Question

我有三个HashSet，它们的访问者来自不同的国家。如果我使用Mexico.retainAll（Cuba）;然后Mexico.retainAll（牙买加）它给了我访问过所有三个国家的所有人员的列表。哪个是：奥利维亚，约翰，威廉

但是当我这样做Mexico.retainAll（Cuba）;然后是Mexico.removeAll（Jamaica），以仅吸引访问过墨西哥和古巴而不是牙买加的人。它给了我一个空的集吗？

设置值如下：

HashSet<String> Mexico = new HashSet<String>();

[Olivia，Zoe，Jennifer，Susan，John，Isabella，William，Jack，Emma和Sophia]

HashSet<String> Cuba = new HashSet<String>();

[Olivia，Tom，Jennifer，Susan，John，William，Jack，Emma，Ava，Sophia，Lily]

HashSet<String> Jamaica = new HashSet<String>();

[Olivia，Tom，John，William，Ava，Jessica，Lily]

public static void main(String[] args) throws FileNotFoundException {   
    visitorPatternNOOP.allThreeCountries();
    visitorPatternNOOP.onlyMexicoCuba();
}


public class VisitorPatternNOOP {
    public void allThreeCountries (){
        Mexico.retainAll(Cuba);
        Mexico.retainAll(Jamaica);
        Iterator<String>  iterator = Mexico.iterator();
        System.out.println("These people has visited all three countries:  ");
        while (iterator.hasNext()){
            System.out.println(iterator.next());
        }
    }

    public  void onlyMexicoCuba (){
        Mexico.retainAll(Cuba);
        Mexico.removeAll(Jamaica);
        Iterator<String> iterator = Mexico.iterator();
        System.out.println("These people have visited Mexico and Cuba Only: ");
        while (iterator.hasNext()){
            System.out.println(iterator.next());
        }
    }
}

输出应该是[Jennifer，Susan，Jack，Emma，Sophia]，但我的设置空了。

Answer 1

Mexico.retainAll对Mexico Set进行了突变，因此您无法依次呼叫allThreeCountries和onlyMexicoCuba，并期望onlyMexicoCuba会看到Mexico集的原始内容。

您应该生成Mexico Set的副本，并对该副本进行变异。

public void allThreeCountries (){
    HashSet<String> result = new HashSet<>(Mexico);
    result.retainAll(Cuba);
    result.retainAll(Jamaica);
    Iterator<String> iterator = result.iterator();
    System.out.println("These people has visited all three countries:  ");
    while (iterator.hasNext()){
        System.out.println(iterator.next());
    }
}

public  void onlyMexicoCuba (){
    HashSet<String> result = new HashSet<>(Mexico);
    result.retainAll(Cuba);
    result.removeAll(Jamaica);
    Iterator<String> iterator = result.iterator();
    System.out.println("These people have visited Mexico and Cuba Only: ");
    while (iterator.hasNext()){
        System.out.println(iterator.next());
    }
}

在其上使用了retainAll方法之后，如何重设HashSet？

1 个答案: