How can I iterate over a HashMap in Java?

There are multiple ways to iterate over a hash map in Java. Let's take a HashMap 

HashMap<String, Integer> map = new HashMap<>();

1. Iterating over key-value pairs

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

    

2. Looping over Keys

   for(String key : map.keySet()) {
      System.out.println("Key: " + key);
    }

    

3. Looping over Values

   for (Integer value : map.values()) {
      System.out.println("Value: " + value);
    }

    

4. Java 8's forEach Method

   map.forEach((key, value) -> {
    System.out.println("Key: " + key + ", Value: " + value);
   });

In Java, there are several ways to iterate over a HashMap, depending on whether you want to access keys, values or both (key–value pairs).

Let’s say you have this map:

Map<String, Integer> map = new HashMap<>();
map.put("A", 10);
map.put("B", 20);
map.put("C", 30);

 1. Iterate using entrySet() (Best and most efficient way)

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

• Efficient because it gives direct access to both key and value.
• Preferred in most real-world scenarios.

2. Iterate using keySet()

for (String key : map.keySet()) {
    System.out.println(key + " = " + map.get(key));
}

• Simple, but slightly less efficient because map.get(key) looks up the value each time.

 3. Iterate using values()

for (Integer value : map.values()) {
    System.out.println(value);
}

• Use this if you only need the values.

4. Using Java 8 forEach() and Lambda

map.forEach((key, value) -> {
    System.out.println(key + " = " + value);
});

• Clean and concise, ideal for modern Java (8+).

5. Using an Iterator (if you need to modify entries while looping)

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

• Use this when you need to remove elements safely during iteration.

 Best Practice

For readability and efficiency, prefer
map.entrySet() or map.forEach() (Java 8+).

Both provide direct access to key-value pairs without extra lookups.

Example (Recommended Modern Way):

map.forEach((key, value) -> System.out.println(key + " -> " + value));

The best and most efficient way to iterate over a HashMap in Java is using the entrySet() or forEach() (Java 8+) method.

Here’s how 

 1. Using entrySet() (most efficient before Java 8)

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

• Directly accesses both key and value.
• Avoids extra lookups (unlike using keySet() + get()).

2. Using forEach() (best for Java 8 and later)

map.forEach((key, value) -> System.out.println(key + " = " + value));

• Clean, modern and concise.
• Internally optimized for performance.

Conclusion:

Use map.forEach() in Java 8+ or map.entrySet() for earlier versions,  both are equally efficient and the best ways to iterate a HashMap.