java - 对理解程序输出感到困惑

Question

这是一个程序

public class MovieTitle {
    public static void main(String[] args) {
        Movie one = new Movie();
        one.title = "I am title";
        one.playIt();
        System.out.println(one.title);
    }
}

class Movie {
    String title;
    void playIt() {
        this.title = "I am title of movie";
    }
}

输出是“我是电影的标题”我试图理解它，但到目前为止我还没有正确理解它。我想知道:为什么它不打印“我是标题”

Answer 1

事件顺序:

// create a new Movie called "one"
Movie one = new Movie();

// at this point, one.title is still null

// set the title to "I am title"
one.title = "I am title";

// call playIt, which in turn ...
one.playIt();
   // sets the title to something else again
   => this.title = "I am title of movie";

如果您在调用 playIt 之前打印了标题，它仍会显示为“我是标题”。