In Java, you can iterate over a HashMap using a for loop or a for-each loop. Here are examples of both methods:

Method 1: Using a for loop

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

public class HashMapIteration {
    public static void main(String[] args) {
        HashMap<String, Integer> map = new HashMap<String, Integer>();
        map.put("John", 25);
        map.put("Alice", 32);
        map.put("Bob", 18);
        map.put("Mary", 27);

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

In the above example, we define a HashMap with String keys and Integer values. We use a for loop to iterate over the key-value pairs of the HashMap.

Inside the loop, we use the entrySet() method of the HashMap class to get a set of all the key-value pairs. We use the Map.Entry interface to represent each key-value pair.

We store the key and value in separate variables and print them to the console using the System.out.println() method.

After running the above code, the output would be:

John => 25
Alice => 32
Bob => 18
Mary => 27

Method 2: Using a for-each loop

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

public class HashMapIteration {
    public static void main(String[] args) {
        HashMap<String, Integer> map = new HashMap<String, Integer>();
        map.put("John", 25);
        map.put("Alice", 32);
        map.put("Bob", 18);
        map.put("Mary", 27);

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

In the above example, we define a HashMap with String keys and Integer values. We use a for-each loop to iterate over the keys of the HashMap.

Inside the loop, we use the keySet() method of the HashMap class to get a set of all the keys. We use the get() method of the HashMap class to get the value corresponding to the current key.

We store the key and value in separate variables and print them to the console using the System.out.println() method.

After running the above code, the output would be:

John => 25
Alice => 32
Bob => 18
Mary => 27

This program iterates over the HashMap and prints each key-value pair to the console using a for loop and a for-each loop.