenericClass
    <E extends MyBoundType,
     T extends AnotherBoundType,
     .......>

This also tells the compiler that when a type argument is passed during an instance creation of this generic type, it will be a subtype of MyBoundType, so it can safely let us access the methods that are defined in that type using the type parameter E. If any other type is passed, then the compiler will issue a compile-time error. The extends keyword specify the bound relation between E and MyBoundType. We will use the same keyword even if E is bounded by an interface type, that is, if MyBoundType is an interface. Here extends means both classical extends and implements.

So, if we use Number as our parameter bound for our last example, the error message will be gone because now the compiler will use Number to erase type parameter E, and it has an intValue method defined in it -

public class MyGenericClass<E extends Number> {
    private E prop;

    public MyGenericClass(E prop) {
        this.prop = prop;
    }

    public E getProp() {
        return this.prop;
    }

    public void printProp() {
        // OK, because toString is defined within Object
        System.out.println(this.prop.toString());
    }

    public int getValue() {
        // Now it compiles just fine!
        return this.prop.intValue();
    }
}

This code will now compile just fine.

Remember our generic Insertion Sort algorithm from the first part of the series? We had declared it like this –

public class InsertionSort<E extends Comparable<E>>

This told the compiler that the type arguments that will be passed here will all implement the Comparable interface, so they will have a compareTo method. As a result, compiler allowed us to do this inside the sort method –

for (int i = 1; i <= elements.length - 1; i++) {
    E valueToInsert = elements[i];
    int holePos = i;

    /**
     * See how we are calling compareTo method
     * on the type parameter?
     */
    while (holePos > 0 &&
            valueToInsert.compareTo(elements[holePos - 1]) < 0) {
        elements[holePos] = elements[holePos - 1];
        holePos--;
    }

    elements[holePos] = valueToInsert;
}

This example also shows that we can pass another generic type as a type parameter bound. In fact we can use all Classes, Interfaces and Enums and even another type parameter as a bound. Only primitive and array types are not allowed as a bound.

Multiple and Recursive Bounds

We can define multiple bounds on a single parameter. In this case we use the & operator to separate them from one another -

public class MyGenericClass<E extends MyBoundClass & MyBoundInterface>

This tells the compiler that the type argument that is passed will be a subclass of MyBoundClass and implement MyBoundInterface. So, we will be able to access all the methods/properties defined in those types.

The Java Language Specification requires us to list the class bound first, otherwise the compiler will throw an error. For example, the following will throw an error –

/**
 * Will throw an error because we are not
 * listing the class bound first.
 */
public class MyGenericClass
    <E extends MyBoundInterface
     & MyBoundClass>

We can also declare recursive bounds, so that a bound can depend on itself too. Consider again our stack example from first part of the series. We declared it like this –

public class InsertionSort<E extends Comparable<E>>

Here, the bound is recursive because E itself depends upon E (the one that is supplied to Comparator). If we pass Integer as a type argument when creating an instance of InsertionSort, then the type argument to Comparable will be Integer too.

We can also declare mutually recursive bounds like this –

public class MyGenericClass
    <T extends FirstType<T, U>,
     U extends SecondType<T, U>>
Java Enum Declaration

Let us now consider an example from the Java API itself. We all know that enumerations in Java are objects of some class, and that class extends the Enum class. The declaration of that class looks like this –

public abstract class Enum<E extends Enum<E>>
    implements Comparable<E>, Serializable

Beginners in Java Generics find the first line very confusing. Before explaining the reasoning behind this weird declaration, let us explore an example.

Suppose that we are going to build a software system which will have various types of vehicles (a vehicle simulation system, perhaps?). The vehicles will have a name and a size. We also want to compare vehicles with each other based on their size. An approach for building the vehicle classes might look like this –

public abstract class Vehicle {
    private String name;
    private double length;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getLength() {
        return length;
    }

    public void setLength(double length) {
        this.length = length;
    }
}

public class Car extends Vehicle implements Comparable<Car> {
    public int compareTo(Car anotherCar) {
        double thisLength = this.getLength();
        double thatLength = anotherCar.getLength();

        if (thisLength > thatLength)
            return 1;
        else if (thisLength < thatLength)
            return -1;

        return 0;
    }

    // other methods and properties
}

public class Bus extends Vehicle implements Comparable<Bus> {
    public int compareTo(Bus anotherBus) {
        double thisLength = this.getLength();
        double thatLength = anotherBus.getLength();

        if (thisLength > thatLength)
            return 1;
        else if (thisLength < thatLength)
            return -1;

        return 0;
    }

    // other methods and properties
}

The problem of the above implementation is pretty obvious – even though the compare method implementations are almost the same among all the subtypes of Vehicle, we still have to maintain different copies of this method. This creates a maintenance problem as now changing the comparison logic forces us to change the code in many places.

