How to modify two Double objects in java function?

Question

My function takes as input two Double objects, and I would like to modify them both inside the function. It seems it is not possible, it seems java copies them inside the function. How is this done in java, in c++ you pass by reference.

Thanks

`Double` objects are immutable. Your approach is impossible. — jlordo, Jan 03 '13 at 16:56
See this question http://stackoverflow.com/questions/40480/is-java-pass-by-reference — Mark Meyer, Jan 03 '13 at 16:56
possible duplicate of [java - passing a double value by reference](http://stackoverflow.com/questions/5749251/java-passing-a-double-value-by-reference) — CoolBeans, Jan 03 '13 at 16:57

kosa · Answer 1 · 2013-01-03T17:12:59.400

12

Wrapper classes are immutable, that is the reason you were unable to modify them. One way is , you may create one class with these two attributes and pass it to the function, inside function, set new values to that object.

edited Jan 03 '13 at 17:12

answered Jan 03 '13 at 16:56

kosa

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score 3 · Answer 2 · answered Jan 03 '13 at 16:57

What you can do is wrap the Double objects into a Wrapper class, and modify the doubles stored in that object. Otherwise, no other way, as the only value you will change is the local argument.

public class DoubleWrapper
{
  private double d1, d2; 
  public void setter(double newdouble){} 
  public double getter() {}
}

score 2 · Answer 3 · answered Jan 03 '13 at 16:58

2

Pass the double values in an array, you can modify array elements in your function.

answered Jan 03 '13 at 16:58

Waqas Ilyas

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score 0 · Answer 4 · answered Jan 03 '13 at 16:57

0

Java is call by value, so you're right: It must be done in another manner.

answered Jan 03 '13 at 16:57

Anders R. Bystrup

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score 0 · Answer 5 · answered Jan 03 '13 at 16:58

There are two issues here:

First, all values in Java are pass-by-value. This means that, when an object X is sent to a function, that function holds a new version of X.

Second, you can't modify a Double value. You have to modify a double, and can convert from Double d to double by running d.doubleValue().

score 0 · Answer 6 · answered Jan 03 '13 at 17:06

In general you shouldn't be modifying arguments in Java (or perhaps any language)

Instead I suggest you return the values changed as an array

public static double[] square(double... doubles) {
     double[] ret = new double[doubles.length];
     for(int i = 0; i < doubles.length; i++)
         ret[i] = doubles[i] * doubles[i];
     return ret;
}

Ideally, you would have a class which wraps these doubles and you call a method to alter them

class MyDoubles {
     double d1, d2;

     public void squareDoubles() {
         d1 *= d1;
         d2 *= d2;
     }

     public String toString() { return "{ " + d1 +", " + d2 + "}"; }
}

How to modify two Double objects in java function?

6 Answers6