Iterating over a HashMap is an essential operation when working with key-value pairs in Java. Since a HashMap does not guarantee any specific order of its elements, the iteration order may appear random. There are several ways to iterate over a HashMap, and each method offers different benefits. In this lesson, we will focus on two common approaches:

• Using keySet(): Iterate over the set of keys and then retrieve the corresponding values using the get() method.
• Using entrySet(): Directly iterate over the key-value pairs by using the set of Map.Entry objects, which provides both the key and the value in each iteration.

1. Iterating Using keySet()

The keySet() method returns a Set view of the keys contained in the HashMap. You can then loop over these keys and call the get() method to access the associated values.

Syntax:

for (K key : map.keySet()) {
    V value = map.get(key);
    // Process key and value
}

Example: Iterating Using keySet()

Example Explanation:

• HashMap Creation:
  • A HashMap<String, Integer> named studentGrades is created and populated with student names as keys and their grades as values.
• Iteration using keySet():
  • The keySet() method retrieves a set of all keys in the map.
  • A for-each loop iterates over each key, and the get() method fetches the corresponding value.
• Outcome:
  • The code prints each key-value pair, demonstrating how to iterate over a HashMap using the keySet approach.

2. Iterating Using entrySet()

The entrySet() method returns a Set view of the mappings (key-value pairs) contained in the HashMap. This approach allows you to directly access both keys and values in a single iteration.

Syntax:

for (Map.Entry<K, V> entry : map.entrySet()) {
    K key = entry.getKey();
    V value = entry.getValue();
    // Process key and value
}

Example: Iterating Using entrySet()

Example Explanation:

• HashMap Creation:
  • A HashMap<String, Integer> named studentGrades is created and populated with key-value pairs.
• Iteration using entrySet():
  • The entrySet() method returns a set of Map.Entry objects, where each entry contains a key and its corresponding value.
  • A for-each loop iterates over these entries, directly accessing keys and values.
• Outcome:
  • The code prints each key-value pair, providing a straightforward and efficient way to iterate over the entire map.

Both methods provide effective means to access all entries in a HashMap. The entrySet() approach is generally preferred for its simplicity and efficiency, as it avoids additional lookups with get(). Understanding these iteration techniques is fundamental for processing and manipulating data stored in maps.