CODEWITHSUNDEEP
javanullpointerexception
Vrushank
Vrushank

Reputation: 2813

Why this code DOES NOT return a NullPointerException?

public class Main
{
   public static void main(String []ar)
   {
      A m = new A();
      System.out.println(m.getNull().getValue());
   }
}

class A
{
   A getNull()
   {
      return null;
   }

   static int getValue()
   {
      return 1;
   }
}

I came across this question in an SCJP book. The code prints out 1 instead of an NPE as would be expected. Could somebody please explain the reason for the same?

Upvotes: 16

Views: 393

Answers (5)

Subhrajyoti Majumder
Subhrajyoti Majumder

Reputation: 41200

System.out.println(m.getNull().getValue()); this line of code is same as System.out.println(A.getValue());

as because getValue() method is static and and all static call initiate at compile time in java. so it does not produce any error once you make getValue() in non static this will produce error as because it will be called at run time

Upvotes: 1

CosminO
CosminO

Reputation: 5226

getNull function returns an A object. getValue is declared as static, and just needs class_name to function, as in A.getValue(). Because getNull returns (in fact) an A object...you will get 1

Upvotes: 1

assylias
assylias

Reputation: 328608

It behaves as it should according to the Java Language Specification:

a null reference may be used to access a class (static) variable without causing an exception.

Upvotes: 18

Michael Borgwardt
Michael Borgwardt

Reputation: 346300

Static method calls are resolved at compile time. The Compiler sees that getNull() has a return value of type A which has a static getValue() method (and no instance method of the same name), so in the bytecode, the actual return value of getNull() is ignored and A.getValue() is called.

Upvotes: 6

Jon Skeet
Jon Skeet

Reputation: 1500515

Basically you're calling a static method as if it were an instance method. That just gets resolved to a static method call, so it's as if you'd written:

A m = new A();
m.getNull();
System.out.println(A.getValue());

IMO the fact that your code is legal at all is a design flaw in Java. It allows you to write very misleading code, with Thread.sleep as an example I always use:

Thread thread = new Thread(someRunnable);
thread.start();
thread.sleep(1000);

Which thread does that send to sleep? The current one, "of course"...

Upvotes: 22

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