Algorithm to find # of occurrences of a substring in a given input string

CountSubs.java

/* Problem: Find # of occurrences of a substring in a given input string

Example Output:
Test string: abcdabceabcfabcabcd
Substring count for abc:5
Substring count for abcd:2
*/
public class CountSubs {
    public static void main(String[] args) {
        String testString = "abcdabceabcfabcabcd";

        System.out.println("Test input string: " + testString);
        String test1 = "abc";
        String test2 = "abcd";
        System.out.println("Substring count for " + test1 + ":" + findSubstringCount(testString, test1));
        System.out.println("Substring count for " + test2 + ":" + findSubstringCount(testString, test2));
    }

    public static int findSubstringCount(String input, String substr) {
        if (input.isEmpty() || input == null || substr.isEmpty() || substr == null) return 0;
        int count = 0, subPrt = 0, subLength = substr.length();
        for (char c : input.toCharArray()) {
            if (substr.charAt(subPrt) == c) {
                subPrt++;
                if (subPrt == subLength) {
                    count++;
                    subPrt = 0;
                }
            } else {
                if (c == substr.charAt(0))
                    subPrt = 1;
                else
                    subPrt = 0;
            }
        }
        return count;
    }
}