java/w8/Cache.java

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

public class Cache<K, V> {
    private final Map<K, V> store = new HashMap<>();
    private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    public void put(K key, V value) {
        if (key == null || value == null)
            throw new IllegalArgumentException("Key and value must not be null");
        lock.writeLock().lock();
        try {
            store.put(key, value);
        } finally {
            lock.writeLock().unlock();
        }
    }

    public V get(K key) {
        lock.readLock().lock();
        try {
            return store.get(key);
        } finally {
            lock.readLock().unlock();
        }
    }

    public boolean containsKey(K key) {
        lock.readLock().lock();
        try {
            return store.containsKey(key);
        } finally {
            lock.readLock().unlock();
        }
    }

    public V remove(K key) {
        lock.writeLock().lock();
        try {
            return store.remove(key);
        } finally {
            lock.writeLock().unlock();
        }
    }

    public void clear() {
        lock.writeLock().lock();
        try {
            store.clear();
        } finally {
            lock.writeLock().unlock();
        }
    }

    public int size() {
        lock.readLock().lock();
        try {
            return store.size();
        } finally {
            lock.readLock().unlock();
        }
    }

    public static void main(String[] args) {
        Cache<String, Integer> cache = new Cache<>();
        cache.put("one", 1);
        cache.put("two", 2);
        System.out.println(cache.get("one"));
        System.out.println(cache.get("two"));
        cache.remove("one");
        System.out.println(cache.containsKey("one"));
        System.out.println("Cache size: " + cache.size());
    }
}