java中出现第n次的子字符串

public static String findNthSubstring(String str, String subStr, int n) {
    int index = -1;
    for (int i = 0; i < n; i++) {
        index = str.indexOf(subStr, index + 1);
        if (index == -1) {
            return null; // 如果找不到第n次出现的子字符串，返回null
        }
    }
    return str.substring(index, index + subStr.length());
}

String str = "Hello, Java is a programming language. Java is widely used.";
String subStr = "Java";
int n = 2;
String result = findNthSubstring(str, subStr, n);
System.out.println(result); // 输出：Java

import java.util.regex.*;

public static String findNthSubstring(String str, String regex, int n) {
    Pattern pattern = Pattern.compile(regex);
    Matcher matcher = pattern.matcher(str);
    int count = 0;
    while (matcher.find()) {
        count++;
        if (count == n) {
            return matcher.group();
        }
    }
    return null; // 如果找不到第n次出现的子字符串，返回null
}

String str = "Hello, Java is a programming language. Java is widely used.";
String regex = "\\bJava\\b";
int n = 2;
String result = findNthSubstring(str, regex, n);
System.out.println(result); // 输出：Java