Java TreeMap merge() Method

The TreeMap.merge() method in Java is used to either insert a new key-value pair into the map or merge a new value with an existing value associated with a key using a specified remapping function. This guide will cover the method’s usage, explain how it works, and provide examples to demonstrate its functionality. We will also cover a real-world use case to show how TreeMap.merge() can be used effectively.

Table of Contents

1. Introduction
2. merge Method Syntax
3. Examples
   • Inserting a New Key-Value Pair
   • Merging Values for an Existing Key
4. Real-World Use Case
   • Example: Updating Scores in a Scoreboard
5. Conclusion

Introduction

The TreeMap.merge() method is a member of the TreeMap class in Java. It allows you to insert a new key-value pair into the map or merge a new value with an existing value using a specified remapping function. If the key is not already present in the map, the method inserts the new key-value pair. If the key is already present, the method merges the new value with the existing value using the remapping function and updates the map with the result.

merge() Method Syntax

The syntax for the merge method is as follows:

public V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)

• Parameters:
  • key: The key with which the specified value is to be associated.
  • value: The value to be merged with the existing value associated with the key.
  • remappingFunction: A function that computes the new value to be associated with the key if the key is already present in the map.
• Returns: The new value associated with the specified key, or null if no value is associated with the key.

Examples

Inserting a New Key-Value Pair

The merge method can be used to insert a new key-value pair into the TreeMap.

Example

import java.util.TreeMap;
import java.util.function.BiFunction;

public class MergeExample {
    public static void main(String[] args) {
        // Creating a TreeMap with String keys and Integer values
        TreeMap<String, Integer> treeMap = new TreeMap<>();

        // Adding initial entries to the TreeMap
        treeMap.put("Ravi", 25);
        treeMap.put("Priya", 30);

        // Merging a new key-value pair into the TreeMap
        treeMap.merge("Vijay", 35, (oldValue, newValue) -> oldValue + newValue);

        // Printing the TreeMap
        System.out.println("TreeMap after merging new key-value pair: " + treeMap);
    }
}

Output:

TreeMap after merging new key-value pair: {Priya=30, Ravi=25, Vijay=35}

Merging Values for an Existing Key

The merge method can be used to merge a new value with an existing value in the TreeMap.

Example

import java.util.TreeMap;
import java.util.function.BiFunction;

public class MergeExistingKeyExample {
    public static void main(String[] args) {
        // Creating a TreeMap with String keys and Integer values
        TreeMap<String, Integer> treeMap = new TreeMap<>();

        // Adding initial entries to the TreeMap
        treeMap.put("Ravi", 25);
        treeMap.put("Priya", 30);

        // Merging a value for an existing key in the TreeMap
        treeMap.merge("Priya", 5, (oldValue, newValue) -> oldValue + newValue);

        // Printing the TreeMap
        System.out.println("TreeMap after merging value for existing key: " + treeMap);
    }
}

Output:

TreeMap after merging value for existing key: {Priya=35, Ravi=25}

Real-World Use Case

Example: Updating Scores in a Scoreboard

A common real-world use case for TreeMap.merge() is updating scores in a scoreboard where new scores need to be added to existing scores.

Example

import java.util.TreeMap;
import java.util.function.BiFunction;

public class ScoreboardManager {
    public static void main(String[] args) {
        // Creating a TreeMap to manage player scores
        TreeMap<String, Integer> scoreboard = new TreeMap<>();

        // Adding initial scores to the TreeMap
        scoreboard.put("Ravi", 50);
        scoreboard.put("Priya", 60);

        // Updating scores using merge method
        scoreboard.merge("Priya", 10, (oldScore, newScore) -> oldScore + newScore);
        scoreboard.merge("Vijay", 20, (oldScore, newScore) -> oldScore + newScore);

        // Printing the updated scoreboard
        System.out.println("Updated Scoreboard: " + scoreboard);
    }
}

Output:

Updated Scoreboard: {Priya=70, Ravi=50, Vijay=20}

In this example, TreeMap.merge() is used to update the scores in a scoreboard, ensuring that new scores are correctly added to existing scores.

Conclusion

The TreeMap.merge() method in Java provides a way to insert new key-value pairs or merge new values with existing values in a TreeMap using a specified remapping function. By understanding how to use this method, you can efficiently manage collections of key-value pairs in your Java applications. The method allows you to handle both the insertion of new pairs and the updating of existing pairs, making it a versatile tool for data management.