java - 如何使用通配符在 Java 中复制通用集合

Question

考虑这个 java 类:

import java.util.Collection;
import java.util.Iterator;
import java.util.LinkedList;


public class NumberSet {

    private Collection<? extends Number> numbers;

    public NumberSet(Collection<? extends Number> numbers) {
        this.numbers = numbers;
    }

    public NumberSet(NumberSet other) {

        //copy other.numbers to this.numbers
        numbers = new LinkedList<>();
        for (Iterator<? extends Number> it = other.numbers.iterator(); it.hasNext();) {
            numbers.add(it.next()); // Here's Syntax Error near `it.next()`
        }

    }
}

for 循环中存在语法错误:

actual argument Number cannot be converted to CAP#1 by method invocation conversion
where E is a type-variable:
    E extends Object declared in interface Collection
where CAP#1 is a fresh type-variable:
    CAP#1 extends Number from capture of ? extends Number

我理解 PECS 的含义，但是我想为这个类实现一个复制构造函数。复制的实例将用作 other 的快照。有什么想法吗？

Answer 1

改变:

 Collection<? extends Number> dst 

收件人:

 Collection<? super Number> dst

不允许使用 添加？延伸。有一种称为 Get 和 Put 的原则。引用自 Generics and Collections从 Philip Wadler 预订:

The Get and Put Principle: use an extends wildcard when you only get values out of a structure, use a super wildcard when you only put values into a structure, and don’t use a wildcard when you both get and put

另请查看 PECS对于这种现象的更普遍的解释。 (不过，我更喜欢 GET 和 PUT 原则，而不是 Producer 和 Consumer 原则，因为它不那么困惑)