Question

It works like this:

Set<Integer> nums = numMap.get(id);
nums.add(new Integer(0));
// now do i have to:
numMap.put(id,nums)?
// or is it already stored?

regards && tia
noircc

Answer 1

You don’t have to put it back, unless you make a deep clone of it. Everything works based on references in Java.

You can always test this by writing a simple program.

public static void main(String... args) {
    Map<Integer, Set<Integer>> numMap = new HashMap<Integer, Set<Integer>>();

    Set<Integer> set = new HashSet<Integer>();
    set.add(10);

    numMap.put(0, set);

    System.out.println("Map before adding is " + numMap);

    set.add(20);

    System.out.println("Map after adding is " + numMap);
}

Which prints

Map before adding is {0=[10]}
Map after adding is {0=[20, 10]}