How to make a class thread-safe in Java?

A thread-safe class is a class that guarantees the internal state of the class as well as returned values from methods, are correct while invoked concurrently from multiple threads.

The HashMap is a non-synchronized collection class. If we need to perform thread-safe operations on it then we must need to synchronize it explicitly.

Example:

import java.util.Collections; import java.util.HashMap; import java.util.Map; import java.util.Set; import java.util.Iterator; public class HashMapSyncExample {    public static void main(String args[]) {       HashMap hmap = new HashMap();       hmap.put(2, "Raja");       hmap.put(44, "Archana");       hmap.put(1, "Krishna");       hmap.put(4, "Vineet");       hmap.put(88, "XYZ");       Map map= Collections.synchronizedMap(hmap);       Set set = map.entrySet();       synchronized(map){             Iterator i = set.iterator();           // Display elements          while(i.hasNext()) {                Map.Entry me = (Map.Entry)i.next();                System.out.print(me.getKey() + ": ");                System.out.println(me.getValue());          }       }    } }

In the above example, we have a HashMap it is having integer keys and String type values. In order to synchronize it we are using Collections.synchronizedMap(hashmap). It returns a thread-safe map backed up by the specified HashMap.

Output:

1: Krishna 2: Raja 4: Vineet 88: XYZ 44: Archana