Why finalize is not giving null pointer exception in the below code?

Question

Why the below code is not giving null pointer exception in the finalize method when the objects are made null??

class Person{  
public int a;
public void finalize(){
    //System.out.println("finalize called"+this.hashCode());
    System.out.println("finalize called"+this.a);
}  
public static void main(String[] args){  
    Person f1=new Person();  
    f1.a=10;
    Person f2=new Person();  
    f1=null;  
    f2=null;  
    System.gc();  
}}

O/P : finalize called0 finalize called10

Which part exactly should give NPE? If you're expecting `this` to be `null`, that's not possible at any time. — Kayaman, Apr 13 '18 at 06:02
f1=null; f2=null; Can you please explain , ie, if f1 and f2 are made null then why in finalize , no NPE is coming ? — Radiance, Apr 13 '18 at 06:05
Yes? You set 2 references to `null`. As you can see your `finalize()` method doesn't try to access either one of those (or do you see `f1` or `f2` in there?). — Kayaman, Apr 13 '18 at 06:06
then why the output is : finalize called0 finalize called10 ? — Radiance, Apr 13 '18 at 06:07
Because you set `f1.a = 10` so f1's finalize is outputting `finalize called10`. f2.a == 0 (default value for an int) and it's finalize is outputting `finalize called0` — Paul Warren, Apr 13 '18 at 06:20
The object don't need to wait the variable to get out of scope to be gc'd. — Onic Team, Apr 13 '18 at 06:30

Jean-Baptiste Yunès · Answer 1 · 2018-04-13T06:16:33.047

Objects are never made null, this is non-sense. References can be made null, and objects can be destroyed (as a consequence).

f1 and f2 are not objects, they are reference to objects. When you write f1=null that just means that this reference does not point to any object anymore and that the object that was previously pointed to has one reference less. The garbage collection (roughly) tracks all manipulations of references you make, and when objects are not referred anymore they are first put in some trash and then recycled or destroyed if needed, but object are existing even in that phase. When recycled/destroyed the machine will call finalize on it just before the recycling/destroying, then object exists at the time finalize is called (if not, how could finalize be called on the object?).

score 1 · Answer 2 · answered Apr 13 '18 at 06:12

Objects can't be made null, only references.

Just because you set f1 and f2 to null, doesn't mean that finalize() would throw a NPE. It's accessing the this reference which can never be null.

f1 --> Object <-- (implicit this accessible from inside the instance)
f2 --> Object <-- (--""--)

f1 = null; f2 = null;

       Object <-- (implicit this) previously f1 referred to this Object
       Object <-- (implicit this) previously f2 referred to this Object

score 0 · Answer 3 · answered Apr 13 '18 at 06:46

class Person{  
public int a;
public void finalize(){
    //System.out.println("finalize called"+this.hashCode());
    System.out.println("finalize called"+this.a);
}  
public static void main(String[] args){  
    Person f1=new Person();  
    f1.a=10;
    Person f2=new Person();  
    f1=null;  
    f2=null;  
    System.out.println("f1 =" + f1 + " f2 = " + f2);
    System.gc(); 
    System.out.println("Below gc");

    System.out.println();
    System.out.println(f1.a);// **here O/P - NPE**
}}

OUTPUT ->

f1 =null f2 = null

Below gc

finalize called0 finalize called10

Exception in thread "main" java.lang.NullPointerException at basic.Person.main(Person.java:19)

Why finalize is not giving null pointer exception in the below code?

3 Answers3