创建有界泛型类

你可以通过在类定义中绑定该类型来限制泛型类中使用的有效类型。给定以下简单类型层次结构：

public abstract class Animal {
    public abstract String getSound();
}

public class Cat extends Animal {
    public String getSound() {
        return "Meow";
    }
}

public class Dog extends Animal {
    public String getSound() {
        return "Woof";
    }
}

没有有界泛型，我们不能创建一个通用的容器类，并且知道每个元素都是动物：

public class AnimalContainer<T> {

    private Collection<T> col;

    public AnimalContainer() {
        col = new ArrayList<T>();
    }

    public void add(T t) {
        col.add(t);
    }

    public void printAllSounds() {
        for (T t : col) {
            // Illegal, type T doesn't have makeSound()
            // it is used as an java.lang.Object here
            System.out.println(t.makeSound()); 
        }
    }
}

使用类定义中的泛型绑定，现在可以实现。

public class BoundedAnimalContainer<T extends Animal> { // Note bound here.

    private Collection<T> col;

    public BoundedAnimalContainer() {
        col = new ArrayList<T>();
    }

    public void add(T t) {
        col.add(t);
    }

    public void printAllSounds() {
        for (T t : col) {
            // Now works because T is extending Animal
            System.out.println(t.makeSound()); 
        }
    }
}

这也限制了泛型类型的有效实例化：

// Legal
AnimalContainer<Cat> a = new AnimalContainer<Cat>();

// Legal
AnimalContainer<String> a = new AnimalContainer<String>();

// Legal because Cat extends Animal
BoundedAnimalContainer<Cat> b = new BoundedAnimalContainer<Cat>();

// Illegal because String doesn't extends Animal
BoundedAnimalContainer<String> b = new BoundedAnimalContainer<String>();