Java-removeIf示例 [英] Java - removeIf example

问题描述

HashMap<Integer, ArrayList<Integer>> cityMap = new HashMap<>();
...    
for (ArrayList<Integer> list : cityMap.values()) {
    int size = list.size();
    if (size > 0) {
        list.removeIf(i -> true);
    }
}

在这种情况下，我不太了解removeIf的作用.特别是部分(i -> true).谢谢您的解释.

I don't quite understand what the removeIf does in this case. Especially the part (i -> true). Thank you for any explanation.

推荐答案

removeIf()的Javadoc指出:

The Javadoc of removeIf() states:

删除此集合中满足给定谓词的所有元素.

Removes all of the elements of this collection that satisfy the given predicate.

您的示例中的谓词始终为true，因为您使用表达式i -> true将列表中的每个整数i映射到true.

The predicate in your example is always true because you map each integer i in your list to trueby the expression: i -> true.

我添加了一个更简单的示例，该示例通过谓词i % 2 == 0删除所有偶数整数并保留所有奇数整数:

I added a simpler example which removes all even integers and keeps all odd integers by the predicate i % 2 == 0:

丑陋的设置:

List<List<Integer>> lists = new ArrayList<List<Integer>>() {{
    add(new ArrayList<>(Arrays.asList(1,2,3,4)));
    add(new ArrayList<>(Arrays.asList(2,4,6,8)));
    add(new ArrayList<>(Arrays.asList(1,3,5,7)));
}};

仅保留奇数:

for (List<Integer> list : lists) {
    list.removeIf(i -> i % 2 == 0);
    System.out.println(list);
}

输出:

[1, 3]
[]
[1, 3, 5, 7]

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