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Trail: Learning the Java Language
Lesson: Generics
Generics
Introduction
Generic Types
Generic Methods and Constructors
Bounded Type Parameters
Subtyping
Wildcards
Type Erasure
Summary of Generics
Questions and Exercises
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>

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