Java 为什么这些继承的类型用作参数化类型不兼容的类型？

我对参数化集合的理解是，如果要使用参数化类型的子类型，则需要将集合声明为

collection，因为您可以将
Box
添加到
Box
，这将违反
bananabox
的完整性

public interface Apple extends Fruit {}

//...

Box<Apple> apples = new Box<>(); // this is legal
Box<? extends Fruit> fruits = apples; // this is legal

Collection<Box<Banana>> bananaBoxes = new ArrayList<>(); 

Collection<Box<? extends Fruit>> boxes = bananaBoxes; //if this were legal...
boxes.add(fruits); //then this would be legal

//and this would be a type violation:
Box<Banana> bananas = bananaBoxes.iterator().next(); 

Apple扩展水果的公共接口{}
//...
苹果盒子=新盒子（）；//这是合法的
Box因为您可以将
框
添加到
框
，这将违反
bananabox
的完整性

public interface Apple extends Fruit {}

//...

Box<Apple> apples = new Box<>(); // this is legal
Box<? extends Fruit> fruits = apples; // this is legal

Collection<Box<Banana>> bananaBoxes = new ArrayList<>(); 

Collection<Box<? extends Fruit>> boxes = bananaBoxes; //if this were legal...
boxes.add(fruits); //then this would be legal

//and this would be a type violation:
Box<Banana> bananas = bananaBoxes.iterator().next(); 

Apple扩展水果的公共接口{}
//...
苹果盒子=新盒子（）；//这是合法的
盒子
error: incompatible types
required: Collection<Box<? extends Fruit>>
found:    Collection<Box<Banana>>

public interface Apple extends Fruit {}

//...

Box<Apple> apples = new Box<>(); // this is legal
Box<? extends Fruit> fruits = apples; // this is legal

Collection<Box<Banana>> bananaBoxes = new ArrayList<>(); 

Collection<Box<? extends Fruit>> boxes = bananaBoxes; //if this were legal...
boxes.add(fruits); //then this would be legal

//and this would be a type violation:
Box<Banana> bananas = bananaBoxes.iterator().next(); 

Collection<? extends Box<? extends Fruit>> boxes = bananaBoxes;