Question

Im sure there is a simple answer to this and I will feel stupid in a few mins but here it goes, it’s late and im tired….

So, can i ask, why does this code…

public class TestProcess {

    final float[] finalFloats = {0.9f,0.8f,0.7f};
    float[] floats;

    public static void main(String[] args) {
        new TestProcess();      
    }

    public TestProcess(){
        floats = finalFloats;
        printTotal();
        floats[0]=0.1f;
        printTotal();
        floats = finalFloats;
        printTotal();
    }

    void printTotal(){
        float count = 0f;
        for(float f : floats){
            count+=f;
        }
        out("Count:"+count);
    }

    void out(String s){
        System.out.println(s);
    }

}

give this output

Count:2.4
Count:1.6
Count:1.6

When i would expect

Count:2.4
Count:1.6
Count:2.4

i feel very stupid…!

This question has also been asked link text, I will post here if it gets answered on the other forum.

Answer 1

Java arrays are mutable reference types.

When you write floats = finalFloats;, you are making the floats field refer to the same array instance as finalFloats.
Therefore, when you write floats[0]=0.1f, you’re also modifying the original finalFloats array.

All the final keyword does is prevent you from assigning finalFloats to point to a new instance (eg, finalFloats = new float[7]); it doesn’t prevent you from mutating the instance.

To make it behave the way you expect, you need to make a copy of the array, like this:

floats = Arrays.copyOf(finalFloats, finalFloats.length);