Why doesn't this change the value

Question

public class SubiectLicentaTrei {
    public static void main(String args[]) {
        double d = 12.34;
        ScadeUnitate su = new ScadeUnitate();
        su.scadeUnitate(d);
        System.out.println(d);
    }
}

class ScadeUnitate {
    public void scadeUnitate(double d) {
        d = d - 1.0;
    }
}

Outputs 12.34 when I expect 11.34.

`su.scadeUnitate(d);` This passes your `double d` ***by value***. — Jonathon Reinhart, Jun 18 '14 at 05:49

Rod_Algonquin · Accepted Answer · 2014-06-18T06:02:35.993

3

Java is pass by value

so when you pass the here

ksu.scadeUnitate(d);

d will be a pass-by-value and when you deduct it here

d = d - 1.0;

d wont reference the value 11.34. but will get destroyed when out-of-scope

solution:

put a return value of the scadeUnitate method

public double scadeUnitate(double d)

get the return value to reference from the returned value.

d = su.scadeUnitate(d);

edited Jun 18 '14 at 06:02

answered Jun 18 '14 at 05:50

Rod_Algonquin

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score 0 · Answer 2 · answered Jun 18 '14 at 05:50

0

Cause d is primitive local variable and keeps new state only in scadeUnitate function. You should rework your function to return new value and reassign d with new value in main function.

answered Jun 18 '14 at 05:50

Viacheslav

5,443
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@ScaryWombat are you sure it will work passing a `Double`? I've just tried it and got the same result. I think it's because of *autoboxing*. – Christian Tapia Jun 18 '14 at 05:52
deleted my misleading question then. – Scary Wombat Jun 18 '14 at 05:56
Seems that it will not work for wrapper types – Viacheslav Jun 18 '14 at 05:56
There is no method for actually setting the value on a instance of `Double` - if you look at the javadocs the methods are static. if you created your own wrapper for a primitive it should work – Scary Wombat Jun 18 '14 at 05:59

score 0 · Answer 3 · answered Jun 18 '14 at 05:52

In Java references to Objects, and primitives are passed by value.

 public class SubiectLicentaTrei {
        public static void main(String args[]) {
            double d = 12.34; // local variable `d`'s value is 12.34                 
            ScadeUnitate su = new ScadeUnitate();
            su.scadeUnitate(d);  // passing 12.34 to constructor.
            System.out.println(d);
        }
    }

    class ScadeUnitate {
        public void scadeUnitate(double d) {  // another local variable `d` (different from original one). Also `d` is of a primitive type (`double`)
            d = d - 1.0;       // the second local variable d being changed. The first d is untouched.
        }
    }

Why doesn't this change the value

3 Answers3