Design DS for balance in Java

Design a data structure to store user IDs and their corresponding balances in an e-Wallet system. The data structure must support insertion, update, and retrieval of balances using user IDs through the following operations.

set(user id, balance):Inserts a new user ID with the given balance if it does not exist, or updates the existing balance if the user ID is already present.
get(user id): This method will return the balance corresponding to the given user id.

Example:

Input:
set(1, 100)
set(2, 2000)
get(1)
set(1, 5000)
get(1)
get(2)
Output:
1 -> 100
1 -> 5000
2 -> 2000

Naive Approach

Follow the below steps to store and manage user balances using a list.

Create a User class with fields id and balance, and a constructor to initialize them.
Maintain an ArrayList<User> in the UserList class to store all user objects.
set(id, balance): Update balance if user exists; otherwise, add a new user.
get(id): Return balance if user exists; otherwise, return -1.
Use set() and get() in the main method to manage user balances.

Java

import java.util.*;

class User {
    int id, balance;

    User(int i, int b) {
        id = i;
        balance = b;
    }
}

// UserList class to maintain a list of users
class UserList {
    ArrayList<User> list;

    UserList() { list = new ArrayList<>(); }

    // Get balance for a given user ID
    int get(int id) {
        for (User u : list) {
            if (u.id == id) return u.balance;
        }
        return -1; 
    }

    // Add new user or update balance if user exists
    void set(int id, int balance) {
        for (User u : list) {
            if (u.id == id) {
                
                // Update existing user
                u.balance = balance; 
                return;
            }
        }
        
        // Add new user
        list.add(new User(id, balance)); 
    }
}

class GfG {
    public static void main(String[] args) {
        UserList users = new UserList();

        users.set(1, 100);  
        users.set(2, 2000);  

        System.out.println(users.get(1));

        users.set(1, 5000); 
        System.out.println(users.get(1));
        System.out.println(users.get(2)); 
    }
}

Output

100
5000
2000

Efficient Approach Using HashMap

Follow the below steps to store and manage user balances using a HashMap.

Create a UserList class with a HashMap<Integer, Integer> to store user IDs as keys and balances as values.
Initialize the HashMap in the constructor.
set(id, balance): insert a new entry or update the balance for the given user ID.
get(id): retrieve and return the balance for the given user ID.
In the main method, use set() to add/update users and get() to fetch their balances.

Java

import java.util.*;

class UserList {

    HashMap<Integer, Integer> map;

    // Constructor initializes the HashMap
    UserList() { 
        map = new HashMap<>(); 
    }

    // Get balance for a given user ID
    int get(int id) { 
        return map.get(id); 
    }

    // Add new user or update balance if user exists
    void set(int id, int balance) { 
        map.put(id, balance); 
    }
}

class GfG {

    public static void main(String args[]) {
        UserList users = new UserList();

        users.set(1, 100);  
        users.set(2, 2000);  

        System.out.println(users.get(1)); 

        users.set(1, 5000);  
        System.out.println(users.get(1)); 
        System.out.println(users.get(2)); 
    }
}

Output

100
5000
2000

Design DS for balance in Java

Naive Approach

Efficient Approach Using HashMap

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