Bounded Type Parameters受限的类型参数

There may be times when you want to restrict the types that can be used as type arguments in a parameterized type. 有时，您可能希望限制可在参数化类型中用作类型参数的类型。For example, a method that operates on numbers might only want to accept instances of Number or its subclasses. 例如，对数字进行操作的方法可能只希望接受Number或其子类的实例。This is what bounded type parameters are for.这就是受限类型参数的用途。

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. 要声明有界类型参数，请列出类型参数的名称，后跟extends关键字，再后跟其上限，在本例中为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).注意，在此上下文中，extends在一般意义上表示“extends”（在类中）或“implements”（在接口中）。

public class Box<T> {

    private T t;          

    public void set(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.set(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:通过修改泛型方法以包含此有界类型参数，编译现在将失败，因为我们调用inspect仍然包含一个String：

Box.java:21: <U>inspect(U) in Box<java.lang.Integer> cannot
  be applied to (java.lang.String)
                        integerBox.inspect("10");
                                  ^
1 error

In addition to limiting the types you can use to instantiate a generic type, bounded type parameters allow you to invoke methods defined in the bounds:除了限制可用于实例化泛型类型的类型外，有界类型参数还允许调用在边界中定义的方法：

public class NaturalNumber<T extends Integer> {

    private T n;

    public NaturalNumber(T n)  { this.n = n; }

    public boolean isEven() {
        return n.intValue() % 2 == 0;
    }

    // ...
}

The isEven method invokes the intValue method defined in the Integer class through n.isEven方法通过n调用Integer类中定义的intValue方法。

Multiple Bounds多重界限

The preceding example illustrates the use of a type parameter with a single bound, but a type parameter can have multiple bounds:前面的示例演示了使用具有单个边界的类型参数，但类型参数可以具有多个边界：

<T extends B1 & B2 & B3>

A type variable with multiple bounds is a subtype of all the types listed in the bound. 具有多个边界的类型变量是边界中列出的所有类型的子类型。If one of the bounds is a class, it must be specified first. 如果其中一个边界是类，则必须首先指定它。For example:例如：

Class A { /* ... */ }
interface B { /* ... */ }
interface C { /* ... */ }

class D <T extends A & B & C> { /* ... */ }

If bound A is not specified first, you get a compile-time error:如果未首先指定绑定A，则会出现编译时错误：

class D <T extends B & A & C> { /* ... */ }  // compile-time error