Java TreeMap自定义比较器怪异行为 [英] Java TreeMap custom comparator weird behaviour

问题描述

我正在尝试创建一个具有排序键的 Map ，该键首先按字母顺序排序，最后按数字顺序排序。为此，我使用了 TreeMap 和自定义的 Comparator ：

I am trying to create a Map with sorted keys, sorted according to alphabetically first, and numerical last. For this I am using a TreeMap with a custom Comparator:

public static Comparator<String> ALPHA_THEN_NUMERIC_COMPARATOR =
    new Comparator<String> () {

        @Override
        public int compare(String first, String second) {
            if (firstLetterIsDigit(first)) {
                return 1;
            } else if (firstLetterIsDigit(second)) {
                return -1;
            }
            return first.compareTo(second);
        }
    };

private static boolean firstLetterIsDigit(String string) {
    return (string == null) ? false : Character.isDigit(string.charAt(0));
}

我编写了以下单元测试来说明问题所在：

I've wrote the following unit test to illustrate what goes wrong:

@Test
public void testNumbericallyKeyedEntriesCanBeStored() {
    Map<String, String> map = new HashMap<>();
    map.put("a", "some");
    map.put("0", "thing");
    TreeMap<String, String> treeMap = new TreeMap<>(ALPHA_THEN_NUMERIC_COMPARATOR);
    treeMap.putAll(map);

    assertEquals("some", treeMap.get("a"));
    assertEquals("thing", treeMap.get("0"));
}

结果：

java.lang.AssertionError: 
Expected :thing
Actual   :null

推荐答案

检查比较器代码。比较 0和 0是否应返回0？不，不是这样，因为您不需要检查字符串是否以数字开头。如果两个字符串都以数字开头，那么您也不会返回正确的顺序。

Check your comparator code. Does comparing "0" and "0" return 0, as it should? No it doesn't, since you don't check for equality if your string starts with a digit. You also don't return proper ordering if two strings both start with digits.

这篇关于Java TreeMap自定义比较器怪异行为的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！