How does the Object Reference change in this code ?

Question

Hi Folks,
This is my code :
package assignments;
class Value
     public int i = 15;
} //Value
public class Assignment15
     public static void main(String argv[])
          Assignment15 t = new Assignment15();
          t.first();
     public void first()
          int i = 5;
          Value v = new Value();
          v.i = 25;
          second(v, i);
          System.out.println(v.i);
     public void second(Value v, int i)
          i = 0;
          v.i = 20;
          Value val = new Value();
          v = val;
          System.out.println(v.i + " " + i);
} // Test
O/P :
15 0
20The output I expected was 15 0 15.
If the value object created in the second() assigns the value 15 to variable v,why does v.i in the last sysout print 20 ?
Thank you for your consideration.

Answer 1

     public void second(Value v, int i)
          i = 0;
          v.i = 20;
          Value val = new Value();
          v = val;
          System.out.println(v.i + " " + i);
     }There are two sources of confusion in this code:
1) for clarity, you shouldn't name second()'s argument "v"; as pbrockway points out, this variable v is a completely different variable from first()'s "v" variable, it just happens to contain a reference to the same object as first()'s "v". Rename the argument to "v2" and it should clarify things (note: this is not a general recommendation, this naming is perfectly valid and makes sense, just not to you yet).
2) You are assigning a new value (the reference denoted by "val") to the argument variable v. That is bad practice (even for seasoned Java developers).Although the Java compiler accepts it gladly, it brings confusion: it does change the value of the "local variable" v2, but doesn't change the value of the variable "v" in method first(), from which v2 was copied when invoking second().
Indeed I've seen coding conventions that recommend to add a "final" specifier in front of method arguments, this way:
       public void second(final Value v, int i)
          i = 0;
          v.i = 20;
          Value val = new Value();
          v = val; // does not compile
          System.out.println(v.i + " " + i);
     }This way the compiler rejects the v=val assignment: if you want to do something similar to your previouscode, you have to introduce a new local variable, that is, local to second()'s body, and clearly unknown to first().
Consider the new code after implementing my suggestions: does it clear the doubts?
package assignments;
class Value
     public int i = 15;
} //Value
public class Assignment15
     public static void main(String argv[])
          Assignment15 t = new Assignment15();
          t.first();
     public void first()
          int i = 5;
          Value v = new Value();
          v.i = 25;
          second(v, i);
          System.out.println(v.i);
     public void second(final Value v2, int i)
          i = 0;
          v2.i = 20;
          Value val = new Value();
          Value anotherValueVariableWhichDoesNotEscapeThisMethod = val;
          System.out.println(v2.i + " " + i);
          System.out.println(anotherValueVariableWhichDoesNotEscapeThisMethod.i + " " + i);
} // Test