To remedy this, we can remove the comparison from the subtypes and move it up in the Vehicle. To do this, we will rewrite those classes as follows –

public abstract class Vehicle implements Comparable<Vehicle> {
    private String name;
    private double length;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getLength() {
        return length;
    }

    public void setLength(double length) {
        this.length = length;
    }

    public int compareTo(Vehicle anotherVehicle) {
        double thisLength = this.getLength();
        double thatLength = anotherVehicle.getLength();

        if (thisLength > thatLength)
            return 1;
        else if (thisLength < thatLength)
            return -1;

        return 0;
    }
}

public class Car extends Vehicle {
    // car-specific methods and properties
}

public class Bus extends Vehicle {
    // bus-specific methods and properties
}

This approach also has a problem. The above implementation will allow us to compare a car with a bus without any errors –

Car car = new Car();
car.setName("Toyota");
car.setLength(2);

Bus bus = new Bus();
bus.setName("Volvo");
bus.setLength(4);

car.compareTo(bus);    // No error

This is certainly a problem, since comparing a bus with a car should not be done using the same logic that is used to compare a car with a car.

How can we solve this? How can we reuse the comparison logic among all the subtypes, while at the same time raising error flags at compile time whenever someone tries to compare two incompatible types?

In the last example the problem occurred because the compareTo method has a parameter which is of type Vehicle. As a result we were able to pass any subtypes of Vehicle to it, like the way we passed a bus to compare with a car. Let’s try to change the type of this parameter so that now this kind of mixing generates an error. If we want to allow the comparison of a car only with a car, then the argument to the compareTo method must be of type Car. If we change it to Car, we will also need to change the type argument that we are passing to Comparable in the Vehicle class declaration –

public abstract class Vehicle implements Comparable<Car> {
    // other methods and properties

    public int compareTo(Car car) {
        // method implementation
    }
}

But then this will not allow us to compare any other types. We will not be able to compare a bus with another bus. To allow this, we will need to change the parameter to be of type Bus. If we declare a new subtype named Cycle, we will also need this method to support this type too! So we can see that the parameter type of this compare method should vary if we need to enforce compatible comparison.

From the above discussion it’s clear that we need to parameterize the parameter type of the compareTo method, and in turn, parameterize the Vehicle class itself. If we do this, we will then be able to pass Car, Bus, and Cycle etc. as its type argument, which in turn will be used as the parameter type of the compare method. In general, after we declare Vehicle as a generic type, all of its subtypes will pass themselves as a type argument while extending from it, so that the parameter type of this compareTo method matches their type –

public abstract class Vehicle<E> implements Comparable<E> {
    // other methods and properties

    public int compareTo(E vehicle) {
        // method implementation
    }
}

/**
* Now this class’s compareTo version will take a Car type
* as its argument.
*/
public class Car extends Vehicle<Car> {}

/**
* Now this class’s compareTo version will take a Bus type
* as its argument.
*/
public class Bus extends Vehicle<Bus> {}

/**
* Doing something like this will now generate a
* compile-time error.
*/
car.compareTo(bus);

This approach solves our last problem that we were facing, but introduces a new one. After converting Vehicle to a generic type and using the type parameter as the parameter type of the compare method, it looks like this –

public int compareTo(E anotherVehicle) {
    double thisLength = this.getLength();

    // Now the following line is an error.
    double thatLength = anotherVehicle.getLength();

    if (thisLength > thatLength)
        return 1;
    else if (thisLength < thatLength)
        return -1;

    return 0;
}

Since we didn’t put any bound on the type parameter, and Object class doesn’t have a getLength method, compiler will generate an error. We get to call this method on a variable of type E only if it’s bounded by Vehicle itself, because then compiler will know that variables of this type will have this method. So our compare method will work only if E is bounded by Vehicle itself! After this modification, the classes look like below –

public abstract class Vehicle<E extends Vehicle<E>>
        implements Comparable<E> {

    private String name;
    private double length;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getLength() {
        return length;
    }

    public void setLength(double length) {
        this.length = length;
    }

    public int compareTo(E anotherVehicle) {
        double thisLength = this.getLength();
        double thatLength = anotherVehicle.getLength();

        if (thisLength > thatLength)
            return 1;
        else if (thisLength < thatLength)
            return -1;

        return 0;
    }
}

public class Car extends Vehicle<Car> {
    // Car-specific properties and methods
}

public class Bus extends Vehicle<Bus> {
    // Bus-specific properties and methods
}

