Java.Collections.What is WeakHashMap?

🧩 What is a WeakHashMap?

A WeakHashMap is a hash table-based map where the keys are stored as weak references.

If a key is no longer referenced elsewhere in your application, then the entry will be automatically removed by the garbage collector — poof 💨

It’s like a self-cleaning map that helps you avoid memory leaks when you’re associating data with objects you don’t “own”.

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🔍 How It Works

• Uses WeakReference for its keys.
• When a key becomes weakly reachable (no strong refs), the GC collects it, and the map removes the entry automatically.
• Internally, it uses a ReferenceQueue to track which keys were GC’d and should be cleaned up.

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🧪 Code Example

import java.util.*;

public class WeakHashMapDemo {
    public static void main(String[] args) {
        Map<Object, String> map = new WeakHashMap<>();
        Object key = new String("weak-key"); // not interned!

        map.put(key, "value");
        System.out.println("Before GC: " + map);

        key = null; // remove strong reference
        System.gc(); // suggest garbage collection

        try { Thread.sleep(500); } catch (InterruptedException ignored) {}

        System.out.println("After GC: " + map); // likely empty now
    }
}

Before GC: {weak-key=value}
After GC: {}

✅ Use Cases

1. Memory-sensitive caches
   • You want to store metadata, but only while the object is still in memory.
2. Class loader metadata
   • Common in frameworks that associate data with classes, but don’t want to prevent class unloading.
3. Listeners or observers
   • If the observer object disappears, you want it removed automatically from the registry.

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🚫 When Not to Use It

• When you need deterministic lifetime of entries — GC is non-deterministic!
• For performance-critical or large-scale apps, WeakHashMap can be slower and less predictable than alternatives like ConcurrentHashMap.

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🧠 Key Differences from HashMap

Feature	HashMap	WeakHashMap
Key reference type	Strong	Weak (WeakReference)
Automatic cleanup	❌ No	✅ Yes, on GC of keys
Allows null key	✅ Yes	✅ Yes
Thread-safe	❌ No	❌ No
GC-aware	❌	✅ Removes entries post-GC

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💡 TL;DR:

WeakHashMap is a special kind of Map where keys are held with weak references — so when a key is no longer used anywhere else, its entry is automatically removed after garbage collection.
Perfect for caches, framework internals, and memory-sensitive metadata.