What situations can lead to an element disappearing from a HashMap collection?

An element can disappear from a HashMap collection in the following situations:

• If the key or value was explicitly removed using remove() methods.
• If WeakHashMap is used, and the key no longer has strong references, the element can be garbage collected.
• Due to errors in the implementation of hashCode() and equals() methods for keys, which can make it impossible to find the element.
• If the collection was modified in another thread without synchronization, unpredictable results, including element disappearance, are possible.

Example with WeakHashMap:

import java.util.WeakHashMap;

public class WeakHashMapExample {
    public static void main(String[] args) throws InterruptedException {
        WeakHashMap<Object, String> map = new WeakHashMap<>();
        Object key = new Object();
        map.put(key, "value");

        System.out.println("Before GC: " + map);
        key = null; // Remove strong reference to key

        System.gc(); // Run garbage collector
        Thread.sleep(1000); // Wait

        System.out.println("After GC: " + map); // Element may disappear
    }
}