Singleton

basic.java

public class SingletonClass {
  private static SingletonClass instance;
  
  private SingletonClass() {}
  
  // basic style
  public static SingletonClass getInstance() {
    if (instance == null) { // <- not thread safe
      instance = new SingletonClass();
    }
    return instance;
  }
}

gistfile1.java

public class SingletonClass {
  private static volatile SingletonClasss instance;
  
  private SingletonClass() {}
  
  // double checking lock
  public static SingletonClass getInstance() {
    if (instance == null) {
      synchronized {
        if (instance == null) {
          instance == new SingletonClass();
        }
      }
    }
    return instance;
  }
}

gistfile2.java

public class SingletonClass {
  private SingletonClass() {}
  
  // lazy and thread safe
  private static class InstanceHolder {
    private static final SingletonClass instance = new SingletonClass();
  }

  public static SingletonClass getInstance() {
    return InstanceHolder.instance;
  }
}

gistfile3.java

public class SingletonClass {
  private static final SingletonClass instance;
  
  private SingletonClass() {}
  
  // when class loaded
  static {
    instance = new SingletonClass();
  }
  
  // thread safe
  public static SingletonClass getInstance() {
    return instance;
  }
}

gistfile4.java

public class SingletonClass {
  private static SingletonClass instance;
  
  private SingletonClass() {}
  
  // it is also thread safe.
  public static synchronized SingletonClass getInstance() {
    if (instance == null) {
      instance = new SingletonClass();
    }
    return instance;
  }
}