Hashmap Example in Java

Introduction

HashMap is a classic Java console program that demonstrates the concept with complete source code and sample output. The Collections Framework provides ArrayList, HashMap, HashSet and related data structures.

This tutorial walks through the program line by line, explains how the logic works, and highlights best practices you can apply in your own code.

Definition

A Hashmap is a Map data structure. Like a list each item that is stored in a hashmap is stored at a particular index. This index is called a hash and it is generated using a hash function. Hash functions accept the object to be stored as an argument and generate a number that is unique to it.

Syntax

HashMap  Variable_name = new HashMap();

HashMap Example Program

import java.util.HashMap;
 
public class HashMapExample {
    public static void main(String a[]){
		HashMap hm = new HashMap();
        hm.put("1","FIRST");
        hm.put("2","SECOND");
        hm.put("3","THIRD");
		hm.put("4",null);
		hm.put(null,"FIFTH");
        System.out.println("hashmap is "+hm);
        System.out.println("Value of 1: "+hm.get("1"));
        System.out.println("Is HashMap empty? "+hm.isEmpty());
        hm.remove("2");
        System.out.println("After removal process, the hashmap is "+hm);
        System.out.println("Size of the HashMap: "+hm.size());
    }
}

Sample Output

hashmap is {null=FIFTH, 3=THIRD, 2=SECOND, 1=FIRST, 4=null}
Value of 1: FIRST
Is HashMap empty? false
After removal process, the hashmap is {null=FIFTH, 3=THIRD, 1=FIRST, 4=null}
Size of the HashMap: 4

When to use

Use this hashmap example when learning or revising core Java syntax.

How it works

1. Execution begins in the main method — the JVM calls this method when you run the class.

2. import java.util.HashMap; imports a class used later in the program.

3. HashMap hm = new HashMap(); updates a variable used in the calculation or output.

4. A println / print call writes text to the console — part of the sample output below.

5. A println / print call writes text to the console — part of the sample output below.

6. A println / print call writes text to the console — part of the sample output below.

7. A println / print call writes text to the console — part of the sample output below.

8. A println / print call writes text to the console — part of the sample output below.

Best Practices

• Use meaningful variable and class names that describe their purpose.
• Compile and run the program locally — modify values to see how output changes.
• Read compiler errors carefully; they usually point to the exact line to fix.

Common Mistakes

• Copying code without understanding each line — practice by changing one statement at a time.
• Mismatching the public class name and the .java filename.
• Forgetting semicolons at the end of statements.