Map / Dictionary — Interview Preparation¶

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class TTLMap<K, V> {
    private static class Entry<V> {
        final V value;
        final long expireAtMs;   // 0 = no expiry

        Entry(V value, long expireAtMs) {
            this.value = value;
            this.expireAtMs = expireAtMs;
        }
    }

    private final Map<K, Entry<V>> map = new HashMap<>();
    private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    /** Puts k=v with ttlMs millisecond lifetime. ttlMs <= 0 means no expiry. */
    public void put(K k, V v, long ttlMs) {
        long exp = ttlMs > 0 ? System.currentTimeMillis() + ttlMs : 0L;
        lock.writeLock().lock();
        try {
            map.put(k, new Entry<>(v, exp));
        } finally {
            lock.writeLock().unlock();
        }
    }

    /** Returns the value, or null if absent or expired. */
    public V get(K k) {
        Entry<V> e;
        lock.readLock().lock();
        try {
            e = map.get(k);
        } finally {
            lock.readLock().unlock();
        }
        if (e == null) return null;
        if (e.expireAtMs > 0 && System.currentTimeMillis() > e.expireAtMs) {
            lock.writeLock().lock();
            try { map.remove(k); } finally { lock.writeLock().unlock(); }
            return null;
        }
        return e.value;
    }

    public void delete(K k) {
        lock.writeLock().lock();
        try { map.remove(k); } finally { lock.writeLock().unlock(); }
    }

    public static void main(String[] args) throws InterruptedException {
        TTLMap<String, Integer> m = new TTLMap<>();
        m.put("a", 1, 50);
        m.put("b", 2, 0);
        System.out.println("a: " + m.get("a"));   // a: 1
        Thread.sleep(60);
        System.out.println("a after expiry: " + m.get("a")); // null
        System.out.println("b: " + m.get("b"));   // b: 2
    }
}