What approaches can be used instead of the synchronized keyword to implement the Singleton pattern in a multithreaded environment?

Instead of using the 'synchronized' keyword to implement Singleton in a multithreaded environment, you can use the following approaches:

1. Initialization-on-demand holder idiom (static inner class)

   This method uses lazy initialization and guarantees thread safety without synchronization.

   public class Singleton {
       private Singleton() {}
   
       private static class Holder {
           private static final Singleton INSTANCE = new Singleton();
       }
   
       public static Singleton getInstance() {
           return Holder.INSTANCE;
       }
   }
   

2. Using enum

   Enum in Java guarantees a single instance and thread safety by default.

   public enum Singleton {
       INSTANCE;
   
       public void someMethod() {
           // ...
       }
   }
   

3. Double-checked locking with volatile

   Allows avoiding full synchronization on each call, synchronizing only during the first creation of the instance.

   public class Singleton {
       private static volatile Singleton instance;
   
       private Singleton() {}
   
       public static Singleton getInstance() {
           if (instance == null) {
               synchronized (Singleton.class) {
                   if (instance == null) {
                       instance = new Singleton();
                   }
               }
           }
           return instance;
       }
   }
   

Each of these approaches provides thread safety without constantly using 'synchronized' on the 'getInstance()' method.