为什么我的JSlider或JLabel不会出现在框架的任何位置？

Question

Need to build a GUI for my second programming class. Part of this includes a J Slider in the east of the frame that should sync up with a text field in the north. However, at first the text field wouldn't show up anywhere on the frame, but eventually the text field and its labeled appeared. Now nothing I'm trying to put in the East side of the frame is appearing at all. I'm very new to Java and especially the swing libraries, please help me understand what I'm doing wrong.

I'm not getting any error messages, and I know my build East function is at least running since I've used print statements to test that. At first the north part of the frame wouldn't show anything so I moved the build North function farther up and it started working. Doing this does nothing for the build East function. I've pasted all of my code below.

    import javax.swing.*;
    import java.awt.BorderLayout;
    import java.awt.*;
    import java.awt.event.*;

public class GUI1 implements ActionListener
{
    //Create window for GUI
    JFrame window;

    public static void main(String [] args)
    {
        GUI1 g = new GUI1();
    }

    public GUI1()
    {
        //Set name and dimensions of window
        window = new JFrame("Program 2");
        window.setBounds(187, 218, 700, 450);

        buildEast();
        buildNorth();

        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setVisible(true); //default is invisible window
    }

    public void buildNorth()
    {
        //Creates panel, and adds panel to north of window
        JPanel north_pane = new JPanel();
        window.add(north_pane, BorderLayout.NORTH);

        //Create and add label to JPanel
        JLabel label = new JLabel ("Year");
        north_pane.add(label);

        //Create and add text field to JPanel in north of window
        JTextField tField = new JTextField(5);
        north_pane.add(tField);
    }

    public void buildEast()
    {
        //Create and add panel to east of window
        JPanel east_pane = new JPanel();

        //Add a title for the slider
        JLabel slideLabel = new JLabel ("Change Year");
        east_pane.add(slideLabel);


        final int YEAR_MIN = 1990;
        final int YEAR_MAX = 2019;
        int selectYear = 2000;
        JSlider yearSlide = new JSlider(JSlider.VERTICAL, YEAR_MIN,    YEAR_MAX, selectYear);

        //Setup for slide labels
        yearSlide.setMajorTickSpacing(4);
        yearSlide.setMinorTickSpacing(2);
        yearSlide.setPaintTicks(true);
        yearSlide.setPaintLabels(true);

        east_pane.add(yearSlide);
    }


    public void actionPerformed(ActionEvent e)
    {
        System.out.println("Click");
    }
}

I expected the J Slider and J Label to at least appear on my frame. No error messages.

Answer 1

In Swing you have to attach any elements you wish to display to a JFrame (often via intermediary JPanels, etc.), which you must then set as visible. As pointed out in comments, you never add the east_pane to the JFrame in your code.

There are two ways you might go about this:

The quick and easy option would be to add the line

window.add(east_pane, BorderLayout.EAST); 

on the line after the declaration of east_pane.

A better option, however, would be to make your buildEast() and buildNorth() methods return JPanel objects and then integrate them into the JFrame inside the GUI1 method like so (note that we can now also scope the window object more narrowly):

public GUI1()
{
    //Set name and dimensions of window
    JFrame window = new JFrame("Program 2");
    window.setBounds(187, 218, 700, 450);

    window.add(buildEast(), BorderLayout.EAST);
    window.add(buildNorth(), BorderLayout.NORTH);

    window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    window.setVisible(true); //default is invisible window
}
public JPanel buildEast()
{
    //Create and add panel to east of window
    JPanel east_pane = new JPanel();
    //Add a title for the slider
    JLabel slideLabel = new JLabel ("Change Year");
    east_pane.add(slideLabel);


    final int YEAR_MIN = 1990;
    final int YEAR_MAX = 2019;
    int selectYear = 2000;
    JSlider yearSlide = new JSlider(JSlider.VERTICAL, YEAR_MIN, YEAR_MAX, selectYear);

    //Setup for slide labels
    yearSlide.setMajorTickSpacing(4);
    yearSlide.setMinorTickSpacing(2);
    yearSlide.setPaintTicks(true);
    yearSlide.setPaintLabels(true);

    east_pane.add(yearSlide);
    return east_pane;
}

Going this route will make your UI much easier to deal with as it grows in size.

The reason Swing works this way (and by extension, the reason you didn't get any errors but were able to use those print statements) is that Swing allows you to dynamically swap out UI elements on the fly. While this may be beyond the scope of your assignment, consider a tabbed application. It may not be (and likely isn't) desirable to destroy a tab each time it becomes hidden, just to have to re-create it moments later (as this takes time, it may contain data or processing logic, etc.). And, obviously, you wouldn't want it to be visible all the time. The answer then is to simply not display that tab until you want to do so.