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Bounded Type Parameters (The Java™ Tutorials > Learning the Java Language > Generics)
Trail: Learning the Java Language
Lesson: Generics
Home Page > Learning the Java Language > Generics
Bounded Type Parameters
There may be times when you'll want to restrict the kinds of types that are allowed to be passed to a type parameter. 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).

/**
 * This version introduces a bounded type parameter.
 */
public class Box<T> {

    private T t;          

    public void add(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.add(new Integer(10));
        integerBox.inspect("some text"); // error: this is still String! 
    }
}
By modifying 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

To specify additional interfaces that must be implemented, use the & character, as in:

<U extends Number & MyInterface>
Previous page: Generic Methods and Constructors
Next page: Subtyping
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