// and in main
Car car = new Car();
car.setName("Toyota");
car.setLength(2);

Bus bus = new Bus();
bus.setName("Volvo");
bus.setLength(4);

car.compareTo(car);       // Works as expected
car.compareTo(bus);       // compile-time error

Even with the above example, a certain kind of type mixing is possible. Rather than discussing it here, I am going to leave it to you to figure it out. If you can’t, check out the next post of this series!

I guess now you know why the Enum class is declared in that way. This kind of recursive bound allows us to write methods in a supertype which will take its subtypes as its arguments, or returns them as return value. I encourage you to check out the source code of the Enum class to find out these methods.

That’s it for today. Stay tuned for the next post!

Resources

  1. Java Generics and Collections
  2. Java Generics FAQs by Angelika Langer

This is a continuation of an introductory discussion on generics, previous parts of which can be found here.

Before jumping into subtyping in generics, I thought it will be good if I talk a little about generic methods and automatic type inference.

Generic Methods

Just like generic class, we can define generic methods in Java.  A generic method is a method which defines its own type parameters. In this case, the type parameter’s scope is limited to the method where it’s declared. To declare a generic method, we just list the type parameters before its return type, like this –

public class MyClass {
    public static <T> void myFirstGenericMethod(T param) {
        System.out.println(param);
    }
}

We can invoke this generic method in the following way –

MyClass. <String>myFirstGenericMethod("Hello World");

If this was an instance method, then we would have called it like this –

myInstance. <String>myFirstGenericMethod("Hello World");

Notice how we have passed the type argument while calling the method. Generally, the rule is as follows –

class/instance_var. <type_argument_list>methodName(argument list…..);

Our above method call will also compile without specifying the type parameters explicitly. This happens because of type inference – the java compiler’s ability to automatically determine the type argument during generic method call/generic instance creation. We will talk more about this in a few moments.

Let’s write another useful generic method which will add all elements of an array into a list –

public class Utils {
    public static <T> void addAll(List<T> myList, T[] arr) {
        for (T elem: arr) {
            myList.add(elem);
        }
    }
}

The above code should be self-explanatory at this point.

Type Inference

Type inference is Java compiler’s ability to look at generic method invocation and instance creation to determine the type arguments that make the code work. This algorithm is capable of determining the type of the arguments and the type that the result is being assigned or returned, provided that there is enough information for the inference algorithm to do so.

According to the official Oracle Java tutorial

The inference algorithm determines the types of the arguments and, if available, the type that the result is being assigned, or returned. Finally, the inference algorithm tries to find the most specific type that works with all of the arguments.

There are two situations where type inference algorithm comes into play –

  1. When an object of a generic type is being created
  2. When a generic method is being invoked
Type Inference during generic instance creation

Generally, type inference for instance creation works like this – first, the compiler tries to deduce the type arguments from the constructor arguments in the object creation expression (the one with the new). If it fails to deduce the type arguments for any of the type parameters from there, it then uses information from the context in which the creation expression appears.

In one of my previous posts, I have demonstrated a short-cut way to create generic instances without specifying the type argument –

MyPrettySimpleGenericClass<Integer> item = new
        MyPrettySimpleGenericClass<>();

Here, we see that the instance that is being created is being assigned to a variable, which is of type MyPrettySimpleGenericClass<Integer>. This line will be valid only if the generic instance being created also has the Integer as its type argument (no, any subtype/supertype of Integer will also not do, we’ll see why in a future post). When type inference algorithm tries to deduce the type argument that will be applicable for this instance creation expression, it first takes a look at the constructor arguments. Since the above code doesn’t have any, it then takes a look at the left hand side of the assignment operator and sees that only Integer can make the statement valid. Thus it automatically passes Integer as the type argument to the generic instance being created.

