Unexpected outputs from two different ways of adding an ArrayList to another

Question

In some program as following:

ArrayList<ArrayList<Integer>> result=new ArrayList<ArrayList<Integer>>();
ArrayList<Integer> temp=new ArrayList<Integer>();

I want to add temp into result, if I use

result.add(new ArrayList<Integer>(temp));

then the final output is correct, but if I use

result.add(temp);

then my final output is wrong. Why? Thanks for the help!

Answer 1

The question does not show what is actually wrong and what the desired output is supposed to be.

This, result.add(new ArrayList<Integer>(temp)) , and this, result.add(temp) will both produce the same contents in result

Without understanding clearly the desired output, I cannot see what is actually wrong with the code.

Further, it is better practice to refer to the ArrayList as a List on the left side and use the <> diamond on the right. Make your coding easier and clearer:

    List<List<Integer>> result = new ArrayList<>();
    List<Integer> temp = new ArrayList<>();
    temp.add(1);
    temp.add(2);
    result.add(temp);
    System.out.println("Result: " + result);

The output is exactly as specified and is a List<List<Integer>> :

Result: [[1, 2]]

Answer 2

When you call:

result.add(new ArrayList<Integer>(temp));

temp is copied into a new ArrayList and the end-result will contain the "untouched" copy.

By calling:

result.add(temp)

you're adding temp itself to the result so the actions you'll preform later-on on temp will be reflected in result (that's the unwanted behavior you described).