Thread: Are method parameters two way traffic?

Can I reset the arguments that are sent through the parameters, from inside the method itself?

Originally Posted by FightingIrishman

Can I reset the arguments that are sent through the parameters, from inside the method itself?

No, as explained by Norm. And even if you could, it would be very bad design. Nobody expects a method to have side effects in code outside the method, let alone mess with variables defined on a higher level.
If you REALLY want to do stuff like this, there's always C 😁

Is it true that with instance/non-static/object methods, parameters are two way traffic?

Pardon me. I think I've realised that the answer is no.

An object of an instance method can set the instance variables of the method. I was confusing this to mean the other direction.

Although Java parameters are defined as "passed by value" it doesn't hold quite so fast with objects as it does with simple types.
If an object is passed as a parameter, that parameter is copied (satisfying pass by value) however that copy is still pointing to the original instance.
Therefore you can call the original instances methods and manipulate the fields of that original instance.
Easier to show than to explain.

public class Main
{
	public static void main(String[] args) {
	    MyClass test = new MyClass(); 
	    test.setN(12);
	    System.out.println(test.getN());
	    changeN(test); 
	    System.out.println(test.getN());	
	    dontChangeN(test);
	    System.out.println(test.getN());	
	}
	private static void changeN(MyClass mc){
	    mc.setN(15); 
	}
	private static void dontChangeN(MyClass mc){
	    MyClass newMC = new MyClass(); 
	    newMC.setN(16); 
	}
 
	public static class MyClass {
	    private int n; 
	    private void setN(int value){
	        this.n = value; 
	    }
	    private int getN(){
	        return this.n; 
	    }
	}
 
}

The output from this is

12
15
15

Here some good explanations of how it works.
I found Dan Carters comment on this quite apt:

Java always passes arguments by value, but what you are passing by value is a reference to an object, not a copy of the object.

In some other languages (like c# and VBA) when passing by value there is an actual copy made and the function has no access to the original object instances methods.