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Language/Type: Java recursion recursive programming
Author: Whitaker Brand (on 2014/02/14)

Write a recursive method called substring that takes as parameters a string, a start index, and an ending index, and that returns a specified substring of the string. You are implementing a recursive alternative to the standard substring method. As with the standard substring method, your method should return the substring that begins at the start index and that extends to the character just before the ending index. For example:

        substring("hello", 0, 2) should return "he"
        substring("hamburger", 4, 8) should return "urge"
        substring("smiles", 1, 5) should return "mile"
        substring("howdy", 3, 3) should return ""

The method should throw an IllegalArgumentException if the start index is negative or if the ending index is greater than the length of the string or if the start index is greater than the ending index. The method should return an empty string if the two indexes are equal. In implementing this method, you are restricted to the following string methods:

        charAt(index)     returns the character at the given index
        equals(other)     returns whether this String is equal to the other
        length()          returns the length of the String

You are not allowed to construct any structured objects other than Strings (no array, StringBuilder, Scanner, etc) and you may not use a while loop, for loop or do/while loop to solve this problem; you must use recursion.

Type your solution here:

This is a method problem. Write a Java method as described. Do not write a complete program or class; just the method(s) above.

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