public class Single {
    private Single() {}

    private static Single instance = new Single();

    private static Single instance(){
        instance = new Single();
        return instance;
    }

}

------解决方案--------------------

Java code

class Single {
  private Single() {}

  // private static Single instance = new Single(); //假如注释掉

  private static Single instance; //这句话没有问题
  // instance = new Single(); //这句活有问题，因为这条语句不能直接放在这里，必须放在某个方法里面或者放在初始化块里面。