public class Singleton {
  private static Singleton instance;

  private Singleton() {
  }

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

public class Client {
  public static void main(String[] args) {
    Singleton s1 = Singleton.getInstance();
    Singleton s2 = Singleton.getInstance();

    if (s1 == s2) {
      System.out.println("同じインスタンスです");
    }
  }
}

C:\JavaHello\Singleton>java Client
同じインスタンスです

C:\JavaHello\Singleton>