double check singleton

singleton.java

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