Java Program to Iterate Over a HashMap

May 27, 2023

What is HashMap in Java ?

A hashmap is a data structure used for storing and organizing key-value pairs. It allows for quick lookup of values associated with a given key, making it a very efficient way to store and retrieve data.
The basic idea behind a hashmap is to use a hash function to compute a unique index for each key-value pair. This index is used to store and retrieve the value associated with that key. The hash function takes the key as input and returns an integer, which is used to determine the index in the array where the value will be stored.

How HashMap Works ?

When a value is added to a hashmap, its key is passed through the hash function to determine the index at which the value should be stored. If that index is already occupied by another key-value pair, a collision occurs. There are various ways to handle collisions, such as using linked lists or other data structures to store multiple values at the same index.

Advantages of Using HashMap :

One of the main advantages of a hashmap is that it allows for very fast lookup of values based on their keys. In a properly designed hashmap, the time required to find a value is independent of the size of the hashmap, making it a scalable data structure for many applications.

Ways to Iterate over HashMap :

1. Using for-each loop with entrySet(): Java provides a for-each loop that can be used to iterate over the entries in a hashmap. The loop iterates over each key-value pair, where each pair is represented as a Map.Entry object.

Syntax :

Map<String, Integer> map = new HashMap<>();
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);

for (Map.Entry<String, Integer> entry : map.entrySet()) {
    String key = entry.getKey();
    int value = entry.getValue();
    System.out.println("Key: " + key + ", Value: " + value);
}

2. Using for-each loop with keySet(): Java also provides a for-each loop that can be used to iterate over the keys in a hashmap.

Syntax :

Map<String, Integer> map = new HashMap<>();
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);

for (String key : map.keySet()) {
    int value = map.get(key);
    System.out.println("Key: " + key + ", Value: " + value);
}

3. Using iterator() method with entrySet(): Java provides an iterator() method that can be used to iterate over the entries in a hashmap.

Syntax :

Map map = new HashMap<>();
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);

Iterator> iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
    Map.Entry entry = iterator.next();
    String key = entry.getKey();
    int value = entry.getValue();
    System.out.println("Key: " + key + ", Value: " + value);
}

Example 1 :

Run

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

public class Main {
    public static void main(String[] args) {
        Map< String, Integer> map = new HashMap<>();
        map.put("A", 1);
        map.put("B", 2);
        map.put("C", 3);

        for (Map.Entry< String, Integer> entry : map.entrySet()) {
            String key = entry.getKey();
            int value = entry.getValue();
            System.out.println("Key: " + key + ", Value: " + value);
        }
    }
}

Output :

Key: A, Value: 1
Key: B, Value: 2
Key: C, Value: 3

Explanation : In this example, we are using a for-each loop to iterate over the entries in the hashmap. We declare a Map.Entry object called "entry" and use the map.entrySet() method to get a Set view of the entries in the hashmap. The for-each loop then iterates over the set, and for each entry, we extract the key and value using entry.getKey() and entry.getValue(), respectively.

Example 2 :

Run

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

public class Main {
    public static void main(String[] args) {
        Map< String, Integer> map = new HashMap<>();
        map.put("A", 1);
        map.put("B", 2);
        map.put("C", 3);

        for (String key : map.keySet()) {
            int value = map.get(key);
            System.out.println("Key: " + key + ", Value: " + value);
        }
    }
}

Output :

Key: A, Value: 1
Key: B, Value: 2
Key: C, Value: 3

Explanation : In this example, we are using a for-each loop to iterate over the keys in the hashmap. We declare a String variable called "key" and use the map.keySet() method to get a Set view of the keys in the hashmap. The for-each loop then iterates over the set, and for each key, we extract the corresponding value using map.get(key).

Example 2 :

Run

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        Map< String, Integer> map = new HashMap<>();
        map.put("A", 1);
        map.put("B", 2);
        map.put("C", 3);

        Iterator< Map.Entry< String, Integer>> iterator = map.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry< String, Integer> entry = iterator.next();
            String key = entry.getKey();
            int value = entry.getValue();
            System.out.println("Key: " + key + ", Value: " + value);
        }
    }
}

Output :

Key: A, Value: 1
Key: B, Value: 2
Key: C, Value: 3

Explanation : In this example, we are using an iterator to iterate over the entries in the hashmap. We declare an Iterator> object called "iterator" and use the map.entrySet().iterator() method to get an iterator over the entries in the hashmap. We then use a while loop to iterate over the entries using the iterator, and for each entry, we extract the key and value using entry.getKey() and entry.getValue(), respectively.

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Java Program to Iterate Over a HashMap

What is HashMap in Java ?

How HashMap Works ?

Advantages of Using HashMap :

Ways to Iterate over HashMap :

Syntax :

Syntax :

Syntax :

Example 1 :

Output :

Example 2 :

Output :

Example 2 :

Output :

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