ArrayList won't return modified after running through a method

Question

I can't get this method to modify an ArrayList then return the modified ArrayList.

public static ArrayList<Integer> sortAndRemoveDuplicates(ArrayList<Integer> list) {
        list = new ArrayList<Integer>(new HashSet<Integer>(list));
        Collections.sort(list);
        //System.out.println(list);
        return list;
}

When I uncomment the System call I can see that the ArrayList is being modified and I know that my method works but when I run the method through main the ArrayList remains unaffected. Why is this? What am I doing wrong?

Why isn't this modified version working?

public static ArrayList<Integer> sortAndRemoveDuplicates(ArrayList<Integer> list) {
    ArrayList<Integer> newList = new ArrayList<Integer>(new HashSet<Integer>(list));
    Collections.sort(newList);
    System.out.println(newList);
    return newList;
}

**How** are you calling it? Are you grabbing the List that is returned? — Hovercraft Full Of Eels, Jul 17 '14 at 20:54
I think your expectations are not met because everything in Java is pass by value. — JamesB, Jul 17 '14 at 20:59

score 1 · Answer 1 · answered Jul 17 '14 at 20:55

1

You are creating new ArrayList from passed list so You need to catch result back

List<Integer> result = sortAndRemoveDuplicates(inputList)

answered Jul 17 '14 at 20:55

jmj

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So do I run my method within my method? – aeipownu Jul 17 '14 at 21:13
you need to reassign result back just that – jmj Jul 17 '14 at 21:19

score 1 · Answer 2 · edited May 23 '17 at 12:13

1

Java passes references by value. You are re-assigning the input reference that points to the initial list to another object. The original list will remain pointing to the original ArrayList object.

You need to assign this newly created list back to the original reference:

originalList = sortAndRemoveDuplicates(originalList);

EDIT: Alternatively, since you are not returning the value to the caller method (main), you can make your method's return type void and modify your input list as follows:

public static void sortAndRemoveDuplicates(List<Integer> list) {
    List<Integer> newList = new ArrayList<Integer>(new HashSet<Integer>(list));
    Collections.sort(newList);
    System.out.println(newList);
    list.clear();
    for(Integer integer : newList) {
        list.add(integer);
    }
}

Note: It is best practice to program to interfaces instead of concrete implementations, i.e. let your newList be declared of type List instead of ArrayList.

edited May 23 '17 at 12:13

Community

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1

answered Jul 17 '14 at 20:55

M A

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Is this within my method? – aeipownu Jul 17 '14 at 21:13
No, in `main` where you call your method. – M A Jul 17 '14 at 21:20
Updated. Your edited method still won't work if you're not assigning the return value to `main`. – M A Jul 18 '14 at 12:12
How would I assign the return value to main? – aeipownu Jul 18 '14 at 23:14

score 0 · Answer 3 · answered Jul 17 '14 at 21:03

I think your expectations are not met because everything in Java is pass by value so the modifications to the list are made to a copy, not your original list. Unless you return and capture the modified copy, you will not see the changes in the calling code on return from the method.

ArrayList won't return modified after running through a method

3 Answers3