hash map and arrayList

Question

look at this small example

import java.util.ArrayList;
import java.util.HashMap;

public class Foo {
public static void main(String[] args) {

    ArrayList<Integer> array = new ArrayList<Integer>();
    HashMap<String, ArrayList<Integer>> foo = new HashMap<String,ArrayList<Integer>>();

    for (int i = 0; i < 20; i++)
        array.add(i);

    foo.put("1", array);
    // array.clear();

    System.out.println(foo.get("1").size());
 }
}

Now, if use array.clear() it automatically delete all values of the arrayList associated with the specified key inside the hashmap

how can I prevent this?

in such a way that:

• I can perform array.clear(); (after entering values ​​in hashmap) and only delete the values of ArrayList<Integer> array

• The values ,​​of the array associated with that key, won't deleted inside the hashmap

if you launch this program,it will print 20 without using array.clear();, 0 instead

Answer 1

It is deleting the maps keys elements because both are same objects. You have to make a shallow copy to make both lists separate

Creating a shallow copy is pretty easy though:

List<Integer> newList = new ArrayList<Integer>(oldList);

Here is the code:

public class Foo {
    public static void main(String[] args) {

        ArrayList<Integer> array = new ArrayList<Integer>();
        HashMap<String, ArrayList<Integer>> foo = new HashMap<String, ArrayList<Integer>>();

        for (int i = 0; i < 20; i++)
            array.add(i);

        ArrayList<Integer> newList = new ArrayList<Integer>(array);

        foo.put("1", newList);
        array.clear();

        System.out.println(foo.get("1").size());
    }

}

Answer 2

foo.get("1") and array refer to the same object. You need foo.put("1", new ArrayList<Integer>(array));.

Answer 3

Just put not the ArrayList itself, but it's copy:

foo.put("1", new ArrayList<Integer>(array));

If you want to put the array itself, you must have it unmodifiable:

Map<String, List<Integer>> foo = ...;
...
List<Integer> unmodifiableArray = Collections.unmodifiableList(array);
foo.put(unmodifiableArray);

Answer 4

The problem is that you do not write "new" objects into that map. In other words your list named "array", it's exactly the same memory address as that list in a map, so... if you change 1, changes appear in both places.