我应该如何计算Java中字符串开头字符出现的次数

Question

我有一个类似于此的字符串,,,foo,bar ，我需要计算java中字符串开头的“，”数量吗？

Answer 1

Have a counter variable which counts the number of occurrences. Then loop through the entire String , using charAt(i) to get the char at position i . Test to see if it's equal to charAt(0) . If it is, increment counter and if it isn't, break out of the loop.

Answer 2

Take a look at the String javadoc . It contains methods you can use to get the length of the String and get characters at certain positions.

Answer 3

If starting characters are known then build a regex pattern and get the first group. First group string will contain the exact match of desired sequence, length of this string is the resultant count.

Answer 4

一个简单的循环（while或for），包含一个if，条件为相等，如果为true，则增加一个计数器。

Answer 5

This quick-n-dirty solution worked for me.

public static void main(String[] args)
{
    String s = ",,,foo,bar";
    int count = 0;
    for (int i = 0; i < s.length() ; i++) {
        if (s.charAt(i) != ',')
            break;
        count++;
    }

    System.out.println("count " + count);
}

Update: just realized that you only need to count the ',' at the beginning of the string and not the middle. I've updated the code to do that.

Answer 6

If you don't want to use any any external jars just write a simple function:

public static int countAtBegin(String str, char c) {
    for (int ret = 0; ret < str.length(); ret++) {
        if (str.charAt(ret) != c)
            return ret;
    }
    return str.length();
}

Answer 7

You can also use regexp:

public static int countCommasAtBegin(String str) {
    Matcher commas = Pattern.compile("^,*").matcher(str);
    if (commas.find()) {
        return commas.group().length();
    } else {
        return 0;
    }
}

but for such trivial task I prefer to use simple loop.