计算字符串中子字符串或字符的出现次数

来自 org.apache.commons.lang3.StringUtils 的 countMatches 方法通常用于计算 String 中子字符串或字符的出现次数：

import org.apache.commons.lang3.StringUtils;

String text = "One fish, two fish, red fish, blue fish";

// count occurrences of a substring
String stringTarget = "fish";
int stringOccurrences = StringUtils.countMatches(text, stringTarget); // 4

// count occurrences of a char
char charTarget = ',';
int charOccurrences = StringUtils.countMatches(text, charTarget); // 3

否则，对于标准 Java API，你可以使用正则表达式：

import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
String text = "One fish, two fish, red fish, blue fish";
System.out.println(countStringInString("fish", text)); // prints 4
System.out.println(countStringInString(",", text)); // prints 3
 

public static int countStringInString(String search, String text) {
    Pattern pattern = Pattern.compile(search);
    Matcher matcher = pattern.matcher(text);
    
    int stringOccurrences = 0;
    while (matcher.find()) {
      stringOccurrences++;
    }
    return stringOccurrences;
}