If constructor arguments are available and if the inference algorithm can determine the applicable type arguments from them, then it completely ignores the assignment context. Let’s consider an example –

List<Number> list = new ArrayList<>(Arrays.asList(0L, 0L));

We are passing long values to the asList method, which takes an arbitrary number of arguments as its input, and returns an object of type List containing those elements. As a result, the inference algorithm determines its return type to be List<Long>, and returns an instance of this type. This returned value is then again used by the inference algorithm to determine the type argument of the ArrayList being created, which is finally set to Long as well. Since List<Long> and List<Number> are totally incompatible type (again, more on this in a future post), the compiler generates an error (although in Java 8 it compiles fine).

What happens if there are no constructor arguments, or assignment of the generic instance being created? Let’s consider an example of that too –

Iterator<String> it = new ArrayList<>().iterator();

In the above example, the object creation expression occurs inside a chain of method calls. As the type inference for object creation expression is only allowed for an assignment context (this is only true for JDK 7 as JDK 8 has improved the inference algorithm a bit by including the method invocation context), the compiler is unable to determine the correct type. As a result, a compile-time error is issued by the compiler.

Similar reasoning can be applied for the following blocks of code –

class TestDrive {
    static void printAll(List<Integer> list) {
        for (Integer i: list) {
            System.out.println(i);
        }
    }

    public static void main(String[] args) {
        printAll(new ArrayList<>());  // Error in Java 7
    }
}

In these cases, the recommended approach is to introduce a variable and assign the object instance to it first. It is then used in the expression replacing the object creation expression.

Type inference generally fails for anonymous inner classes. Consider another example –

Comparator<Integer> cmp = new Comparator<>() {

    @Override
    public int compare(Integer o1, Integer o2) {
        return o1.compareTo(o2);
    }
};

In the above example, the type inference cannot determine the applicable type arguments from the context, and will throw a compilation error.

Type Inference during generic method invocation

Type inference algorithm for generic methods works in the same way as the instance creation. Here, type inference first observes the method arguments to determine the type parameter, and then observes the context in which it is being used.

If we recall our myFirstGenericMethod method, we can see that the type inference is capable of determining the appropriate type argument by observing the string argument. Same thing is true for our addAll utility method.

What happens if the method doesn’t have any arguments which correspond to its type parameter? In this case the compiler tries to determine the type from the method invocation context. Let’s write a method which will create lists for us if we specify the initial capacity –

static <T> List<T> createList(int capacity) {
    List<T> list = new ArrayList<>(capacity);
    return list;
}

We can call it in the following way –

List<Integer> list = createList(10);

Our utility method doesn’t have any arguments which correspond to the type parameter. Hence inference algorithm cannot deduce anything by observing the arguments in the method invocation. It then tries to observe the calling context and sees that the result returned by the method is being assigned to a reference of type List<Integer>, and determines the type argument to the method to be Integer.

What if the above method is not being called from an assignment context? The compiler cannot determine the type argument! If we call our previous printAll method like this –

printAll(createList(10));

then the compiler will throw an error (JDK 8 can determine the type argument). We have to specify the type argument explicitly in this case, or introduce a variable to hold the value returned by the first method, and then pass it as argument to the latter.

We can declare generic methods in a generic class declaration too. What happens if the type parameter of the generic method matches with the type parameter of the generic class? I leave this to the reader to experiment!

Lastly, the Java Specification says that we can only specify type arguments when we call generic methods using dot notation. As a result, calling our createList method like this will result in a compile-time error (obviously from inside the class where it’s defined) –

<Integer>createList(5);  // Error, won’t compile

In this case, if the generic method is a static one, we use class name to invoke the method. If it’s an instance method, we use an instance reference (or this / super) to call it.

That’s it for today. Stay tuned for the next post!

Resources
  1. Java Generics and Collections by Naftalin, Philip Wadler
  2. Angelika Langer: Java Generics Faqs

This is a continuation of my introductory tutorial on Generics. Previous posts in this series can be found here.

Why should we use Generics

I intended to write about this after I’ve demonstrated some generics examples so that their usefulness become apparent to the readers. So let’s see what the official tutorial of Java has to say about the usefulness of generics, or, why should we use them -

  1. Stronger type checks at compile time
  2. Elimination of casts
  3. Enabling programmers to implement generic algorithms

The first two are very closely related. We have already seen how generics allow us to write generic algorithms. It’s time to focus on the first two.

