Copying a HashMap in Java

I am trying to keep a temporary container of a class that contains member :

HashMap<Integer,myObject> myobjectHashMap

A class called myobjectsList

Then I do

myobjectsListA = new myobjectsList();
myobjectsListB = new myobjectsList();

then: Add some hashmap items to A (like 2)

then:

myobjectListB = myobjectListA; //B has 2

then: Add hashmap items to A (like 4 more)

then: return A to the items stored in B

myobjectListA = myobjectListb;

But when I do this, B grows with A while I am adding hashmap items to A. A now has 6 items in it because B had 6.

I want A to still have the original 2 at the end after last assignment. In C++ I would use copy with objects, what is the java equivalent?

Added: OK I left something out explaining this. MyObjectsList does not contain the HashMap, it is derived from a class MyBaseOjbectsList which has the HashMap member and MyObjectsList extends MyBaseOjbectsList. Does this make a difference?

Solution 1:

If you want a copy of the HashMap you need to construct a new one with.

myobjectListB = new HashMap<Integer,myObject>(myobjectListA);

This will create a (shallow) copy of the map.

Solution 2:

You can also use

clone()

Method to copy all elements from one hashmap to another hashmap

Program for copy all elements from one hashmap to another

import java.util.HashMap;

public class CloneHashMap {    
     public static void main(String a[]) {    
        HashMap hashMap = new HashMap();    
        HashMap hashMap1 = new HashMap();    
        hashMap.put(1, "One");
        hashMap.put(2, "Two");
        hashMap.put(3, "Three");
        System.out.println("Original HashMap : " + hashMap);
        hashMap1 = (HashMap) hashMap.clone();
        System.out.println("Copied HashMap : " + hashMap1);    
    }    
}

source : http://www.tutorialdata.com/examples/java/collection-framework/hashmap/copy-all-elements-from-one-hashmap-to-another

Solution 3:

The difference is that in C++ your object is on the stack, whereas in Java, your object is in the heap. If A and B are Objects, any time in Java you do:

B = A

A and B point to the same object, so anything you do to A you do to B and vice versa.

Use new HashMap() if you want two different objects.

And you can use Map.putAll(...) to copy data between two Maps.