DoubleCheckedLocking.java

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