Failed update list value in a for-loop but succeed in list with object value

Question

I tried to run a for-loop and update a double value in a list , and I found that I failed. I search on the web and I know that when getting double value by List#get(i) method is simply get a copy of the value, such that will not update a value in a list, just like the code below.

        List<Double> list1 = new ArrayList<Double>();
        list1.add(10.0);
        for(int i = 0; i < list1.size() ; i++){
            Double value = list1.get(i);
            value = value/100;
            System.out.println(list1.get(i));
        }

        //Result 10.0

the only way is use the List#set(i, value) method.

Now I add a Test class

class Test{
    Double value;

    public Test(Double value){
        this.value = value;
    }
}

and do the similar thing with Test class

        List<Test> list2 = new ArrayList<Test>();
        Test test = new Test(10.0);
        list2.add(test1);

        for(int i = 0; i < list2.size() ; i++){
             Test test = list2.get(i);
             test.value = test.value / 100;
             System.out.println(test.value);
        }

        //Result 0.1

My question is : Why the value in a list will update. Isn't the test object in a for loop still a copy of the original one ? Is it because of primitive data type or reference difference?

Isn't the test object in a for loop still a copy of the original one? - No, what's being returned by list2.get(i) in this case is a reference to the original Test object. — azurefrog
– azurefrog, Commented Jul 29, 2014 at 17:44

Elliott Frisch · Accepted Answer · 2014-07-29 17:41:35Z

3

Actually, the issue here is that the primitive wrapper types are immutable.

List<Double> list1 = new ArrayList<Double>();
list1.add(10.0);
for(int i = 0; i < list1.size() ; i++){
  Double value = list1.get(i);
  value = value/100;                      // <-- creates a new value!
  System.out.println(list1.get(i));
}

You could use the shorter,

List<Double> list1 = new ArrayList<Double>();
list1.add(10.0);
for(int i = 0; i < list1.size() ; i++){
  list1.set(i, list1.get(i) / 10);
  System.out.println(list1.get(i));
}

answered Jul 29, 2014 at 17:41

Elliott Frisch

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1 Comment

Will Over a year ago

great, thanks for your help, I know this method, but I totally forget the immutable and mutable thing.

Eran · Accepted Answer · 2014-07-29 17:40:17Z

2

The difference is between immutable objects (such as Float and String) and mutable objects (such as your Test class).

Mutable objects can be updated. The list contains a reference to the object. When you change the content of that object, the list still refers to the same object, which is now changed.

answered Jul 29, 2014 at 17:40

Eran

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1 Comment

Will Over a year ago

thanks for your help, you teach me the immutable and mutable concept clearly!

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