类如何获取自身的实例？

Question

Today, I was working on a tutorial with integrating buttons into a ground up built game engine. This is the code that requests win.

public class ButtonEvent{

    private Window win;

    public ButtonEvent(){

        this.win = Window.getWindow();

    }

    /*Other methods*/

}

I watched the whole video through, but the guy who went over the class never mentioned what he did to his Window class to do this. In that video he happened to click the window class and then click the next class and in that time, when I paused the video, I saw his getWindow() method. Which I wrote below.

public class Window extends Canvas{

public Window win; //I tried doing this

    /*Other methods*/ 

    public static Window getWindow(){
        return win;
    }

}

Can someone explain to me how he does this? I know the question is kind of vague, but it's all I have to work with.

Thanks!

Answer 1

Use the singleton pattern to ensure that only one instance of Window is created and used throughout the application. Note that singletons are considered an "anti-pattern" meaning that there's usually a better way to go about it, such as using container-managed objects (see PicoContainer, Spring IoC, etc).

public class Window {
    private static Window INSTANCE = null;

    public static Window getWindow() {
        if (INSTANCE == null) {
            INSTANCE = new Window();
        }
        return INSTANCE;
    }
}

Answer 2

Change

public Window win;

to

private static Window win = new Window();

And you will have the proper local visibility for getWindow() to see the Window field, and it will be initialized by calling the implicit empty constructor that the compiler inserts (that's the assignment of new Window ).