Strings.java

/*
 * The only String methods that you are allowed to use in this assignment
 *  are String.charAt(int) and String.length().
 */

public class Strings {

    /*
     * Tests two given strings for equality without using isEqual methods
     * 
     * Returns true if and only if the two given strings represents the 
     * same sequence of characters, false otherwise.
     * 
     */
    public static boolean isEqual(String s1, String s2) {
	if(s1.length() != s2.length()){
	    return false;
	}
	for(int i = 0; i< s1.length();i++){
	    if (s1.charAt(i) != s2.charAt(i)){
		return false;
	    }
	}
	return true;
    }

    /*
     * Returns a new string that is a substring of a given String s. 
     * The substring begins at the specified start index (index) and extends
     * to the number of characters defined by length.
     * 
     * For example: substring("Computer Science", 4, 6) returns "uter S"
     */
    public static String substring(String s, int index, int length) {
	String result = "";

	for (int i = 0; i < length; i++)
	    result += s.charAt(index + i);

	return result;

    }

    /*
	 * Returns the index within a given String s of the first occurrence of 
	 * the specified substring sub. If no occurrence of sub exists within s, 
	 * then -1 is returned.
	 * 
	 * Examples:
	 * indexOf("abracadabra", "c") returns 4
	 * indexOf("abracadabra", "s") returns -1
	 */
    public static int indexOf(String s, String sub) {
	for (int i = 0; i < s.length() - sub.length() + 1; i++)
		if (isEqual(substring(s, i, sub.length()), sub))
			return i;

	return -1;
	
	
	
    }
}