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

Answer 1

It would not compile due to 'Index Out Of Bounds Exception',

It does compile. You get a runtime Exception when you try to execute the code.

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

Eventually, the question is that how is this possible that we can reach this index we should not?

Read the API for the substring(...) method. It explains how the method works and what the parameter means.

In this case it returns an empty String.

Answer 2

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, ...

Answer 3

In this case details of implementation of methods charAt and substring are helpful to know.

charAt as described in this tutorial can be described as method that:

returns the character located at the String's specified index. The string indexes start from zero.

That is why calling:

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

gives

Exception in thread "main" java.lang.StringIndexOutOfBoundsException:
String index out of range: 5

Because maximum index is, as you perfectly described, only 4.

substring is described in java documentation and explains your exact situation here in the "Examples" section which states that :

"emptiness".substring(9) returns "" (an empty string)

For additional reference, here is the source code for java's substring(int index) implementation, where you can see that it uses java's substring(int beginInndex, int endIndex) method.

Answer 4

Refer to the substring method at Java String API (http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#substring(int))

A String ".length()" method will return the number of characters in the string.

if you count "HELLO" from 0->4, it will still amount to 5 characters (the length), which is the number returned when you call "Hello".length().

If you want to access the last character of a String, use (Stringobj.length()-1); Accessing the last element of a String or Array through utilizing its .length() or .size() method calls is standardized across Java.