static variable initialization java

Question

how to initialize a private static member of a class in java.

trying the following:

public class A {
   private static B b = null;
   public A() {
       if (b == null)
         b = new B();
   }

   void f1() {
         b.func();
   }
}

but on creating a second object of the class A and then calling f1(), i get a null pointer exception.

On which line of code do you get the null pointer exception? — Greg Hewgill
– Greg Hewgill, Commented Oct 29, 2009 at 8:52
I think you should correct variables names, the static a is an instance of B and this is confusing, also you should post class B, maybe the NPE gets raised there. — Alberto Zaccagni
– Alberto Zaccagni, Commented Oct 29, 2009 at 8:55
this is just sample code. in the actual code i am using a different library. i get an error in the java.util.Collections.sort function. the stack trace shows that func has been called and some internal calls in the lib. is there a problem with the above code. — Rohit Banga
– Rohit Banga, Commented Oct 29, 2009 at 8:56
@KLE, because of some network problem this got posted twice. i'll delete the other one. — Rohit Banga
– Rohit Banga, Commented Oct 29, 2009 at 8:58

sfussenegger · Accepted Answer · 2009-10-29 10:56:24Z

80

The preferred ways to initialize static members are either (as mentioned before)

private static final B a = new B(); // consider making it final too

or for more complex initialization code you could use a static initializer block:

private static final B a;

static {
  a = new B();
}

edited Oct 29, 2009 at 10:56

answered Oct 29, 2009 at 8:58

sfussenegger

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9 Comments

Rohit Banga Over a year ago

i used a static initializer block as the constructor of B throws an exception. still i get the same error. the first call to the library function works but not the second one.

rsp Over a year ago

As I see it the preferred way of initialising static members depends on the actual situation. Software which creates all static members via this pattern takes a performance hit during application startup. For non-trivial situations I favor a lazy initialisation pattern for just that reason.

sfussenegger Over a year ago

@rsp: You're right with your performance concern (to be correct it's not on startup but when class is loaded though - which might be the same but needn't be). I'd still consider this way of initializing static member preferred as doing initialization lazily adds complexity to the code - this should be avoided except for good reason. Performance might be one such reason.

sfussenegger Over a year ago

@iamrohitbanga: consider adding more code and the stacktrace of your NullPointerException to get help.

Rohit Banga Over a year ago

the code would be even more confusing. i did this: if (b == null) str = "error"; str is set to "error" it seems to be an API specific problem.

|

KLE · Accepted Answer · 2009-10-29 09:06:59Z

5

Your code should work. Are you sure you are posting your exact code?

You could also initialize it more directly :

    public class A {

      private static B b = new B();

      A() {
      }

      void f1() {
        b.func();
      }
    }

edited Oct 29, 2009 at 9:06

answered Oct 29, 2009 at 8:53

KLE

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