use case of singleton class in Java

singleton

public class ApplicationCache {
  
  //NO Lazy Initialization
  private static final ApplicationCache INSTANCE = new ApplicationCache();
  
  //Check the INSTANCE to avoid the reflect call
  private ApplicationCache() {
    if(INSTANCE != null)
      throw new IllegalStateException("Exist Already");
  }
}

class ConnectionPool {
  
  //Lazy Initialization
  private static volatile ConnectionPool instance;
  
  public static ConnectionPool getInstance() {
    if(instance == null) {
      //synchronized on class level lock
      synchronized(ConnectionPool.class) {
        if(instance == null) {
          instance = new ConnectionPool();
        }
      }
    }
    
    return instance;
  }
  
  private ConnectionPool() {
    if(instance != null) {
      throw new IllegalStateException("Exist Already");
    }
  }
  
}

//Use Enum maybe the best solution
public enum Connection {
  INSTANCE;
  
  public void connect() {
    
  }
}