How do I copy the contents of one ArrayList into another?

You can use such trick:

myObject = new ArrayList<Object>(myTempObject);

or use

myObject = (ArrayList<Object>)myTempObject.clone();

You can get some information about clone() method here

But you should remember, that all these ways will give you a copy of your List, not all of its elements. So if you change one of the elements in your copied List, it will also be changed in your original List.

There are no implicit copies made in java via the assignment operator. Variables contain a reference value (pointer) and when you use = you're only coping that value.

In order to preserve the contents of myTempObject you would need to make a copy of it.

This can be done by creating a new ArrayList using the constructor that takes another ArrayList:

ArrayList<Object> myObject = new ArrayList<Object>(myTempObject);

Edit: As Bohemian points out in the comments below, is this what you're asking? By doing the above, both ArrayLists (myTempObject and myObject) would contain references to the same objects. If you actually want a new list that contains new copies of the objects contained in myTempObject then you would need to make a copy of each individual object in the original ArrayList

originalArrayList.addAll(copyArrayList);

Please Note: When using the addAll() method to copy, the contents of both the array lists (originalArrayList and copyArrayList) refer to the same objects or contents. So if you modify any one of them the other will also reflect the same change.

If you don't wan't this then you need to copy each element from the originalArrayList to the copyArrayList, like using a for or while loop.