Java.Collections.What is weak reference ?

In Java, a weak reference is a type of reference that does not prevent an object from being garbage collected.

✅ In plain terms:

If an object is only weakly referenced (i.e., no strong references point to it), then the GC is free to collect it at any time — even if it’s still “reachable” through that weak reference.

import java.lang.ref.WeakReference;

public class WeakRefExample {
    public static void main(String[] args) {
        String strong = new String("Hello");
        WeakReference<String> weak = new WeakReference<>(strong);

        System.out.println("Before GC: " + weak.get()); // "Hello"

        strong = null;  // Remove strong reference
        System.gc();    // Suggest garbage collection

        // Wait a bit to give GC a chance
        try { Thread.sleep(500); } catch (InterruptedException e) {}

        System.out.println("After GC: " + weak.get()); // null (if GC ran)
    }
}

📦 Reference Types in Java

Reference Type	GC Behavior	Example Use Case
Strong	Never collected unless unreachable	Normal variables
Weak	Collected as soon as no strong refs	WeakHashMap, caches
Soft	Collected only when memory is low	Memory-sensitive caches
Phantom	Used for final cleanup (rare)	Advanced GC tracking

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✅ Use Cases for Weak References

1. WeakHashMap:
   • Keys are weak references.
   • When a key is no longer strongly referenced, it’s automatically removed from the map.
2. Caches:
   • Store data that can be discarded if not in use anymore.
3. Avoiding memory leaks:
   • When you want to refer to something without preventing it from being garbage collected.

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⚠️ Gotchas

• You can’t rely on when the GC will clear weak references.
• weakRef.get() can return null at any time, so always check before using.

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🧠 TL;DR:

A weak reference lets you point to an object without preventing it from being garbage collected.
Useful for caches, observers, and memory-sensitive structures.