Java8：使用字符串的字符数创建HashMap

问题描述

想知道的是，比计算给定字符串的字符数更简单的方法如下：

Wondering is there more simple way than computing the character count of a given string as below?

String word = "AAABBB";
    Map<String, Integer> charCount = new HashMap();
    for(String charr: word.split("")){
        Integer added = charCount.putIfAbsent(charr, 1);
        if(added != null)
            charCount.computeIfPresent(charr,(k,v) -> v+1);
    }

    System.out.println(charCount);

推荐答案

计算字符串中每个字符出现的最简单方法，具有完整的Unicode支持（Java 11 +）：

Simplest way to count occurrence of each character in a string, with full Unicode support (Java 11+):

String word = "AAABBB";
Map<String, Long> charCount = word.codePoints().mapToObj(Character::toString)
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
System.out.println(charCount);

输出

{A=3, B=3}

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更新：对于Java 8+（不支持补充平面中的字符，例如emoji）：

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UPDATE: For Java 8+ (doesn't support characters from supplemental planes, e.g. emoji):

Map<String, Long> charCount = IntStream.range(0, word.length())
        .mapToObj(i -> word.substring(i, i + 1))
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

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更新2：同样适用于Java 8 +。

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UPDATE 2: Also for Java 8+.

我想错了，以为 Java 8中的接口，因此对于在Java 8中显示为以用于更高版本。

I was mistaken, thinking that codePoints() wasn't added until Java 9. It was added in Java 8 to the CharSequence interface, so it doesn't show in javadoc for String in Java 8, and shows as added in Java 9 for later versions of the javadoc.

不过，方法直到Java 11才被添加，因此要使用方法，我们可以使用在Java 8中：

However, the Character.toString​(int codePoint) method wasn't added until Java 11, so to use the Character.toString​(char c) method, we can use chars() in Java 8:

Map<String, Long> charCount = word.chars().mapToObj(c -> Character.toString((char) c))
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

或者对于完整的Unicode支持，包括补充平面，我们可以使用和构造函数，在Java 8中：

Or for full Unicode support, incl. supplemental planes, we can use codePoints() and the String(int[] codePoints, int offset, int count) constructor, in Java 8:

Map<String, Long> charCount = word.codePoints()
        .mapToObj(cp -> new String(new int[] { cp }, 0, 1))
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

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