如何使用GridBagLayout使所有组件出现在JPanel的中心？

Question

I have below code. But I want all the components to appear in the center of the JPanel. I have already spent countless hours on this but failed every time. Can someone please help?

package tg.com.bugtracker;

import java.awt.*;

import javax.swing.*;

@SuppressWarnings("serial")
public class LoginPanel extends JPanel {    
    LoginPanel() {
        GridBagLayout layout = new GridBagLayout();
        setLayout(layout);
        GridBagConstraints constraints = new GridBagConstraints();
        constraints.gridx = 0;
        constraints.gridy = 0;
        constraints.anchor = GridBagConstraints.LAST_LINE_END;
        add(new JLabel("Username"), constraints);
        constraints.gridx = 1;
        constraints.gridy = 0;
        constraints.anchor = GridBagConstraints.LAST_LINE_START;
        JComboBox<String> combobox = new JComboBox<>();
        combobox.setPreferredSize(new Dimension(250, 20));
        add(combobox, constraints);
        constraints.gridx = 0;
        constraints.gridy = 1;
        constraints.anchor = GridBagConstraints.FIRST_LINE_END;
        add(new JLabel("Password"), constraints);
        constraints.gridx = 1;
        constraints.gridy = 1;
        constraints.anchor = GridBagConstraints.FIRST_LINE_END;
        JTextField textfield = new JTextField();
        textfield.setPreferredSize(new Dimension(250, 20));
        add(textfield, constraints);
    }
}

Answer 1

One fairly easy way to center a group of components in an area of a GUI is to lay them out in a panel & create a (or another) panel with GridBagLayout . Then place the panel with the components into the GBL as a single component with no constraint. Job done!

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As an aside, when I created an MCVE (Minimal Complete Verifiable Example) of that code, I dropped an instance of LoginPanel into the CENTER of a BorderLayout and it sure seems to be centered.

General tip

See Should I avoid the use of set(Preferred|Maximum|Minimum)Size methods in Java Swing? (Yes.)

The size of a text field can be suggested by the number of columns and the font size. A combo box will take the size of the content, which comes down to the widest combo item and the renderer used.

Answer 2

Ok, so first of all I quite don't understand your question, I think you want to put them in the center, but I don't know why you're using anchors. The anchors you're using will make the objects appear in the Line End, or in the Line Start, but you want them in the center so you should use:

constraints.anchor = GridBagConstraints.CENTER; //BIG MOTHER OF GOD REDUNDANCE

Which is a redundance because the default value for anchor is Center, and you shouldn't use setBounds, and I don't know why you are setting the size, the grid will always be of the size of the largest object in the column. If what you're trying to do there is fill the grid you should use:

constraints.fill = GridBagConstraints.HORIZONTAL;

Or vertical or Both depending the direction where you want to fill it. So if you want to all be displayed at the Center here's your "corrected code":

package tg.com.bugtracker;

import java.awt.*;

import javax.swing.*;

@SuppressWarnings("serial")
public class LoginPanel extends JPanel {    
    LoginPanel() {
        GridBagLayout layout = new GridBagLayout();
        setLayout(layout);
        GridBagConstraints constraints = new GridBagConstraints(); 
        constraints.gridx = 0;
        constraints.gridy = 0;
        constraints.anchor = GridBagConstraints.CENTER; //Redundance but I leaved it for you to see the change.
        add(new JLabel("Username"), constraints);
        constraints.gridx = 1;
        constraints.gridy = 0;
        JComboBox<String> combobox = new JComboBox<>();
        combobox.setPreferredSize(new Dimension(250, 20));
        add(combobox, constraints);
        constraints.gridx = 0;
        constraints.gridy = 1;
        add(new JLabel("Password"), constraints);
        constraints.gridx = 1;
        constraints.gridy = 1;
        JTextField textfield = new JTextField();
        textfield.setPreferredSize(new Dimension(250, 20));
        add(textfield, constraints);

    }
}

Now all the objects should appear in the center, I recommend you this post here in Stack Overflow about GridBagLayout, is quite interesting and it has common errors and covers almost all about this awesome layout.