Copying a HashMap in Java

Question

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.

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

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.

There is a small (HUGE) understatement here. If you want to copy a HashMap with nested structures, HashMap.putAll() will copy by reference, because it doesn't know how to exactly copy your object. For example:

import java.util.*;
class Playground {
    public static void main(String[ ] args) {
        Map<Integer, Map<Integer,List<Float>>> dataA = new HashMap<>();
        Map<Integer, Map<Integer,List<Float>>> dataB = new HashMap<>();

        dataA.put(1, new HashMap<>());
        dataB.putAll(dataA);

        assert(dataB.get(1).size() == 0);

        dataA.get(1).put(2, new ArrayList<>());

        if (dataB.get(1).size() == 1) { // true
            System.out.println(
                "Sorry object reference was copied - not the values");
        }
    }
}

So basically you will need to copy the fields yourself like here

List <Float> aX = new ArrayList<>(accelerometerReadingsX);
List <Float> aY = new ArrayList<>(accelerometerReadingsY);

List <Float> gX = new ArrayList<>(gyroscopeReadingsX);
List <Float> gY = new ArrayList<>(gyroscopeReadingsY);

Map<Integer, Map<Integer, Float>> readings = new HashMap<>();

Map<Integer,List<Float>> accelerometerReadings = new HashMap<>();
accelerometerReadings.put(X_axis, aX);
accelerometerReadings.put(Y_axis, aY);
readings.put(Sensor.TYPE_ACCELEROMETER, accelerometerReadings);

Map<Integer,List<Float>> gyroscopeReadings = new HashMap<>();
gyroscopeReadings.put(X_axis, gX);
gyroscopeReadings.put(Y_axis, gY);
readings.put(Sensor.TYPE_GYROSCOPE, gyroscopeReadings);