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<\/CODE> or its subclasses. This is what bounded type parameters<\/I> are for.
\nTo declare a bounded type parameter, list the type parameter’s name, followed by the extends<\/CODE> keyword, followed by its upper bound<\/I>, which in this example is Number<\/CODE>. Note that, in this context, extends<\/CODE> is used in a general sense to mean either “extends” (as in classes) or “implements” (as in interfaces). <\/P>
\n<\/P>
\n
\/**
\n * This version introduces a bounded type parameter.
\n *\/
\npublic class Box<T> {<\/p>\n private T t; <\/p>\n
public void add(T t) {
\n this.t = t;
\n }<\/p>\n
public T get() {
\n return t;
\n }<\/p>\n
public <U extends Number<\/B>> void inspect(U u){
\n System.out.println(“T: ” + t.getClass().getName());
\n System.out.println(“U: ” + u.getClass().getName());
\n }<\/p>\n
public static void main(String[] args) {
\n Box<Integer> integerBox = new Box<Integer>();
\n integerBox.add(new Integer(10));
\n integerBox.inspect(“some text”); \/\/ error: this is still String!<\/B>
\n }
\n}
\n<\/PRE><\/BLOCKQUOTE>By modifying our generic method to include this bounded type parameter, compilation will now fail, since our invocation of inspect<\/CODE> still includes a String<\/CODE>:
\nBox.java:21: <U>inspect(U) in Box<java.lang.Integer> cannot
\n be applied to (java.lang.String)
\n integerBox.inspect(“10”);
\n ^
\n1 error
\n<\/PRE><\/BLOCKQUOTE>
\nTo specify additional interfaces that must be implemented, use the &<\/CODE> character, as in:
<\/P>
\n<U extends Number & MyInterface>
\n<\/PRE><\/BLOCKQUOTE><\/BLOCKQUOTE>
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