Learn to create clone of a HashMap in Java. We will see the java programs to create shallow copy and deep copy of a HashMap.
Read More : A guide to cloning in Java
1. Clone HashMap – shallow copy
1.1. HashMap clone() method
The best way to create shallow clone of hashmap is to use it’s clone() method. It returns a shallow copy of the map. The keys and values themselves are not cloned; and point to same object in memory as in original map.
import java.time.LocalDate;
import java.util.HashMap;
public class JavaHashMapCloningExample
{
@SuppressWarnings("unchecked")
public static void main(String[] args)
{
HashMap<Integer, Employee> employeeMap = new HashMap<>();
employeeMap.put(1, new Employee(1l, "Alex", LocalDate.of(1990, 01, 01)));
employeeMap.put(2, new Employee(2l, "Bob", LocalDate.of(1990, 02, 01)));
//Shallow clone
HashMap<Integer, Employee> clonedMap = (HashMap<Integer, Employee>) employeeMap.clone();
//Same as employeeMap
System.out.println(clonedMap);
System.out.println("\nChanges reflect in both maps \n");
//Change a value is clonedMap
clonedMap.get(1).setName("Charles");
//Verify content of both maps
System.out.println(employeeMap);
System.out.println(clonedMap);
}
}
Program output.
{1=Employee [id=1, name=Alex, dob=1990-01-01], 2=Employee [id=2, name=Bob, dob=1990-02-01]}
Changes reflect in both maps
{1=Employee [id=1, name=Charles, dob=1990-01-01], 2=Employee [id=2, name=Bob, dob=1990-02-01]}
{1=Employee [id=1, name=Charles, dob=1990-01-01], 2=Employee [id=2, name=Bob, dob=1990-02-01]}
1.2. Clone HashMap using Java 8 Stream API
If you want little customization in values copied to cloned HashMap, or you simply want to modify the cloning process for individual key-value pairs, then we can use Java 8 stream api.
In this approach, iterate over hashmap entryset using stream api, perform customization of values and then collect the pairs in a new HashMap.
HashMap<Integer, Employee> employeeMap = new HashMap<>();
employeeMap.put(1, new Employee(1l, "Alex", LocalDate.of(1990, 01, 01)));
employeeMap.put(2, new Employee(2l, "Bob", LocalDate.of(1990, 02, 01)));
// Shallow clone
Map<Integer, Employee> clonedMap = employeeMap.entrySet()
.stream()
//perform customization
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
// Same as employeeMap
System.out.println(clonedMap);
Program output.
{1=Employee [id=1, name=Alex, dob=1990-01-01], 2=Employee [id=2, name=Bob, dob=1990-02-01]}
2. How to deep clone HashMap
The most effective way to deep clone a Java object is serialization. The same applies to deep clone a HashMap as well. Here, we are using Google Gson library to serialize the HashMap and deserialize to create HashMap deep copy.
HashMap<Integer, Employee> employeeMap = new HashMap<>();
employeeMap.put(1, new Employee(1l, "Alex", LocalDate.of(1990, 01, 01)));
employeeMap.put(2, new Employee(2l, "Bob", LocalDate.of(1990, 02, 01)));
//Deep clone
Gson gson = new Gson();
String jsonString = gson.toJson(employeeMap);
Type type = new TypeToken<HashMap<Integer, Employee>>(){}.getType();
HashMap<Integer, Employee> clonedMap = gson.fromJson(jsonString, type);
System.out.println(clonedMap);
//--------------------------------------
System.out.println("\nChanges DO NOT reflect in other map \n");
//Change a value is clonedMap
clonedMap.get(1).setName("Charles");
//Verify content of both maps
System.out.println(employeeMap);
System.out.println(clonedMap);
Program output.
{1=Employee [id=1, name=Alex, dob=1990-01-01], 2=Employee [id=2, name=Bob, dob=1990-02-01]}
Changes DO NOT reflect in other map
{1=Employee [id=1, name=Alex, dob=1990-01-01], 2=Employee [id=2, name=Bob, dob=1990-02-01]}
{1=Employee [id=1, name=Charles, dob=1990-01-01], 2=Employee [id=2, name=Bob, dob=1990-02-01]}
Let me know if you have question regarding how to deep clone a HashMap or create shallow copy of a HashMap in Java.
Happy Learning !!