Identity Hashmap Example in Java

Introduction

Identity 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

Identity HashMap is a HashTable based implementation of Map Interface. Normal Hashmap compares keys using ‘.equals’ method. But Identity Hashmap compares its keys using ’==’ operator. Hence ‘a’ and new String(‘a’) are considered as 2 different keys. The initial size of Identity hashmap is 21 while the initial size of normal Hashmap is 16.

Syntax

IdentityHashMap<key-variable-type,value-variable-type> identityHashMap = new IdentityHashMap<key-variable-type,value-variable-type>();

Identity HashMap Example Program

import java.util.IdentityHashMap;

public class IdentityHashMapExample {

    public static void main(String args[]) {

        IdentityHashMap<String, String> identityHashMap = new IdentityHashMap<String, String>();

        identityHashMap.put("a", "Apple");
        identityHashMap.put(new String("a"), "Aeroplane");
        identityHashMap.put("b", "Ball");
        identityHashMap.put(new String("b"), "Bat");
        identityHashMap.put("c", "Cat");

        for (String str : identityHashMap.keySet()) {
            System.out.println("Key : " + str + " and Value : " + identityHashMap.get(str));
        }

        System.out.println("Size of map is : " + identityHashMap.size());
        System.out.println("Here 'a' and new String('a') are considered as separate keys");
    }
}

Sample Output

Key : a and Value : Apple
Key : b and Value : Ball
Key : c and Value : Cat
Key : b and Value : Bat
Key : a and Value : Aeroplane
Size of map is : 5
Here 'a' and new String('a') are considered as separate keys

When to use

Use this identity 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.IdentityHashMap; imports a class used later in the program.

3. IdentityHashMap<String, String> identityHashMap = new IdentityHashMap<String, String>(); 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. Compare your console output with the sample output for Identity HashMap to confirm the program behaves correctly.

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.