ConcurrentMap.java

import java.util.HashMap;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class ConcurrentMap<K,V> extends HashMap<K,V> {

    private final Lock readLock;
    private final Lock writeLock;

    public ConcurrentMap() {
        ReadWriteLock readWriteLock = new ReentrantReadWriteLock();
        readLock = readWriteLock.readLock();
        writeLock = readWriteLock.writeLock();
    }

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

    public V put(K key, V value) {
        try {
            writeLock.lock();
            return super.put(key, value);
        }finally {
            writeLock.unlock();
        }
    }
}

SingletonThreadsafeMap.java

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

public class SingletonThreadsafeMap {

    private Map<String, String> map = new HashMap<String, String>();
    private ReadWriteLock readWriteLock = new ReentrantReadWriteLock();

    private static TestMap INSTANCE = new TestMap();
    private Lock readLock = readWriteLock.readLock();;
    private Lock writeLock = readWriteLock.writeLock();

    public static TestMap getInstance() {
        return INSTANCE;
    }

    public String get(String key) {
        try {
            readLock.lock();
            return map.get(key);
        } finally {
            readLock.unlock();
        }
    }

    public String put(String key, String value) {
        try {
            writeLock.lock();
            return map.put(key, value);
        }finally {
            writeLock.unlock();
        }
    }
}