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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>
