How should I implement a substring function in java?

Question

Consider you have been told to implement a kind of the java String.substring method.
The signature of the method is as follows:

public static boolean isSubstring(String i_StringForSearch, String i_SubStringToFind)

Here is my solution, but I feel it's still not the best elegant solution I could have.(Many canonical if else's)
What do you think? Would you do it in another way?

public static boolean isSubstring(String i_StringForSearch, String i_SubStringToFind)
    {
        int strForSearchIndex = 0;
        int subStrToFindIndex = 0;
        boolean endOfStringToSearch = false;
        boolean foundSubString = false;
        boolean isThereASequenceOfMatching = false;


        while(!endOfStringToSearch && !foundSubString)
        {
            if(strForSearchIndex == i_StringForSearch.length())
            {
                endOfStringToSearch = true;
            }

            else if(i_StringForSearch.charAt(strForSearchIndex) == i_SubStringToFind.charAt(subStrToFindIndex))
            {
                isThereASequenceOfMatching = true;
                if(subStrToFindIndex == i_SubStringToFind.length() -1 )
                {
                    foundSubString = true;
                }
                subStrToFindIndex++;
                strForSearchIndex++;
            }

            else if(i_StringForSearch.charAt(strForSearchIndex) != i_SubStringToFind.charAt(subStrToFindIndex))
            {
                if(isThereASequenceOfMatching)
                {
                    subStrToFindIndex = 0;
                    isThereASequenceOfMatching = false;
                }
                strForSearchIndex++;
            }
        }

       return foundSubString;
    }

You may want to move this to codereview.stackexchange.com for better responses! (and also tag it as homework if it is so)! — Vikdor
– Vikdor, Commented Sep 3, 2012 at 17:37
That looks more like contains than substring to me. substring has a very clearly understood purpose, which is very different from what's here. — Jon Skeet
– Jon Skeet, Commented Sep 3, 2012 at 17:38
When you are told to implement something which is the same as or can use a common built API, it doesn't make sense to rewrite it from scratch. — Peter Lawrey
– Peter Lawrey, Commented Sep 3, 2012 at 17:41
Because It's a good exercise , and if you were the man who are told to write it in the first time what would you say then? — JavaSa
– JavaSa, Commented Sep 3, 2012 at 17:42

user207421 · Accepted Answer · 2012-09-03 22:02:32Z

1

Look up the Boyer-Moore and Knuth-Morris-Pratt algorithms. In tests many years ago I found BM to be slightly faster.

answered Sep 3, 2012 at 22:02

user207421

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2 Comments

JavaSa Over a year ago

I just saw that complexity of KMP algo is Θ(n+m) when n is size of String/Text to search for, and m is the size of the substring to search. Boyer-Moore is Θ(nm) so it is supposed to be worse. But What about my solution complexity? Doesn't it Θ(n+m) also?

user207421 Over a year ago

@JavaSa I don't know what you're asking for now. You asked for the most elegant solutions, there they are. Unless you can show yours is better than either it's of no interest. It's up to you.

Aditi Paliwal · Accepted Answer · 2018-12-08 19:02:25Z

0

public static boolean isSubstring(final String i_StringForSearch, final String i_SubStringToFind) {
    int j = 0;
    for (int i = 0; i < i_StringForSearch.length(); i++) {
        if (i_StringForSearch.charAt(i) == i_SubStringToFind.charAt(j)) {
            j++;
            if (j == i_SubStringToFind.length()) {
                return true;
            }
        }
    }
    return false;
}

answered Dec 8, 2018 at 19:02

Aditi Paliwal

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