java - Passing object reference to a method -

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if employee reference made null in changedetails(), variable id value retained , nullpointerexception not thrown (code 1) may because pass copy of object's reference, in code 2 why variables value has changed

code 1:

public class javapassing {     public static void changedetails(employee e)     {         e=null;     }      public static void main(string args[])     {         employee emp = new employee("vishal",7);         changedetails(emp);         system.out.println(emp.id);     } } 

code 2:

public class javapassing {     public static void changedetails(employee e)     {         e.id=9;     }      public static void main(string args[])     {         employee emp = new employee("vishal",7);         changedetails(emp);         system.out.println(emp.id);     } } 

      --------  -->| object |<-- b       --------  a.id = 10; // property of object modified b.id = 10; // property of object modified here  b = null ; // b set null       --------  -->| object |     b (reference null)       -------- 

here when set b null, a not modified continue point object in heap.

and why not throw nullpointerexception if access id reference a. confused between reference of object , object in memory.

in case, a emp , b e.

you can find explanation in question.