java关于“的一些用法问题？”；？超级水果“；

我遇到了一个问题，例如：

水果类

public class Fruit extends Food {

    public static void main(String[] args) {
        Plate<? super Fruit> plate = new Plate<>(new Food());
        plate.setItem(new Apple());
        plate.setItem(new Food());
    }


    static class Apple extends Fruit {

    }
}

public class Food {
}

板材类'

public class Plate<T> {

    private T item;

    public Plate(T t) {
        item = t;
    }

    public T getItem() {
        return item;
    }

    public void setItem(T item) {
        this.item = item;
    }
}

是错误吗

这两种方法有什么区别

---

-就这些，谢谢

这条线上发生了两件事：

Plate<? super Fruit> plate = new Plate<>(new Food());

您正在设置一个盘子的项目。这个

盘子

可以是任何东西的盘子，只要它是

水果

或

水果

的超类。这意味着传递

食物

对象将不起作用。为什么？那么，如果

plate

实际上是一个

plate

？有可能，不是吗？编译器不知道

因此，您可以传递到

plate.setItem

的唯一对象是

水果

和

水果

的子类，这是

在这里使用

super

时，您创建了一些可以被消费但不能产生指定类型的东西。这正是你想要做的

为了简化一下，这里有一个示例，说明如何处理对象

Plate<? super Fruit> plate = new Plate<>(...);
Fruit fruit = plate.getItem(); // GOOD !
Food food = plate.getItem(); // GOOD ! Because even a Fruit can be assigned to a Food reference so it's OK !
Apple apple = plate.getItem(); // BAD ! No insurance it is an apple, we just know it is a Fruit

plate.setItem(new Fruit()); // GOOD !
plate.setItem(new Apple()); // GOOD ! Because we know it's always a Fruit, and Apple extends Fruit
plate.setItem(new Food()); // BAD ! No insurance we're not dealing with a Fruit, and a Food object can't be assigned to a Fruit reference (without casting)

Plate
Plate
Plate<? super Fruit> plate = new Plate<>(new Food());

plate.setItem(new Food()) 

Plate<? super Fruit> plate = new Plate<>(...);
Fruit fruit = plate.getItem(); // GOOD !
Food food = plate.getItem(); // GOOD ! Because even a Fruit can be assigned to a Food reference so it's OK !
Apple apple = plate.getItem(); // BAD ! No insurance it is an apple, we just know it is a Fruit

plate.setItem(new Fruit()); // GOOD !
plate.setItem(new Apple()); // GOOD ! Because we know it's always a Fruit, and Apple extends Fruit
plate.setItem(new Food()); // BAD ! No insurance we're not dealing with a Fruit, and a Food object can't be assigned to a Fruit reference (without casting)