how to alter variable value in java? Why some work, some fail?

Question

I'm trying to alter the variable value in java? The following code works, it changes x,y value from 0 to 100:

public void tricky(Point arg1)
{
  arg1.x = 100;
  arg1.y = 100;
}
public static void main(String [] args)
{
  Point pnt1 = new Point(0,0);
  System.out.println("X: " + pnt1.x + " Y: " +pnt1.y); 
  tricky(pnt1);
  System.out.println("X: " + pnt1.x + " Y:" + pnt1.y); 
}

But why the following code doesn't work?

public class Solution {
    public static void tricky(int arg1)
    {
      arg1 = 100;
    }
    public static void main(String [] args)
    {
      int pnt1 = 0;
      System.out.println(pnt1); 
      tricky(pnt1);
      System.out.println(pnt1); 
    }
}

score 1 · Answer 1 · answered Sep 30 '14 at 08:37

1

Java is a pass by value language. Variables that you pass to a method cannot be changed by that method beyond the scope of the method.

answered Sep 30 '14 at 08:37

Eran

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Not entirely correct. Java is pass by value for primitive types, but pass by reference for objects. – Kurt Du Bois Sep 30 '14 at 08:41
1

@KurtDuBois It's pass by value for object references too, which means you can't change the object reference passed to the method, but can change the state of the object. – Eran Sep 30 '14 at 08:43
For objects, you pass the object references as a value. – EpicPandaForce Sep 30 '14 at 08:45

score 0 · Answer 2 · answered Sep 30 '14 at 08:45

public static void tricky(int arg1)
{
  arg1 = 100;
}

arg1 is a copy of the value that you provide for this method. It is not the actual variable, it is a copy of its value.

In the case of

public void tricky(Point arg1)
{
  arg1.x = 100;
  arg1.y = 100;
}

You send an Object over using a reference and modify its class variables. Thus, you are actually modifying the value, not a copy of it.

However, to demonstrate the difference,

public void tricky(Point arg1)
{
   Point arg2 = new Point(50,50);
   arg1 = arg2;
}

The variable you provided the method will not change outside of this method. You can do that only if you say

arg1 = tricky(arg1);

public Point tricky(Point arg1)
{
   Point arg2 = new Point(arg1.x / 2, arg1.y / 2);
   return arg2;
}

Aka return the new value as a return value.

how to alter variable value in java? Why some work, some fail?

2 Answers2