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.
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.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Dec 15, 2021 (edit link).
