Question

I have following LinkHashMap,

LinkedHashMap<String, ArrayList<String>> test1=new ...

in a loop I assign the key value pairs like this,

ArrayList<String> temp=new ...
//start iteration
temp.add("some strings")
test1.put("some string", temp);
temp.clear()//temp is cleared for next iteration

here temp is a temporary list I create just before and add something to it.
But the problem is that when I clear temp, the temp passed to test1 is also cleared, that means it is copied by reference. How can I use temp inside test1 still maintaining its reference. I know it is a basic concept, but i am new to java and did not get through all its features.

Answer 1

That’s because you are not really copying the List, temp. You are copying the pointer (“reference” in Java lingo).

There are two simple solutions here:

1. Do an actual copy of the List using new ArrayList<String>(temp) as what you store in the map.
2. Instead of doing temp.clear(), do temp = new ArrayList<String>().

The second option is going to perform the best because it’s not making a copy of anything; it’s simply replacing the reference with a fresh ArrayList.