Find out if s2 is a substring of s1

Substring.java

/**
 * Created by shail on 4/11/17.
 */
public class Substring {
    static boolean isStartsWith(int start, String s1, String s2){
        int n = s2.length();

        for(int i1 = start, i2 = 0; i2 < n; i1++, i2++)
            if(s1.charAt(i1) != s2.charAt(i2))
                return false;

        return true;
    }

    static boolean isSubstring(String s1, String s2){
        int n1 = s1.length();
        int n2 = s2.length();

        if(n2 == 0)
            return true;

        if(n2 > n1)
            return false;

        for(int i = 0; i < s1.length(); i++)
            if(isStartsWith(i, s1, s2))
                return true;

        return false;
    }
    
    public int strStr(String haystack, String needle) {
        for(int i = 0; ; i++) 
            for(int j = 0; ; j++) {
                if(j == needle.length())
                    return i;
                if((i+j) == haystack.length())
                    return -1;
                if(haystack.charAt(i+j) != needle.charAt(j))
                    break;
            }
    }

    public static void main(String args[]){
        System.out.println(isSubstring("abc", "abc"));
        System.out.println(isSubstring("abcd", "abc"));
        System.out.println(isSubstring("qabc", "abc"));
        System.out.println(isSubstring("qabcd", "abc"));
        System.out.println(isSubstring("abcabcd", "abcd"));
        System.out.println(isSubstring("aabcdbc", "abc"));
        System.out.println(isSubstring("abcd", "abcde"));
        System.out.println(isSubstring("aabcdbc", ""));
        System.out.println(isSubstring("aabcdbc", "c"));
    }
}