Java Null behaviour

Question

I am trying to understand how null works in Java.

If we assign null to any object, what happens actually behind the scene? Does it assign a memory location address pointing to a null "object" or something else?

I've tried the following program and I've come to understand that all nulls point to same location.

But can anybody tell me how Java throws NullPointerException and how null works in Java?

class Animal{
}

class Dog{
}


public class testItClass {

    public static void main(String args[]){
        Animal animal=null;
        Dog dog=null;

        if(((Object)dog) == ((Object)animal))
            System.out.println("Equal");
    }

}

Output

Equal.

possibly duplicate question http://stackoverflow.com/questions/2430655/java-does-null-variable-require-space-in-memory — Drogba, Jan 31 '13 at 07:52
I think the `NullPointerException` is thrown by the language runtime, when the code tries to access members of a `null`. — Alvin Wong, Jan 31 '13 at 07:59

score 4 · Accepted Answer · edited May 23 '17 at 11:53

if we assign null to any object what it actually is it some memory location in heap OR anything else.

One should distinguish reference and object. You can assign null to a reference. Objects are normally created in heap using new operator. It returns you a reference to an object.

A a = new A();

object with type A is created in heap. You are given back reference a. If now you assign

a = null;

the object itself still reside in heap, but you would not be able to access it using reference a.

Note that object might be garbage collected later.

UPD:

I have created this class to see byte code of it (first time to me):

public class NullTest {
    public static void main (String[] args) {
        Object o = new Object();
        o = null;
        o.notifyAll();
    }
}

And it produces:

C:\Users\Nikolay\workspace\TestNull\bin>javap -c NullTest.class
Compiled from "NullTest.java"
public class NullTest {
  public NullTest();
    Code:
       0: aload_0
       1: invokespecial #8                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #3                  // class java/lang/Object
       3: dup
       4: invokespecial #8                  // Method java/lang/Object."<init>":()V
       7: astore_1
       8: aconst_null
       9: astore_1
      10: aload_1
      11: invokevirtual #16                 // Method java/lang/Object.notifyAll:()V
      14: return
}

You can see that set null to a reference results:

8: aconst_null
9: astore_1

List of byte code instructions

Basically it puts value of null to the top of stack and then saves to the reference. But this mechanism and reference implementation is internal to JVM.

How is reference to java object is implemented?

Yes you are right regarding reference and Object and I know it. I am asking what exactly happens when we assign Null to any reference. is it some memory location or Anything else ? — Ganesh, Jan 31 '13 at 08:37
@Ganesh, I have updated the answer. Does it make more clear? It might be set as zero memory location, but it is internal to JVM and JVM specific. — Nikolay Kuznetsov, Jan 31 '13 at 11:35

score 1 · Answer 2 · answered Jan 31 '13 at 07:58

The following declares a reference to an object of class Animal (or a subclass thereof), and initializes it to null:

Animal animal = null;

Here, the reference itself takes up some space. Since there is no associated object, there's no further memory cost.

If you try to use a null reference by accessing the object:

Animal cat = null;
if (cat.equals(dog)) { ... } // throws NPE

you'll get a NullPointerException.

It is, however, OK to operate on the null reference itself, as long as you don't try to dereference it:

Animal cat = null;
Animal dog = null
if (cat == dog) { ... }      // works fine

Saveen · Answer 3 · 2013-09-17T10:42:44.947

1

Null in java is designed based on NULL Object pattern.

NULL Object means Non-existent Object. If we try to access any resource from this Non Existent Object we should not get any thing, this logically make sense. But practically the programming languages like Java will create instance of NullPointerException class and populate the required information into the Object and throw at calling statement.

edited Sep 17 '13 at 10:42

answered Sep 17 '13 at 09:42

Saveen

11
2

score 0 · Answer 4 · answered Jan 31 '13 at 07:55

Null is in fact a memory reference to nothing (hence null). All object references in code are pointers to locations in memory where the physical data is kept. If an object is null then it has no reference, or points at nothing in memory. This is why null cannot be used for anything else except checking whether it is null.

Java will throw a null exception when an attempt is made to manipulate an object that has no physical memory reference.

score 0 · Answer 5 · answered Jan 31 '13 at 07:59

It outputs true because null == null

Null just means nothing.

Animal animal=null;

Defines a reference to an Animal object, but is not yet assigned to one, so it points to nothing.

If you are experienced with C/C++, it is like setting a pointer to NULL or nullptr.

score 0 · Answer 6 · edited Nov 03 '13 at 09:09

0

When an object is set to null then basically it has no memory reference in the JVM. Also, == comparison between two null objects would also yield a true value.

edited Nov 03 '13 at 09:09

zero323

322,348
103
959
935

answered Jan 31 '13 at 08:02

Anugoonj

575
3
9

score 0 · Answer 7 · answered Jan 31 '13 at 08:06

There is only one null in Java. It is the same in all cases it arises, regardless to the variable and even regardless of the class. Compiler will not allow to compare directly but you can cast:

String x = null;
ArrayList a = null;
if ((Object) x == (Object) a) {
  System.out.println("Do not have onther nulls, only ME");
}

score 0 · Answer 8 · answered Jul 25 '14 at 23:57

On the bytecode level there is an instruction to push a "null" to the local stack (and then assign it to a variable slot or field).

Most Java Virtual Machines are actually using real (machine) pointers assigned to null. So if they are used they internally trap the signal (SIGSEGV) and convert it into a NPE. You can actually test this by sending a "kill -SEGV" to a Java PID, it will generate random NullPointerExceptions most of the time.

Java Null behaviour

8 Answers8