Does Java's HashMap Guarantee Retrieval Order?

Is Hash Map Retrieval Order Preserved?

In Java, HashMaps employ a specific algorithm to store and retrieve data, leaving some users wondering about the order in which values are retrieved. As we delve into a code snippet, we'll uncover the truth behind HashMap retrieval order.

import java.util.HashMap;

public class HashMapExample {
   public static void main(String[] args) {
       HashMap<Integer, String> hashmap = new HashMap<Integer, String>();
       hashmap.put(1, "apple" );
       hashmap.put(2, "lemon" );
       hashmap.put(3, "orange" );
       hashmap.put(4, "banana" );
       hashmap.put(5, "litchi" );
       hashmap.put(6, "mango" );
       hashmap.put(7, "papaya" );

       System.out.println(hashmap.size());

       for (String key : hashmap.values()) {
           System.out.println(key);
       }
   }
}

Output:

7
apple
lemon
orange
banana
litchi
mango
papaya

Surprising Behavior:

Surprisingly, the values are retrieved in the same order they were inserted. However, this observation might not always hold true.

The Truth:

According to the HashMap Javadoc, there is no guarantee that the retrieval order will remain consistent over time. This means that the retrieval order can change arbitrarily.

Need Consistent Ordering?

If consistent ordering is crucial, one can opt for alternative data structures such as LinkedHashMap or TreeMap.

• LinkedHashMap: Maintains insertion or access order (LIFO)
• TreeMap: Maintains comparison order based on the key

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