What is the difference between synchronize and concurrent collection in Java?

1 ) Internal implementation and performance

Collections.SynchronizedMap

SynchronizedMap is a static inner class of utility class java.util.Collections. It is quite similar to Hashtable and it acquires lock on entire Map instance.

But it internally wraps all the methods of Map interface with synchronized keyword. For example, here is the internal put() and get() method implementation of SynchronizedMap :

public V get(Object key) {
 synchronized(mutex) {return m.get(key);}
 }
 public V put(K key, V value) {
 synchronized(mutex) {return m.put(key, value);}
 }

SynchronizedMap acquires lock on entire Map object which provides thread-safety, but not good performance as at a time only one thread can access that Map instance.

ConcurrentHashMap

To overcome slow performance issue, ConcurrentHashMap was introduced in Java 5 along with other concurrent classes like CountDownLatch, CyclicBarrier, CopyOnWriteArrayList, BlockingQueue within java.util.Concurrent package.

More than one threads can read and write concurrently in ConcurrentHashMap and still it provides thread-safety.

ConcurrentHashMap divides the Map instance into different segments. And each thread acquires lock on each segment. By default it allows 16 threads to access it simultaneously without any external synchronization i.e. by default concurrency level is 16. We can also increase or decrease the default concurrency level while creating ConcurrentHashMap by using below constructor :

ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)

2) Null Key and Null Values

ConcurrentHashMap doesn't allow null keys and null values, whereas SynchronizedMap may allow null keys and null values based on the original collection class being passed inside it