HashMap and iterate over its entries in java

HashMap and iterate over its entries in java. involves several approaches depending on what you need to accomplish. Here, I’ll demonstrate three common methods . HashMap and iterate over its entries in java:

1. Iterating using entrySet()

This method is efficient because it accesses both the key and value in each iteration.

java
import java.util.;

public class HashMapIterationProgram {

    public static void main(String[] args) {
        // Creating a HashMap
        Map<String, Integer> map = new HashMap<>();
        map.put("John", 215);
        map.put("Jane", 310);
        map.put("Doe", 410);

        // Iterating over the Hash-Map using entry-Set()
        for (Map.Entry<String, Integer> entry : map.entrySet()) {
            String key = entry.getKey();
            Integer value = entry.getValue();
            System.out.println(key + " => " + value);
        }
    }
}

Explanation:
`entrySet()` provides a set containing all mappings stored within the map.
– We iterate over each Map.Entry<String, Integer> in the set obtained from entrySet().
– For each entry, getKey() retrieves the key, and getValue() retrieves the corresponding value.
– We print each key-value pair.

2. Iterating using keySet()

This approach allows iterating over keys and then fetching corresponding values using get(key).

java
import java.util.;

public class HashMapIterationProgram {

    public static void main(String[] args) {
        // Creating a HashMap
        Map<String, Integer> map = new HashMap<>();
        map.put("rahul", 251);
        map.put("shubham", 301);
        map.put("Gaurav", 410);

        // Iterating over the HashMap using keySet()
        for (String key : map.keySet()) {
            Integer value = map.get(key);
            System.out.println(key + " ===> " + value);
        }
    }
}

Explanation:
– keySet() returns a set view of the keys contained in the map.
– We iterate over each key using the enhanced for loop.
– For each key, get(key) retrieves the corresponding value.
– We print each key-value pair.

3. Iterating using Java 8’s forEach() with lambda expressions

This method provides a concise way to iterate over the entries.

java
import java.util.;

public class HashMapIterationProgram{

    public static void main(String[] args) {
        // Creating a HashMap
        Map<String, Integer> map = new HashMap<>();
        map.put("Laptop", 2500);
        map.put("Mouse", 3000);
        map.put("Charger", 4000);

        // Iterat over the Hash-Map using forEach() and lambda-expression 
        map.forEach((key, value) -> {
            System.out.println(key + " => " + value);
        });
    }
}

Explanation:
– forEach() is a new method in Java 8 that accepts a lambda expression taking two parameters (key and value).
– The lambda expression System.out.println(key + ” => ” + value); is applied to each key-value pair in the map .

Summary

– entrySet(): Preferred when you need both keys and values.
– keySet(): Useful if you only need keys and can fetch values using get(key).
– forEach(): Concise for simple iterations in Java 8 and later versions.