`HashMap`

For C# programs, we do not use a HashMap as a collection of key-value pairs. Instead we use a Dictionary. Java programs use HashMap, and this functionality can be translated to C#.

Methods, notes

The put() method on HashMap can be changed to the Add() method on Dictionary. And we can use TryGetValue instead of getOrElse.

Example program

To begin, we have a program in C# that adds 3 string keys with 3 int values to a Dictionary. We get one of the values. Then we try to get a value that does not exist.

Detail We use a foreach-loop over the Dictionary to iterate the KeyValuePairs. A KeyValuePair is similar to a Java Entry.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        // Create Dictionary.
        Dictionary<string, int> hash = new Dictionary<string, int>();

        // Add some data.
        hash.Add("diamond", 500);
        hash.Add("ruby", 200);
        hash.Add("pearl", 100);

        // Get value that exists.
        int value1 = hash["diamond"];
        Console.WriteLine("get DIAMOND: " + value1);

        // Get value that does not exist.
        hash.TryGetValue("coal", out int value2);
        Console.WriteLine("get COAL: " + value2);

        // Loop over items in collection.
        foreach (KeyValuePair<string, int> pair in hash)
        {
            Console.WriteLine("KEY: " + pair.Key);
            Console.WriteLine("VALUE: " + pair.Value);
        }
    }
}get DIAMOND: 500
get COAL: 0
KEY: diamond
VALUE: 500
KEY: ruby
VALUE: 200
KEY: pearl
VALUE: 100

Java `HashMap`

For comparison, here is a Java program that creates a HashMap. It adds 3 string keys with 3 Integer values to the collection. It uses get() and getOrElse() to access values.

Detail For the loop, it uses for-loop over the collection returned by entrySet(). We iterate over each Entry.

import java.util.HashMap;
import java.util.Map.Entry;

public class Program {
    public static void main(String[] args) {

        // New HashMap.
        HashMap<String, Integer> hash = new HashMap<>();

        // Add to collection.
        hash.put("diamond", 500);
        hash.put("ruby", 200);
        hash.put("pearl", 100);

        // Get values from collection.
        int value1 = hash.get("diamond");
        System.out.println("get DIAMOND: " + value1);

        // Try to get a key that does not exist.
        int value2 = hash.getOrDefault("coal", -1);
        System.out.println("get COAL: " + value2);

        // Display entire HashMap.
        for (Entry<String, Integer> entry : hash.entrySet()) {
            System.out.println("KEY: " + entry.getKey());
            System.out.println("VALUE: " + entry.getValue());
        }
    }
}get DIAMOND: 500
get COAL: -1
KEY: diamond
VALUE: 500
KEY: ruby
VALUE: 200
KEY: pearl
VALUE: 100

Notes, language comparison

We can see the Dictionary and HashMap are almost exactly the same collection. They have different names, and the syntax is a bit different.

However On a conceptual level they are equivalent. We can use either with just some syntax adjustments.

A summary

Typically no HashMap is used in C#. And in Java we do not use a "Dictionary" as often. But the collections has the same purpose, and can be used in almost the same way.