Java – Update the Key in a HashMap

In Java, a HashMap is a class that implements the Map interface, it stores key-value pairs in a hash table, which allows for fast lookups, insertions, and deletions. There are several ways to update the key in a HashMap.

Here, we will explore the following:

• By removing and adding the new key/value pair with the updated key
• Using for-loop
• Using Java 8 Streams
• Using the forEach method

Update the key in a HashMap by removing and adding the new key/value pair with the updated key

Let’s create one HashMap, populate it with some key/value pairs, and replace one of the keys.

public class Test {

  public static void main(String[] args) {

    Map<String, Integer> map = new HashMap<>();
    map.put("one", 1);
    map.put("two", 2);
    map.put("three", 3);
    map.put("four", 4);

    // modify the key "two"
    map.remove("two");
    map.put("twoUpdated", 2);

    System.out.println(map);
  }
}

Update the key in a HashMap using Java for-loop

We can iterate over a HashMap and populate a new one with modified keys.

public class Test {

  public static void main(String[] args) {
    Map<String, Integer> map = new HashMap<>();
    map.put("one", 1);
    map.put("two", 2);
    map.put("three", 3);
    map.put("four", 4);

    // create a new HashMap
    Map<String, Integer> updatedMap = new HashMap<>();

    // iterate over original map and put the key/values pair into a new one with modified key "two"
    for (Map.Entry<String, Integer> entry : map.entrySet()) {

      if (entry.getKey().equals("two")) {
        updatedMap.put("twoUpdatedKey", entry.getValue());
      } else {
        updatedMap.put(entry.getKey(), entry.getValue());
      }
    }

    System.out.println(updatedMap);
  }
}

Update the key in a HashMap using Java 8 Streams

If we need to update all the keys, we can use the Streams API to create a new HashMap of the existing one, and during that process, we can change the keys.

import java.util.*;
import java.util.stream.Collectors;

public class Test {

  public static void main(String[] args) {

    Map<String, Integer> map = new HashMap<>();
    map.put("one", 1);
    map.put("two", 2);
    map.put("three", 3);
    map.put("four", 4);

    // converting all keys to uppercase
    map = map.entrySet().stream()
            .collect(Collectors.toMap(e -> e.getKey().toUpperCase(), Map.Entry::getValue));

    System.out.println(map);
  }
}

Change the key in a HashMap using the forEach method

We can also use Java 8 forEach method to change the keys in a HashMap. We will also create a new one and populate it with updated key/value pairs from the original one.

public class Test {

  public static void main(String[] args) {
    Map<String, Integer> map = new HashMap<>();
    map.put("one", 1);
    map.put("two", 2);
    map.put("three", 3);
    map.put("four", 4);

    // create a new HashMap
    Map<String, Integer> updatedMap = new HashMap<>();

    // convert the keys to uppercase and put into a new map with corresponding values
    map.forEach((key, value) -> updatedMap.put(key.toUpperCase(), value));

    System.out.println(updatedMap);
  }
}

Happy coding!