Substring method in String class reaches the index it isn't supposed to [duplicate]

Question

I apologize if title is not clear.

Now, in strings index starts at 0. So for instance:

Index    0   1   2   3   4
String   H   E   L   L   O

In this case, the last index is 4.

If I would try to do something like this:

System.out.println("hello".charAt(5));

It would throw an 'Index Out Of Bounds Exception', as it should.

However, if I try to run the following code:

System.out.println("hello".substring(5));

It does run! I wrote similar code, noticed this later and could not figure it out.

Eventually, the question is that how is this possible that we can reach this index we should not? It allows us to write:

stringObj.substring(stringObj.length());

and it doesn't look very safe as stringObj.length() return us an integer that is one more than what the last index is.

Is it made intentionally and does this have a purpose? If so, what is this purpose and what is the benefit it brings?

Answer 1

It is easier to work with substring when you think of indexes like this (like letters are between indexes)

         H E L L O
        0 1 2 3 4 5    <- acceptable range of indexes for "HELLO"
        ^         ^
        |         |
      start      end = length
   (min index) (max index)

When you substring from start to end you get only characters between these indexes.

So in case of "HELLO".substring(2,4) you will get part

 L L 
2 3 4

which returns "LL".

Now in case of substring(5) it acts same as substring(5,length) which means in this case it is substring(5,5). So since 5 index exists in our "model" as acceptable value, and since there are no characters between 5 and 5 we are getting empty string as result.

Similar situation happens in case of substring(0,0) substring(1,1) and so on as long as indexes are acceptable by our model.

StringIndexOutOfBoundsException happens only when we try to access indexes which are not acceptable, which means:

• negative ones: -1, -2, ...
• greater than length. Here: 6 7, ...