Is the "this" reference ever null in Java

Question

What is the value of this? somewhere I read that in C# (this==null) is possible. But what about in Java? That, will the following fragment ever return true?

 if(this!=null)
 {
     return false;
 }
 else
 {
     return true;
 }

For those who (like me) can' grok the idea that `this` *could be* `null`, read this Q&A - http://stackoverflow.com/questions/3143498/why-check-this-null — Stephen C, Jun 29 '13 at 09:58

NPE · Answer 1 · 2013-06-29T11:42:56.557

7

if(this!=null)

The above always evaluates to true, meaning that the first branch of your if will always get executed, and the function always returns false.

edited Jun 29 '13 at 11:42

answered Jun 29 '13 at 09:35

NPE

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@StinePike Certainly. Did someone suggest otherwise? In this answer? Your point? – user207421 Jun 29 '13 at 10:26
@EJP .. oh at first npe wrote that it is not valid.. I didn't see that he changed the text.. So as he modified, I have deleted that comment :) – stinepike Jun 29 '13 at 10:28
@EJP: At first the question asked about `this=!null`, to which I said the syntax was invalid. Then the `=!` got changed to `!=`, so I edited the answer to match the question. Hence the confusion. – NPE Jun 29 '13 at 11:41

score 1 · Answer 2 · edited Jun 29 '13 at 09:50

1

this can never be null. Because this refers to the self instance of the object. And it is only accessed when a object is already created.

so the else block is unreachable.

edited Jun 29 '13 at 09:50

keyser

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answered Jun 29 '13 at 09:36

stinepike

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score 1 · Answer 3 · answered Jun 29 '13 at 09:38

1

"this" can never be null in Java

.....?

if(this!=null)
{
 return false;
}

answered Jun 29 '13 at 09:38

Ganesh Rengarajan

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score 1 · Answer 4 · answered Jun 29 '13 at 09:39

1

this means current object which can never be null.

answered Jun 29 '13 at 09:39

M Sach

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Suresh Atta · Answer 5 · 2013-06-29T10:13:00.270

0

Within an instance method or a constructor, this is a reference to the current object .So it never be null.

edited Jun 29 '13 at 10:13

answered Jun 29 '13 at 09:35

Suresh Atta

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Never false, you mean. The test is != null. – user207421 Jun 29 '13 at 09:57
No,@EJP the instance.I mentioned this also now.It retruns always false. – Suresh Atta Jun 29 '13 at 09:59
What 'instance'? The test in the OP's code is (this!= null). This test can never be false. 'Since it is a valid syntax' has nothing to do with it either way. – user207421 Jun 29 '13 at 10:01
@EJP - false is true for small enough values of true :-) – Stephen C Jun 29 '13 at 10:02
It is now neither right nor wrong, just meaningless. You have failed to express yourself clearly. @StephenC *de minimis curat non lex.*. :-| – user207421 Jun 29 '13 at 10:07
True EJP..Let me know,if you found any mistake. – Suresh Atta Jun 29 '13 at 10:13
Not much of an improvement. It isn't English, and it doesn't really explain anything. 'this' refers to the current object, which *is* the current context, to the extent that 'current context' means anything. – user207421 Jun 29 '13 at 10:15
Normally I would upvote a corrected answer that I had downvoted, but it shouldn't take four edits, several of which made an already meaningless answer worse, to get to a correct answer, so I am making an exception. If you don't know the answer, or can't express it clearly, leave it to others. – user207421 Jun 29 '13 at 10:32
sure EJP,thanks for the suggestion. English in not my native language.I tried to give an answer. – Suresh Atta Jun 29 '13 at 10:33

score 0 · Answer 6 · answered Jun 29 '13 at 09:40

"this" keyword refers to the "that" object, which you are referring to..

class Sample
{
  int age; 
  Sample(int age)
  {
    this.age = age;   // this.age -> the variable a in the that current instance
  }
  public void display()
  {
     System.out.println(age);  //age here is actually this.age
  }
}
public class XYZ
{
 public static void main(String[] args)
 {
   Sample a,b;
   a.display();
   b.display();
 }
}

score 0 · Answer 7 · answered Jun 29 '13 at 15:17

0

Just think logically - that's like saying If I don't exist....

The thing that currently has control of the code has to exist, otherwise the code wouldn't be running in the first place.

answered Jun 29 '13 at 15:17

tckmn

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Don't make generalizations like that. In some languages, `this` may in some circumstances be null. – newacct Jun 29 '13 at 20:04

Is the "this" reference ever null in Java

7 Answers7