How should substring() work?

Question

I do not understand why Java's [String.substring() method](http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html#substring(int,%20int%29) is specified the way it is. I can't tell it to start at a numbered-position and return a specified number of characters; I have to compute the end position myself. And if I specify an end position beyond the end of the String, instead of just returning the rest of the String for me, Java throws an Exception.

I'm used to languages where substring() (or substr()) takes two parameters: a start position, and a length. Is this objectively better than the way Java does it, and if so, can you prove it? What's the best language specification for substring() that you have seen, and when if ever would it be a good idea for a language to do things differently? Is that IndexOutOfBoundsException that Java throws a good design idea, or not? Does all this just come down to personal preference?

Answer 1

There are times when the second parameter being a length is more convenient, and there are times when the second parameter being the "offset to stop before" is more convenient. Likewise there are times when "if I give you something that's too big, just go to the end of the string" is convenient, and there are times when it indicates a bug and should really throw an exception.

The second parameter being a length is useful if you've got a fixed length of field. For instance:

// C#
String guid = fullString.Substring(offset, 36);

The second parameter being an offset is useful if you're going up to another delimited:

// Java
int nextColon = fullString.indexOf(':', start);
if (start == -1)
{
    // Handle error
}
else
{
    String value = fullString.substring(start, nextColon);
}

Typically, the one you want to use is the opposite to the one that's provided on your current platform, in my experience :)