Collections.sort（）方法上的IllegalArgumentException [英] IllegalArgumentException on Collections.sort() method

问题描述

我有用于字符串的比较器，这些字符串已转换为日期。当我将此比较器传递给Collections.sort（）方法时，我得到 java.lang.IllegalArgumentException：比较方法违反了它的常规约定！。

I have comparator for strings, which are converted to date. When I pass this comparator to Collections.sort() method I get java.lang.IllegalArgumentException: Comparison method violates its general contract!.

我已经阅读了一些有关此异常的文章，但是我真的不明白为什么会出现此异常。知道吗？

I have read some articles about this exception, but I don't really understand why this exception appears. Any idea ?

private SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy HH:mm");   

    Comparator<String> comparator = new Comparator<String>() {
                    @Override
                    public int compare(String o1, String o2) {
                        if (o1 == null && o2 == null) {
                            return 0;
                        }
                        if (o1 == null) {
                            return 1;
                        }
                        if (o2 == null) {
                            return -1;
                        }
                        try {
                            Date first = sdf.parse(o1);
                            Date second = sdf.parse(o2);
                            return first.compareTo(second);
                        } catch (Exception ignored) {
                            return 0;
                        }
                    }
                };

推荐答案

如果引发异常，则返回0。这意味着无论何时无法解析任何参数，都将被视为相等。考虑以下示例：

In case an exception is thrown, you return 0. That means whenever any of the arguments cannot be parsed, both are considered equal. Think of this example:

a = "01/01/2015"
b = "01/01/2016"
c = "xxx"

然后您得到

comparator.compare(a,c) = 0
comparator.compare(b,c) = 0

但是

comparator.compare(a,b) != 0

解决方案：尝试分别解析每个字符串，并在例外情况下使用 null 像这样：

Solution: Try parsing each of the Strings separately, and on exception use null like this:

私有SimpleDateFormat sdf = new SimpleDateFormat（ dd / MM / yyyy HH：mm）;

private SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy HH:mm");

Comparator<String> comparator = new Comparator<String>() {
    @Override
    public int compare(String o1, String o2) {
        Date first;
        try {
            first = sdf.parse(o1);
        } catch (Exception ignored) {
            first = null;
        }
        Date second;
        try {
            second = sdf.parse(o2);
        } catch (Exception ignored) {
            second = null;
        }

        if (first == second) {
            return 0;
        }
        if (first == null) {
            return 1;
        }
        if (second == null) {
            return -1;
        }
        return first.compareTo(second);
    }
};

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