String concat() question required

[PETER CARTER]

Given:
11. String a = null;
12. a.concat(�abc�);
13. a.concat(�def�);
14. System.out.println(a);
Why it is :
Exception in thread "main" java.lang.NullPointerException at X.main(X.java:12)
I think it should be "abcdef".

Thanks !!

[David Ulicny]

Look into the documentation how concat is defined. It returns the String. Remember that String is IMMUTABLE, so there is no way how to modified it.
Your exmaple is
null.concat();
so it will throws the exception.

[Sandeep Jindal]

Hi Peter,

I think you are new to java/OOP technology.
The answer is in your question only.
You are writing String a = null;
then calling a method of String object on an object that is null.
This is what a null pointer exception is.

In other words, you are trying to c0ncatenate something to an "a" string object, what the jvm will think in which object to concatenate??

I think u just give a thought and it will be clear.

Regards
Sandeep Jindal

[PETER CARTER]

David,
Thanks !!