WeakHashMap

// ── WeakHashMap — entries removed when key is GC'd: ──────────────────
WeakHashMap<Object, String> wmap = new WeakHashMap<>();

Object key1 = new Object();   // strong reference
Object key2 = new Object();   // strong reference

wmap.put(key1, "value1");
wmap.put(key2, "value2");
System.out.println("Size: " + wmap.size());   // 2

// Remove strong reference to key2:
key2 = null;

// Suggest GC (not guaranteed — illustrative):
System.gc();
Thread.sleep(100);   // give GC time to run

// After GC collects key2's object, the entry is removed:
System.out.println("Size: " + wmap.size());   // 1 or 2 — GC timing uncertain
System.out.println("key1 present: " + wmap.containsKey(key1));  // true
// key2's entry may already be gone

// ── String literals vs new String — important distinction: ────────────
WeakHashMap<String, Integer> strMap = new WeakHashMap<>();

String literal = "hello";         // interned — JVM holds strong reference
strMap.put(literal, 1);
literal = null;                    // release our reference
System.gc();
// "hello" literal STILL in map — JVM's string pool holds it strongly

String dynamic = new String("hello");  // NOT interned — no other strong ref
strMap.put(dynamic, 2);
dynamic = null;                   // release our reference
System.gc();
// dynamic "hello" MAY be GC'd and entry removed

// ── Practical: object metadata without memory leak: ───────────────────
public class ObjectMetadataStore<T> {
    private final WeakHashMap<T, Map<String, Object>> metadata =
        new WeakHashMap<>();

    public void set(T obj, String key, Object value) {
        metadata.computeIfAbsent(obj, k -> new HashMap<>()).put(key, value);
    }

    public Object get(T obj, String key) {
        Map<String, Object> m = metadata.get(obj);
        return m == null ? null : m.get(key);
    }
    // When obj is GC'd, its metadata entry is automatically removed.
    // No manual cleanup needed. No memory leak.
}