无法解析在公共方法中启动的对象？

Question

I'm just starting out with java and I was working on a new GUI. I made a method that makes my JFrame, sets its default location, close operation, background colour etc.

I made a separate method for this to keep it out of the main code for the sake of tidiness. I already know how to solve this method if I just put all of those things in the main method.

This is the code:

public class Main {

    public static void makeWindow(){
        JFrame mainWindow = new JFrame();
        mainWindow.setVisible(true);
        mainWindow.setPreferredSize(new Dimension(400,400));
        mainWindow.pack();
        mainWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainWindow.setLocationRelativeTo(null);
    }

    public static void main(String args[]){
        makeWindow();
        JPanel mainPanel = new JPanel();
        mainWindow.add(mainPanel);
    }
}

I get an error in my main method saying that "mainWindow" can't be resolved. Why is this? is it because I made the mainWindow object in the other method? Is there a way to resolve this issue without putting everything from the makeWindow() method into the main method?

Answer 1

That won't work because mainWindow only exists in the context of your makeWindow method. You can make it a static field instead:

public class Main {

    private static JFrame mainWindow; // declare it here so it can be seen from your main method.

    public static void makeWindow(){
        mainWindow = new JFrame();
        mainWindow.setVisible(true);
        mainWindow.setPreferredSize(new Dimension(400,400));
        mainWindow.pack();
        mainWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainWindow.setLocationRelativeTo(null);
    }

    public static void main(String args[]){
        makeWindow();
        JPanel mainPanel = new JPanel();
        mainWindow.add(mainPanel);
    }
}

EDIT

As @Xing commented (credits shared), you can return the mainWindow from your makeWindow method:

public class Main {

    public static JFrame makeWindow(){
        JFrame mainWindow = new JFrame();
        mainWindow.setVisible(true);
        mainWindow.setPreferredSize(new Dimension(400,400));
        mainWindow.pack();
        mainWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainWindow.setLocationRelativeTo(null);
        return mainWindow;
    }

    public static void main(String args[]){
        JFrame mainWindow = makeWindow();
        JPanel mainPanel = new JPanel();
        mainWindow.add(mainPanel);
    }
}

Answer 2

create mainWindow instance/ reference globally

private static JFrame mainWindow;

public static void makeWindow(){
    mainWindow = new JFrame();
}