How can an infinite loop be created using HashMap?

Creating an infinite loop with HashMap is possible due to its concurrent modification features during resizing (rehashing) by multiple threads simultaneously. Without proper synchronization, two threads can modify the linked lists in HashMap buckets in such a way that a cycle is formed.

During rehashing, elements from the old bucket array are transferred to a new one. In single-threaded mode, the order of elements in the linked list of a bucket is preserved. In multi-threaded mode, if thread A starts rehashing and thread B modifies (adds/removes) elements or also begins rehashing, problems can arise.

Consider a scenario with two threads adding elements to HashMap as it approaches the threshold for rehashing. Thread A might see the old array and start transferring elements, while thread B might see the new array and also start transferring or modifying. During the transfer, HashMap uses linked lists. In multi-threaded mode, without synchronization, the atomicity of operations is broken.

A cycle might look like this:

1. HashMap is nearly full, rehashing is required.
2. Thread A begins rehashing, iterating over elements in the old bucket and creating new nodes for the new array.
3. Before Thread A finishes transferring element X, the scheduler switches to Thread B.
4. Thread B also begins rehashing or modifies the same bucket.
5. As a result of incorrect concurrent modification, node Y, which should point to null (end of list), might start pointing to node X (already processed by Thread A) or to itself.

This leads to an infinite loop during subsequent get() or put() operations in that bucket, as traversing the linked list becomes endless.

Such issues are fixed in ConcurrentHashMap, which uses more complex and safe synchronization mechanisms to ensure thread safety.

// Example (NOT recommended for use in multithreaded applications without synchronization)
// Demonstration of a possible cycle scenario.
// In real conditions, reproducing this is difficult without specific conditions and tools.
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class HashMapLoopExample {
    private static final ManInBlack hatGuy = new ManInBlack(1);
    private static final ManInBlack jacketGuy = new ManInBlack(2);

    // Class for keys with the same hash code
    static class ManInBlack {
        private final int id;

        ManInBlack(int id) {
            this.id = id;
        }

        // Override equals to be unequal
        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            ManInBlack that = (ManInBlack) o;
            return id == that.id;
        }

        // Return the same hash code for demonstration
        @Override
        public int hashCode() {
            return 1; // Always return 1
        }

        @Override
        public String toString() {
            return "ManInBlack{" + "id=" + id + '}';
        }
    }

    public static void main(String[] args) throws InterruptedException {
        final Map<ManInBlack, String> map = new HashMap<>(2); // Small initial size

        ExecutorService executor = Executors.newFixedThreadPool(2); // Two threads

        executor.submit(() -> {
            while (true) {
                // Attempt to add elements that may cause rehashing
                map.put(hatGuy, "Hat");
                map.put(jacketGuy, "Jacket");
                // If a cycle occurs, the next operation may hang
                // or throw an error after a long execution.
                // System.out.println("Thread 1: Put operation completed.");
            }
        });

        executor.submit(() -> {
             while (true) {
                 // Another thread also interacts with the map,
                 // increasing the chance of concurrent modification.
                 // get operations may hang if a cycle occurs.
                 map.get(hatGuy);
                 map.get(jacketGuy);
                 // System.out.println("Thread 2: Get operation completed.");
             }
        });

        // Detecting deadlock or cycle in real scenarios is difficult.
        // A timer can be set to check if execution is stuck.

        // For demonstration, let the threads run for some time
        Thread.sleep(5000); // Let threads run for 5 seconds

        executor.shutdownNow(); // Forcefully terminate threads
        System.out.println("Script finished.");
    }
}