Point number one – stronger type checks at compile time

Generics were specifically introduced to work with the Java Collections API. Before generics era, Lists and ArrayLists didn’t use generics, and were used like this -

/* Create a list to hold some
 * string values.
 */
List myList = new ArrayList();

myList.add("Hello ");
myList.add("World ");
myList.add("from Java");
myList.add(1);    // Oh no...

Iterator it = myList.iterator();
while(it.hasNext()) {
  String s = (String ) it.next();
  System.out.print(s);
}

If you are new to Collections, don’t worry, they are not that hard to learn. For the time being, you can simply consider myList to be something like an array, or a collection, where we can add elements using the add method, and later traverse them using a standard iterator.

This is how we used collections in the pre-generic era. To be used as a collection for any types, the add method expects an Object as its argument and casts/boxes any other types to it. As you can see from the above code, one can easily put integers into a collection of strings if they are not careful enough, just like we did in line number 9. In this case the program will compile without any trouble, but we will get a ClassCastException at runtime at line number 13, because we cannot cast an integer into a string.

Some people might consider my example far-fetched, saying that the scenario described above is less likely to occur in real world. I would like to tell them that the above scenario is a very simplified one. For example, you might call a library method in your program which returns a collection of strings, and you didn’t know it in advance and put integers in that list, resulting in runtime exception. If this is also not convincing, consider the Date class in Java which is defined in two different packages – java.util and java.sql, and mixing them together in the same collection will likely result in a runtime exception. It was programmer’s responsibility to make sure that a collection didn’t contain elements from various types, and programmers did make those mistakes.

When we use generics, the above problem doesn’t occur. Now it’s the compiler’s responsibility to make sure that a collection doesn’t get polluted by containing elements from more than one type. When we try to put an integer into a collection of strings, compiler will generate an error at compile time, rather than at runtime -

/* Create a list to hold some
 * string values.
 */
List<String> myList = new ArrayList<String>();

myList.add("Hello ");
myList.add("World ");
myList.add("from Java");
myList.add(1);    // Now this is a compile time error

Iterator<String> it = myList.iterator();
while(it.hasNext()) {
  String s = it.next();
  System.out.print(s);
}

Using generics in this way ensures that our list will contain only elements of a certain type, and we will not run into any runtime exception. From the java best practice guidelines, we know that catching and fixing errors at compile time is much easier than catching them at runtime (more on this in a future article).

Point number two – elimination of casts

If you check out the second example, you’ll see that the cast at line number 13 is no longer required. This is because when we used generics, we have explicitly told the compiler that this collection will only contain a collection of strings. We will add only strings to it, and we will retrieve only string values from it. If you could check the method signature of the next method now (just like I did using Eclipse by hovering the cursor over this method), you’ll see that it too returns a string value where in the first example it returned an object of type Object. Eliminating cast this way also helps us to reduce the possibility of encountering any runtime exception (although implicitly this is a consequence of the first point).

Point number three – enabling programmers to implement generic algorithms

I have already explained this point in the previous two articles. If like to visit them again, here is the first one and here is the second one :-).

With the above three usefulness, I’d like to mention another one which I have described below.

Point number four – enabling programmers to write clear and concise code

Let’s write a simple program – put five numbers into a list and add them together. Here’s how we did it before generics were introduced -

List intList = Arrays.asList(new Integer[] {
    new Integer(1), new Integer(2), new Integer(3),
    new Integer(4), new Integer(5)
});
int sum = 0;

Iterator it = intList.iterator();
while(it.hasNext()) {
    sum += ((Integer) it.next()).intValue();
}

System.out.println("The sum is: " + sum);

Reading this code is not too easy. It’s also not expressive, as we cannot tell what types of elements we intend to store in our list.

If we write the same code using generics, then it becomes something like this -

List<Integer> intList = Arrays.asList(1, 2, 3, 4, 5); // We can do this
                                                      // because of Autoboxing
int sum = 0;

for (int i: intList) {
    sum += i;
}

System.out.println("The sum is: " + sum);

You can probably read this code without much explanation. It’s clear and much shorter from it’s non-generic counterpart. Here we have made use of couple of other Java language features – the for-each loop and autoboxing/unboxing. Indeed generics work well with these features.

I hope you are now convinced about generics :-).

That’s it for today. Hope to write another post real soon. Stay tuned!

Additional Resources