创建我自己的myindexof方法

Question

I'm a beginner in Java and received homework to create a replica of the indexOf method which receives a string as a parameter. I have to check if the string received is a substring of the original string, and if so, I have to return the index of it. For example: if the original string is "mother", str == "other" will return 1. If str is not a substring then return -1. I have to create it using only the methods length() and/or charAt() of String class.

I'm stuck on it for a long time. I've tried many sorts of codes but nothing succeeds...

For example:

public int myIndexOf1(String str)
{
    String objectStr = this._st;
    Word w3 = new Word(objectStr);
    Word w4 = new Word(str);
    char[] array = w3.toCharacterArray();

    int firstShowIndex = 0;
    int length = array.length;
    int max = objectStr.length() - str.length(); 
    for (int index = 0; index < max; index++)
    {
        for (int indexSubstring = 0; indexSubstring < str.length(); indexSubstring++)
        {
            if (objectStr.charAt(index) == str.charAt(indexSubstring))
            {
                firstShowIndex = index;
                break;
            }
            else
                firstShowIndex = -1;
        }
    }

    return firstShowIndex;
}

Please help! Thanks in advance!

Answer 1

Here's a solution that I came up with:

Note : It is not within the context of a class that contains a String as a private member as your does, but you can adapt it.

public static int myIndexOf (String mainStr, String otherStr)
{
    // either is null
    if (mainStr == null || otherStr == null)
    {
        return -1;
    }

    int len = mainStr.length();
    int lenOfOther = otherStr.length();

    // special case: both strings are empty
    if (len == 0 && lenOfOther == 0)
    {
        return 0;
    }

    // for each char in the main string
    for (int i = 0; i < len && len - i >= lenOfOther; i++)
    {
        // see if we can match char for char in the otherStr
        int k = 0;
        while (k < lenOfOther && mainStr.charAt(k + i) == otherStr.charAt(k))
        {
            k++;
        }
        if (k == lenOfOther)
        {
            return i;
        }
    }

    // nothing found
    return -1;
}

Usage :

public static void main(String[] args)
{
    String mainStr = "mother";
    String otherStr = "other";

    int index = myIndexOf(mainStr, otherStr);
    System.out.println("My Index: " + index);

    // Just for a sanity check
    System.out.println("String Index: " + mainStr.indexOf(otherStr));
}

Outputs :

My Index: 1
String Index: 1