Having trouble with passing parameters to methods

[Angela Li]

I can't get b to print out to be 32 and I keep getting 16.

public class Help {
	public static void doubler(int a) {
        a *= 2;
    }
    
	

    public static void main(String[] args) {
    	int b=16;
    	doubler(b);
        System.out.println("b is " +b); //b should be 32
    }
}

I also tried this but it also didn't work.

public class Help {
	static int b =16;
	public static void doubler(int a) {
        a *= 2;
    }
    
	

    public static void main(String[] args) {
    	Help obj1 = new Help();
    	doubler(obj1.b);
        System.out.println("b is " + obj1.b); //b should be 32
    }
}

[Henry Wong]

https://coderanch.com/how-to/java/CallByReferenceVsCallByValue

[fred rosenberger]

All variables in java are 'pass by value'. So when you pass b into your method, you pass in the value 16, which then gets assigned to your 'a'. So 'b' is pointing to one spot in memory that holds the value 16, and a is pointing to a DIFFERENT spot that also holds 16.

you do a *= 2;, which does double what a is pointing to, but the value b points to is untouched.

[Praveen Kumar M K]

Hi Angela,

Please go through the link that Henry has provided, that should solve your problem.

For the specific example that you have posted, instead of passing objl.b, try and see what happens when you pass objl itself.

    public static void doubler(Help objl) {  
        objl.b *= 2;  
    }

[Winston Gutkowski]

Angela Li wrote:I also tried this but it also didn't work...

Try this instead:

public class Help {
   private int b = 16; // Note: NOT static

   public static int doubler(int a) { // Note: returns an int
      return a * 2; // return the result
   }

   public static void main(String[] args) {
      int anotherB = 16;
      Help obj1 = new Help();
      // ALL of the following will print out 32
      System.out.println( "b * 2 is " + doubler(anotherB) );
      System.out.println( "b * 2 is " + doubler(obj1.b) );
      anotherB = doubler(obj1.b);
      System.out.println( "b * 2 is " + anotherB );
   }
}

Lessons:
1. Methods, in order to be useful, generally return something, so don't use void unless you know it's correct (eg, for a 'setter' method, or for main() itself).
2. Resist the urge to make everything static.
3. Make instance fields private unless you have a good reason not to.
4. It may be a bit early yet, but try to get into the habit of writing classes, not main() methods.
5. Read what the others have already told you about 'pass by value'.

HIH

Winston