在Java中計算字符串中的字符出現次數

Question

我試圖讓它打印有多少個字母“a”。 它一直給我0 ...有什么幫助嗎？

JFileChooser chooser = new JFileChooser();
if (chooser.showOpenDialog(null) == JFileChooser.APPROVE_OPTION) {
    File myfile = chooser.getSelectedFile();
    try {

        Scanner in = new Scanner(myfile);
        String word = in.nextLine();
        int counter = 0;
        for (int i = 0; i < word.length(); i++) {
            if (word.charAt(i) == 'a') {
                counter++;
            }
        }

        System.out.println("# of chars: " + counter);
    } catch (IOException e) {
        System.err.println("A reading error occured");
    }
}

Answer 1

一種簡單的（一行）計數出現的字符的方法是：

int count = word.replaceAll("[^a]", "").length();

這將用空格替換不是“ a”的每個字符-有效地將其刪除-留下一個僅包含原始字符串的“ a”字符的字符串，然后得到該字符串的長度。

Answer 2

除了讀取文件的任何問題外，還請嘗試使用StringUtils countMatches。 它已經在通用語言中，請記住也要使用它。

例如

int count = StringUtils.countMatches(word, "a");

Answer 3

JFileChooser chooser = new JFileChooser();
if (chooser.showOpenDialog(null) == JFileChooser.APPROVE_OPTION) {
File myfile = chooser.getSelectedFile();
try {

    Scanner in = new Scanner(myfile);

    int counter = 0;
while(in.hasNextLine()){
 String word = in.nextLine();
    for (int i = 0; i < word.length(); i++) {
        if (word.charAt(i) == 'a') {
            counter++;
        }
    }
}

    System.out.println("# of chars: " + counter);
} catch (IOException e) {
    System.err.println("A reading error occured");
}
}

Answer 4

    /* Most common string occurrence related solutions using java 8 */
    
    //find all character occurrences in a string
    String myString = "test";
    List<Character> list = myString.chars().mapToObj(c -> (char)c).collect(Collectors.toList());
    list.stream().distinct().forEach(c -> System.out.println(c + " = " + Collections.frequency(list, c)));

    //find one specific character occurrence in a string
    String myString = "test";
    char search = 't';
    long count = myString.chars().filter(c -> c == search).count();
    System.out.println(count);

    //find all unique characters in a string
    String myString = "test";
    List<Character> list = myString.chars().mapToObj(c -> (char)c).collect(Collectors.toList());
    list.stream().filter(c -> Collections.frequency(list,c) == 1).forEach(System.out::println);

    //find first unique character in a string
    String myString = "test";
    List<Character> list = myString.chars().mapToObj(c -> (char)c).collect(Collectors.toList());
    char firstUniqueChar = list.stream().filter(c -> Collections.frequency(list,c) == 1).findFirst().get();
    System.out.println(firstUniqueChar);