An Introduction to Generics in Java – Part 5

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

An Introduction to Generics in Java – Part 4

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