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I understand that trying to compare the value of an Object that's null with something will result in a NullPointerException, e.g.,

String bob;
if(bob.equals("blahblahblah")){do_something();}

and that it's a good idea to use nested checks like so:

if(!bob.equals(null)){
    if bob.equals("blah blah blah"){
      do_something();
    }
}

But if I haven't given bob any value yet, I get an error when I run the above saying bob hasn't been initialized - since I'm comparing bob to null, I should be able to run this without a problem, shouldn't I?

Thanks.

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  • 1
    ofcourse bob is null : stackoverflow.com/questions/218384/… Commented Mar 28, 2014 at 17:09
  • You are right that you can compare bob to null, but you would use the == operator in that case. For string comparisons (on non null values, meaning after a null check) you use the .equals method. its not the comparison that throws the exception, its the call to a .equals method on an object (bob) which is null. Commented Mar 28, 2014 at 17:12
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    what is the difference ? Commented Mar 28, 2014 at 17:15
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    @JonK Java is not C. In Java variables are either initialized by user/programmer, or have default values (false for boolean, 0 for numeric values, null for references). If variable will not be initialized and tried to use, code will not compile. BTW only class fields are set up by compiler to have default value, local variables (the ones declared in method need to be initialized by user/programmer). Commented Mar 28, 2014 at 17:23
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    @JonK Most problematic sentence in your comment is that "The memory that the JVM allocated to that variable holds whatever it held before the JVM grabbed it for the variable". You are saying for example about case where int i; /*some time passes*/ i=42; In this case memory for i before i=42 could in fact not reset, so it can be any set of bits, but we can't check it because Java explicitly demands to initialize it before it can be used (as jls-4.12.5 describes). Commented Mar 28, 2014 at 19:12

6 Answers 6

Reset to default
3

if bob is null you can't do bob.equals - you need to check 

if (bob != null)

like this

if (bob != null) {
    if (bob.equals("blah blah blah")){

          do_something();
    }
}

A nice way to get around it is 

if ("blah blah blah".equals(bob)) {

}
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2 
!bob.equals(null)

You are trying to call a method on a null object, thus you'll get errors. Try this instead:

bob != null
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2

Imagine writing 

null.equals(null)

Would you expect it to work? This doesn't work for the same reason. As other stated, you can use reference compare (==) for this, since there is only 1 null.

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1

As Shreyos pointed out, you never initialize bob so you get the NPE. You can safeguard against this by using Yoda Conditions:

if("blahblahblah".equals(bob)){do_something();}

Your string literal can never be null so the equals() comparison will return false when bob is, in fact, null. Many scoff and joke about using such techniques but they can be handy in many cases.

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1

your bob is null and you can not call equal() on that either change your program to 

if(bob != null){
    if bob.equals("blah blah blah"){
      do_something();
    }
}

OR

initialize bob like String bob = new String("some string");

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5 Comments

I don't know about this initialized keyword. Is that Java?
I don't see initialized in the list of Java keywords: docs.oracle.com/javase/tutorial/java/nutsandbolts/…
It's not about spelling. There is no "initialize" or "initialized" keyword in Java. Is that Scala or Groovy or something?
Oh. Remove the code markdown for initialize though and that'll clear up any potential confusion. I see what you were saying.
oh I am sorry I did not mean initialize a keyword I was just trying to say initialize some value in it.
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Your problem is this:

if(!bob.equals(null)){
    if bob.equals("blah blah blah"){
      do_something();
    }
}

Assuming you declared this with String bob; The compiler will default that to String bob = null;

null is a pointer (reference) all by itself. The method String.equals() is a method that is referenced in the String class. Since you didn't create a object of type String, the class doesn't exist and therefore neither does the method.

If you're going to test to see if something is null (as you will when you eventually write production code) you're going to use the equals operator like this:

if( null != bob ){ //operations on bob }

EXTRA DETAIL:

Now initialization is like this, When you say something like:

String bob = "foo";

The compiler is actually going to write something like String bob = new String(foo);

This should tell you then that initialization means object creation.

I say "something like" because the following is also possible:

String bob = "foo" + "bar";

The compiler is going to write

String bob = new StringBuilder("foo").append("bar").toString();
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