混淆理解程序输出

Question

Here is a program

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";
    }
}

The output is "I am title of movie" I am trying to understand it but till now I do not understand it properly. I want to know: Why does it not print "I am title"

Answer 1

Sequence of events:

// 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";

If you printed the title before calling playIt , it would still show as "I am title".

Answer 2

Does this illustrate why:

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

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

Here's the corresponding output:

java MovieTitle
null
I am title
I am title of movie

Answer 3

Cause you set when you call playIt() the title to "I am title of movie"; Try changing the order of the two lines

one.title = "I am title";
one.playIt();

Answer 4

If you trace out the calls, it should become fairly obvious.

Movie one = new Movie();

title will be NULL at this point (ie, it has had no value assigned)

one.title = "I am title";

Now your Movie object one has the title "I am title"

one.playIt();

Calls the playIt() method on one, which sets the title of "this" (which is one) to "I am title of movie"

Answer 5

You asign "I am title" to the title varible in object one of type Movie .

When you call method playIt() in the same object, varible title gets asigned another value "I am title of movie" .

If you invert the lines like so

one.playIt(); 
one.title = "I am title";

The output is going to be "I am title" , because you set this value, after you call playIt() method.