Java: Implement String method contains() without built-in method contains()

Question

I'm trying to implement String method contains() without using the built-in contains() method.

Here is what I have so far:

public static boolean containsCS(String str, CharSequence cs) {

    char[] chs = str.toCharArray();
    int i=0,j=chs.length-1,k=0,l=cs.length();

    //String      str = "Hello Java";
    //                   0123456789
    //CharSequence cs = "llo";

    while(i<j) {
        if(str.charAt(i)!=cs.charAt(k)) {
            i++;
        }
        if(str.charAt(i)==cs.charAt(k)) {

        }
    }

    return false;
}

I was just practicing my algorithm skills and got stuck.

Any advice?

Answer 1

Using Only 1 Loop

I did some addition to Poran answer and It works totally fine:

 public static boolean contains(String main, String Substring) {
    boolean flag=false;
    if(main==null && main.trim().equals("")) {
        return flag;
    }
    if(Substring==null) {
        return flag;
    }

    char fullstring[]=main.toCharArray();
    char sub[]=Substring.toCharArray();
    int counter=0;
    if(sub.length==0) {
        flag=true;
        return flag;
    }

    for(int i=0;i<fullstring.length;i++) {

        if(fullstring[i]==sub[counter]) {
            counter++;
        } else {
            counter=0;
        }

        if(counter==sub.length) {
            flag=true;
            return flag;
        }

    }
    return flag;

}

Answer 2

This should work fine..I am printing execution to help understand the process.

public static boolean isSubstring(String original, String str){
    int counter = 0, oLength = original.length(), sLength = str.length();
    char[] orgArray = original.toCharArray(), sArray = str.toCharArray();
    for(int i = 0 ; i < oLength; i++){
        System.out.println("counter at start of loop " + counter);
        System.out.println(String.format("comparing %s with %s", orgArray[i], sArray[counter]));
        if(orgArray[i] == sArray[counter]){
            counter++;
            System.out.println("incrementing counter " + counter);
        }else{
            //Special case where the character preceding the i'th character is duplicate
            if(counter > 0){
                i -= counter;
            }
            counter = 0;
            System.out.println("resetting counter " + counter);
        }
        if(counter == sLength){
            return true;
        }
    }
    return false;
}