java泛型如何深复制,如何使用通配符在Java中复制泛型集合

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()`

}

}

}

循环内部存在此语法错误：

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,但我想为这个类实现一个copy-constructor.复制的实例将被用作其他的快照.任何的想法？

解决方法:

变化：

Collection extends Number> dst

至 ：

Collection super Number> dst

添加是不允许使用？延伸.有一些被称为Get and Put原则的东西.引自Philip Wadler的Generics and Collections书：

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

另请参阅PECS以获得有关此现象的更常见解释. (但是,我更喜欢GET和PUT原则而不是生产者和消费者原则,因为它不那么令人困惑)

标签：java,generics

来源： https://codeday.me/bug/20190519/1137312.html