How to Efficiently Count Substring Occurrences in a String?

Finding Substring Occurrences in a String

In the following code, our goal is to determine the number of times the substring findStr appears within the string str:

String str = "helloslkhellodjladfjhello";
String findStr = "hello";
int lastIndex = 0;
int count = 0;

while (lastIndex != -1) {
    lastIndex = str.indexOf(findStr, lastIndex);

    if (lastIndex != -1)
        count++;

    lastIndex += findStr.length();
}

System.out.println(count);

However, this algorithm may fail to terminate under certain circumstances. The issue lies in the fact that lastIndex = findStr.length() might cause the algorithm to search beyond the end of the string. To resolve this, we can instead use the following approach:

String str = "helloslkhellodjladfjhello";
String findStr = "hello";
int count = StringUtils.countMatches(str, findStr);
System.out.println(count);

This code utilizes the StringUtils.countMatches method from Apache Commons Lang, which provides a more robust and efficient solution for counting substring occurrences.

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