使用此关键字来引用实例变量，该实例变量的名称被局部变量隐藏

public class Main {

  int num = 2017; // An instance variable


  void printNum(int num) {/*时代Java 提供*/

    System.out.println("Parameter num: " + num);

    System.out.println("Instance variable num: " + this.num);

  }


  public static void main(String[] args) {

    Main a = new Main();

    a.printNum(2000);

  }

}