How to Copy a Map in Java - Step by Step Examples

How to Copy a Map in Java ?

Answer

To copy a map in Java, you can use the putAll() method or the constructor that takes another map as a parameter to create a new map with the same key-value pairs as the original map. These methods provide straightforward ways to duplicate a map.

✐ Examples

1 Copying a Map Using the putAll() Method

We can copy a map in Java using the putAll() method, which adds all key-value pairs from the original map to a new map.

For example,

We start by importing the java.util.HashMap class, which provides the necessary functions and data structures for working with maps.
We declare and initialize a map named originalMap with some key-value pairs. In this example, the map has string keys and integer values.
We declare an empty map named copiedMap.
We use the putAll() method to add all key-value pairs from originalMap to copiedMap.
We print the copied map to the console using the System.out.println function to verify the copy.

Java Program

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        // Declare and initialize a map
        HashMap<String, Integer> originalMap = new HashMap<>();
        originalMap.put("one", 1);
        originalMap.put("two", 2);
        originalMap.put("three", 3);

        // Declare an empty map
        HashMap<String, Integer> copiedMap = new HashMap<>();

        // Copy the map using the putAll() method
        copiedMap.putAll(originalMap);

        // Print the copied map
        System.out.println("Copied Map: " + copiedMap);
    }
}

Output

Copied Map: {one=1, two=2, three=3}

2 Copying a Map Using the Copy Constructor

We can also copy a map in Java using the constructor that takes another map as a parameter, which creates a new map with the same key-value pairs as the original map.

For example,

We start by importing the java.util.HashMap class, which provides the necessary functions and data structures for working with maps.
We declare and initialize a map named originalMap with some key-value pairs. In this example, the map has string keys and integer values.
We use the copy constructor to create a new map named copiedMap that contains the same key-value pairs as originalMap.
We print the copied map to the console using the System.out.println function to verify the copy.

Java Program

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        // Declare and initialize a map
        HashMap<String, Integer> originalMap = new HashMap<>();
        originalMap.put("one", 1);
        originalMap.put("two", 2);
        originalMap.put("three", 3);

        // Copy the map using the copy constructor
        HashMap<String, Integer> copiedMap = new HashMap<>(originalMap);

        // Print the copied map
        System.out.println("Copied Map: " + copiedMap);
    }
}

Output

Copied Map: {one=1, two=2, three=3}

3 Copying a Map and Modifying Values

We can copy a map in Java and modify its values during the copying process by iterating over the original map and applying transformations to the values.

For example,

We start by importing the java.util.HashMap class, which provides the necessary functions and data structures for working with maps.
We declare and initialize a map named originalMap with some key-value pairs. In this example, the map has string keys and integer values.
We declare an empty map named copiedMap.
We use a for-each loop to iterate over originalMap and add each key-value pair to copiedMap, modifying the values during the copying process (e.g., multiplying each value by 2).
We print the copied map to the console using the System.out.println function to verify the copy.

Java Program

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        // Declare and initialize a map
        HashMap<String, Integer> originalMap = new HashMap<>();
        originalMap.put("one", 1);
        originalMap.put("two", 2);
        originalMap.put("three", 3);

        // Declare an empty map
        HashMap<String, Integer> copiedMap = new HashMap<>();

        // Copy the map and modify values
        for (HashMap.Entry<String, Integer> entry : originalMap.entrySet()) {
            copiedMap.put(entry.getKey(), entry.getValue() * 2);
        }

        // Print the copied map
        System.out.println("Copied Map: " + copiedMap);
    }
}

Output

Copied Map: {one=2, two=4, three=6}

Summary

In this tutorial, we learned How to Copy a Map in Java language with well detailed examples.

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How to Copy a Map in Java - Step by Step Examples