为什么GridBagConstraints不起作用？

Question

I have JFrame form which must contain TextField somewhere around center of the form, I am using GridBagLayout to set TextField position on the frame.

The code like this below:

public static JFrame enterFrameDraw(JFrame frame){
    JButton btnEnter = new JButton("Sign in!");
    JLabel loginLabel = new JLabel("Login!");
    GridBagLayout gbl = new GridBagLayout();
    JTextField textFieldLogin = new JTextField();
    textFieldLogin.setBackground(Color.GRAY);
    btnEnter.addActionListener(e -> {
        try{
            Chat.btnEnterHandler(frame);
        } catch (Exception e2){
            e2.printStackTrace();
        }
    });


    frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    frame.setTitle(FRM_TITLE);
    frame.setLocation(FRM_LOC_X, FRM_LOC_Y);
    frame.setSize(FRM_WIDTH, FRM_HEIGHT);
    frame.setLayout(new GridBagLayout());
    GridBagConstraints c = new GridBagConstraints();
    c.anchor = GridBagConstraints.NORTH;
    c.fill = GridBagConstraints.NONE;
    c.gridheight = 1;
    c.gridwidth = GridBagConstraints.REMAINDER;
    c.gridx = GridBagConstraints.RELATIVE;
    c.gridy = GridBagConstraints.RELATIVE;
    c.insets = new Insets(40, 0, 0, 0);
    c.ipadx = 0;
    c.ipady = 0;
    c.weightx = 0.0;
    c.weighty = 0.0;
    gbl.setConstraints(textFieldLogin, c);
    frame.add(textFieldLogin, c);
    frame.setResizable(false);
    frame.setVisible(true);

    return frame;
}

And result seems like: Image

Could the problem lies in the "frame.add()" instead of simple "add()"?

UPDATE:

Method enterFrameDraw() after modification. Still doesn't work.

public static JFrame enterFrameDraw(JFrame frame){
    JButton btnEnter = new JButton("Sign in!");
    JLabel loginLabel = new JLabel("Login!");
    GridBagLayout gbl = new GridBagLayout();
    JTextField textFieldLogin = new JTextField();
    textFieldLogin.setBackground(Color.GRAY);
    btnEnter.addActionListener(e -> {
        try{
            Chat.btnEnterHandler(frame);
        } catch (Exception e2){
            e2.printStackTrace();
        }
    });


    frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    frame.setTitle(FRM_TITLE);
    frame.setLocation(FRM_LOC_X, FRM_LOC_Y);
    frame.setSize(FRM_WIDTH, FRM_HEIGHT);
    frame.setLayout(new GridBagLayout());
    GridBagConstraints c = new GridBagConstraints();
    c.anchor = GridBagConstraints.NORTH;
    c.fill = GridBagConstraints.HORIZONTAL;
    c.gridheight = 1;
    c.gridwidth = GridBagConstraints.REMAINDER;
    c.gridx = GridBagConstraints.RELATIVE;
    c.gridy = GridBagConstraints.RELATIVE;
    c.insets = new Insets(0, 40, 0, 40);
    c.ipadx = 0;
    c.ipady = 0;
    c.weightx = 0.5;
    c.weighty = 0.0;
    gbl.setConstraints(textFieldLogin, c);
    frame.add(textFieldLogin);
    frame.setResizable(false);
    frame.setVisible(true);
    return frame;
}

UPDATE 2: What i want to have on the form. Image: This one

Answer 1

As far as I can tell there are two problems.

1. You need to set your constraints' fill to horizontal:

   c.fill = GridBagConstraints.HORIZONTAL;

2. You need to set the constraints' weightx to a non-zero value:

   c.weightx = 0.5;

Edit 1:

There's 2 ways you can centralize the content:

1. Define the constraints' insets to push it in from the left and right, like so:

   c.insets = new Insets(0, 40, 0, 40);

The value 40 here is just an example. You could change the parameters to whatever suits.

2. You can use javax.swing.Box to create empty grid cells. To keep your content in the center, you can create an empty 'strut' on either side of the middle components:

    c.gridx = 0; c.gridy = 0; c.weightx = 0.5; frame.add(Box.createHorizontalStrut(100), c); c.gridx = 1; frame.add(textFieldLogin, c); c.gridx = 2; frame.add(Box.createHorizontalStrut(100), c); 

Again, the value of 100 here is just an example, you should tweak the parameters to your needs.

It would be worthwhile reading up on things like Struts and Glues in the context of gridbaglayouts, they're really useful in organizing your content the way you want it.

Edit 2:

To add more components below the text field that's in the center, you should use the second approach of adding horizontal struts to the cells in the first row.

The idea is that putting a strut in a cell fills up space that would be used by a component, if there was a component in that cell. So by specifying the width of a horizontal strut in the first row, any following components with the same x value as the strut will share the same width as the strut.

So in this case, your method might look something like this:

public static JFrame enterFrameDraw(JFrame frame){

    frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    frame.setResizable(false);
    frame.setSize(500, 500);
    frame.setLayout(new GridBagLayout());

    GridBagConstraints c = new GridBagConstraints();
    c.fill = GridBagConstraints.HORIZONTAL;

    c.gridx = 0;
    c.gridy = 0;
    c.weightx = 1;
    frame.add(Box.createHorizontalStrut(80), c);

    JTextField textFieldLogin = new JTextField();
    textFieldLogin.setBackground(Color.GRAY);
    c.gridx = 1;
    c.gridy = 0;
    frame.add(textFieldLogin, c);

    c.gridx = 2;
    c.gridy = 0;
    frame.add(Box.createHorizontalStrut(80), c);

    JLabel label = new JLabel("Label");
    c.gridx = 0;
    c.gridy = 1;
    frame.add(label, c);

    frame.setVisible(true);

    return frame;
}

Here a label is added to the second row, and it does not appear in the center.