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 ofNumber
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 isNumber
. Note that, in this context,extends
is used in a general sense to mean either "extends" (as in classes) or "implements" (as in interfaces).
By modifying our generic method to include this bounded type parameter, compilation will now fail, since our invocation of/** * 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! } }inspect
still includes aString
:Box.java:21: <U>inspect(U) in Box<java.lang.Integer> cannot be applied to (java.lang.String) integerBox.inspect("10"); ^ 1 errorTo specify additional interfaces that must be implemented, use the
&
character, as in:
<U extends Number & MyInterface>