In Collection Framework, only some of the classes are thread-safe or synchronized. If you need to work on a multi-threaded environment then you have to convert these non-synchronized type classes to Synchronized type collection.

Synchronized Collection

These are the classes only are synchronized:

• Vector
• HashTable

As a solution java.util.Collections class provides some static methods to make them Synchronized.

Collections Class Synchronized Methods

All these methods are static:

Collection synchronizedCollection(Collection c)	Returns a synchronized collection.
List synchronizedList(List list)	Returns a synchronized list.
Map<K,V> synchronizedMap(Map<K,V> m)	Returns a synchronized map .
NavigableMap<K,V> synchronizedNavigableMap(NavigableMap<K,V> m)	Returns a synchronized navigable map.
NavigableSet synchronizedNavigableSet(NavigableSet s)	Returns a synchronized navigable set .
static Set synchronizedSet(Set s)	Returns a synchronized set .
SortedMap<K,V> synchronizedSortedMap(SortedMap<K,V> m)	Returns a synchronized sorted map .
SortedSet synchronizedSortedSet(SortedSet s)	Returns a synchronized sorted set.

Example: Synchronized Collections

In this example creating the blank type of collection and making it Synchronized. You can assign the same with collection objects.

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

public class CollectionSunchronizationExample {

	public static void main(String[] args) {
		Collection c = Collections.synchronizedCollection(new ArrayList());
	    List list = Collections.synchronizedList(new ArrayList());
	    Set s = Collections.synchronizedSet(new HashSet());
	    Map m = Collections.synchronizedMap(new HashMap());

	}

}

References

• https://docs.oracle.com/javase/8/docs/api/java/util/Collections.html