jpanel not displaying well with jframe set to grid

问题:

the program below is to position a jpanel at the top left conner of jframe with gridbaglayout but instead a very small box is displayed in center of jframe. when I set the layout of jframe to null, the jpanel displays fine. can someone tell me why the jpanel is compressed to the center of frame with gridbaglayout? i really need to use gridbag. please help

import java.awt.*;
import javax.swing.*; //swing package

public class Major {

    //defining the constructor
    public Major() {
        JFrame maFrame = new JFrame("The main screen"); //creating main Jframe
        JPanel headPanel = new JPanel(); //creating the header panel
        maFrame.setSize(900, 700); //setting size
        maFrame.setBackground(Color.LIGHT_GRAY); //setting color of frame
        Container container = maFrame.getContentPane();
        container.setLayout(new GridBagLayout()); //setting layout of main frame
        GridBagConstraints cns = new GridBagConstraints(); //creating constraint
        cns.gridx = 0;
        cns.gridy = 0;
        maFrame.setLocationRelativeTo(null); //centering frame
        headPanel.setBackground(Color.WHITE);
        headPanel.setSize(200, 150);
        container.add(headPanel, cns);
        maFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); //setting the default close operation of JFrame
        maFrame.setVisible(true); //making the frame visible
    }

//defining the main method
    public static void main(String[] args) {
        new Major(); //instantiating the class
    }
}

回答1:

Seems like you forgot to provide, weightx and weighty constraints to your GridBagConstraints, As you provide them, you will see your JPanel.

Here I had modified your code with those Constraints.

And never use this line, headPanel.setSize(200, 150);, as I had commented it out, since the constraints I had mentioned will sort this out for you.

Adding a new Code with image :

 import java.awt.*;
 import javax.swing.*; //swing package

 public class Major 
 {


    //defining the constructor
    public Major() 
    {

        JFrame maFrame = new JFrame("The main screen"); //creating main Jframe
        JPanel headPanel = new JPanel(); //creating the header panel
        maFrame.setBackground(Color.LIGHT_GRAY); //setting color of frame
        Container container = maFrame.getContentPane();
        container.setLayout(new GridBagLayout()); //setting layout of main frame
        GridBagConstraints cns = new GridBagConstraints(); //creating constraint
        cns.gridx = 0;
        cns.gridy = 0;
        //cns.gridwidth = 3;
        //cns.gridheight = 4;
        cns.weightx = 0.3;
        cns.weighty = 0.7;
        cns.anchor = GridBagConstraints.FIRST_LINE_START;
        cns.fill = GridBagConstraints.BOTH;
        maFrame.setLocationRelativeTo(null); //centering frame
        headPanel.setBackground(Color.WHITE);
        container.add(headPanel, cns);

        JPanel panel = new JPanel();
        panel.setBackground(Color.BLUE);
        cns.gridx = 1;
        cns.gridy = 0;
        //cns.gridwidth = 7;
        //cns.gridheight = 4;
        cns.weightx = 0.7;
        cns.weighty = 0.7;
        cns.anchor = GridBagConstraints.PAGE_START;
        cns.fill = GridBagConstraints.BOTH;
        container.add(panel, cns);

        JPanel panel1 = new JPanel();
        panel1.setBackground(Color.DARK_GRAY);
        cns.gridx = 0;
        cns.gridy = 1;
        cns.gridwidth = 2;
        //cns.gridheight = 4;
        cns.weightx = 1.0;
        cns.weighty = 0.3;
        cns.anchor = GridBagConstraints.LAST_LINE_START;
        cns.fill = GridBagConstraints.BOTH;
        container.add(panel1, cns);     

        //JButton button = new JButton("BUTTON");
        //headPanel.add(button);

        maFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); //setting the default close operation of JFrame
        maFrame.pack();
        maFrame.setVisible(true); //making the frame visible
    }

    //defining the main method
    public static void main(String[] args) 
    {
        Runnable runnable = new Runnable()
        {
            public void run()
            {
                new Major(); //instantiating the class
            }
        };
        SwingUtilities.invokeLater(runnable);       
    }
}

here is the output :

回答2:

You must set the weightx and weighty of at least one GridBagConstraint to some value greater than 0.0!

The weight attributes are are used to indicate what happens with extra space if the whole layout is smaller than the available space. If all weights (for one direction) are zero, the default value, the whole layout is centered. If at least one weight is greater than zero, the extra space is distributed to the columns or rows in proportion to its weight, so the layout will occupy all available space.