ArrayList Object assignment

Question

I have two separate objects ArrayList<String> in two separate packages Top and top10. I assign the value of top10 to Top in my activity. And now if I remove an element from Top it also gets removed from top10. I don't know why is this happening? I feel totally dumbfounded. Is there something I don't know about java? Or is it android?

This is my activity code:

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

// ServiceCall is the name of the class where top10 is initialized.

Top = ServiceCall.top10;

System.out.println("top WR: "+ServiceCall.top10);

if(Top.get(0).equals("Please Select")) Top.remove(0);   

System.out.println("top WR: "+ServiceCall.top10);

The second printed out statement has one element less than the one before.

Top is reference to top10 in servicecall, so when you call remove(0), it removes element at 0, that is why you are seeing one less. — kosa, Jul 25 '12 at 17:46
Right! I do. I guess I was totally treating it as a variable in my mind. Thanks — Harsh, Jul 25 '12 at 17:53

score 2 · Answer 1 · answered Jul 25 '12 at 17:46

You are pointing to the same Object.

Top = ServiceCall.top10;

is not creating a new Object, but making a reference to the other one, hence all changes in both pointers will be reflected in the same Object.

You'll have to create a new one passing the other one as parameter:

List<String> Top = new ArrayList<String>(ServiceCall.top10);

score 1 · Accepted Answer · answered Jul 25 '12 at 17:47

You are pointing Top to top10, not creating a new list (your initializer is effectively unused right now, as you are just repointing it to the other list.)

You should do:

ArrayList<String> Top = new ArrayList<String>(ServiceCall.top10);

This will create a shallow copy.

ArrayList Object assignment

2 Answers2