CODEWITHSUNDEEP
javaarraylist
Wodlo
Wodlo

Reputation: 937

Why am I removing an element from the wrong ArrayList?

I am seeing some behaviour that I can't quite make sense of and was wondering if someone would be kind enough to explain exactly what is going on here. This is the code I currently have.

public class MyClass {

    ArrayList<String> myList = new ArrayList<String>();

    public MyClass() {
        populate();
        removeData(myList);
    }

    private void populate() {
        myList.add("Some data");
        myList.add("Some more data");
        myList.add("Even more data");
    }

    private void removeData(ArrayList<String> list) {
        ArrayList<String> temp = new ArrayList<String>();
        temp = list;
        temp.remove("Some data");
    }
}

Now for some reason, after I run this code, data is being removed from the ArrayList "myList". Why is this happening even though I am only supposed to be removing data from a variable inside the method "removeData" and not from the field "myList"?

Upvotes: 5

Views: 92

Answers (4)

Rahman
Rahman

Reputation: 3795

ArrayList<String> temp = new ArrayList<String>();
    temp = list;

In the first line you are creating new list. Till that point its fine. But in the next line you are referring list to temp. So earlier object to which temp was referring will be garbage collected temp will be referring to the same object which was created Myclass constructor

Upvotes: 1

Nathaniel Jones
Nathaniel Jones

Reputation: 1849

Java works with references, so when you assigned temp = list, both variables pointed to the same object. Instead, you want to make a copy of list:

private void removeData(ArrayList<String> list) {
    ArrayList<String> temp = new ArrayList<String>(list); // Make a copy
    temp.remove("Some data");
}

Upvotes: 1

Zaphod Beeblebrox
Zaphod Beeblebrox

Reputation: 679

Change your code to: 

private void removeData(ArrayList<String> list) {
    ArrayList<String> temp = new ArrayList<String>(list);
    temp.remove("Some data");
}

and you'll get your desired behaviour (though it doesn't really make much sense). Assigning list to temp does not copy the items in the list, but assigns just a reference to the same area of memory.

Upvotes: 3

Suresh Atta
Suresh Atta

Reputation: 122026

 temp = list;

Even though you are removing inside the method, you are still pointing to a instance member, hence it can see the changes.

You need to create a new list with that values if you don't want to affect the instance member.

Upvotes: 6

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