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This chapter is from the book

Bounded Type Parameters

There may be times when you want to restrict the types that can be used as type arguments in a parameterized type. For example, a method that operates on numbers might only want to accept instances of Number or its subclasses. This is what bounded type parameters are for.

To declare a bounded type parameter, list the type parameter’s name, followed by the extends keyword, followed by its upper bound, which in this example is Number. Note that, in this context, extends is used in a general sense to mean either extends (as in classes) or implements (as in interfaces):

public class Box<T> {

    private T t;

    public void set(T t) {
        this.t = t;
    }

    public T get() {
        return t;
    }

    public <U extends Number> void inspect(U u){
        System.out.println("T: " + t.getClass().getName());
        System.out.println("U: " + u.getClass().getName());
    }

    public static void main(String[] args) {
        Box<Integer> integerBox = new Box<Integer>();
        integerBox.set(new Integer(10));
        integerBox.inspect("some text"); // error: this is still String!
    }
}

If we modify our generic method to include this bounded type parameter, compilation will now fail, since our invocation of inspect still includes a String:

Box.java:21: <U>inspect(U) in Box<java.lang.Integer> cannot
  be applied to (java.lang.String)
                        integerBox.inspect("10");
                                  ^
1 error

In addition to limiting the types you can use to instantiate a generic type, bounded type parameters allow you to invoke methods defined in the bounds:

public class NaturalNumber<T extends Integer> {

    private T n;

    public NaturalNumber(T n)  { this.n = n; }

    public boolean isEven() {
        return n.intValue() % 2 == 0;
    }

    // . . .
}

The isEven method invokes the intValue method defined in the Integer class through n.

Multiple Bounds

The previous example illustrates the use of a type parameter with a single bound, but a type parameter can have multiple bounds:

<T extends B1 & B2 & B3>

A type variable with multiple bounds is a subtype of all the types listed in the bound. If one of the bounds is a class, it must be specified first, as follows:

Class A { /* . . . */ }
interface B { /* . . . */ }
interface C { /* . . . */ }

class D <T extends A & B & C> { /* . . . */ }

If bound A is not specified first, you get a compile-time error:

class D <T extends B & A & C> { /* . . . */ }  // compile-time error

Generic Methods and Bounded Type Parameters

Bounded type parameters are key to the implementation of generic algorithms. Consider the following method, which counts the number of elements in an array T[] that are greater than a specified element elem:

public static <T> int countGreaterThan(T[] anArray, T elem) {
    int count = 0;
    for (T e : anArray)
        if (e > elem)  // compiler error
            ++count;
    return count;
}

The implementation of the method is straightforward, but it does not compile because the greater than operator (>) applies only to primitive types such as short, int, double, long, float, byte, and char. You cannot use the greater than operator to compare objects. To fix the problem, use a type parameter bounded by the Comparable<T> interface:

public interface Comparable<T> {
    public int compareTo(T o);
}

The resulting code is as follows:

public static <T extends Comparable<T>> int countGreaterThan(T[] anArray, T elem) {
    int count = 0;
    for (T e : anArray)
        if (e.compareTo(elem) > 0)
            ++count;
    return count;
}
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