Why does GridBagLayout center my components instead of putting it in the corner?

Question

So far I managed to avoid using the GridBagLayout (by hand code) as much as possible, but I could not avoid it this time and I am reading the SUN's tutorial GridBagLayout So far it is not going well. I think I am missunderstanding something.
For example I try the following code (similar to the one in SUN's post):

public class MainFrame extends JFrame { 


    public static void main(String args[]) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    MainFrame frame = new MainFrame();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the frame
     */
    public MainFrame() {
        super();
        setBounds(100, 100, 500, 375);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        Container mainContainer = getContentPane();

        mainContainer.setLayout(new GridBagLayout());       

        //add label
        JLabel someLabel = new JLabel("Label 1:");
        GridBagConstraints constraints = new GridBagConstraints();

        constraints.gridx = 0;
        constraints.gridy = 0;
        //constraints.anchor = GridBagConstraints.FIRST_LINE_START;
        //constraints.weightx = 0.5;
        mainContainer.add(someLabel, constraints);      

        JTextField someText = new JTextField(30);

        constraints = new GridBagConstraints();

        constraints.gridx = 1;
        constraints.gridy = 0;
        constraints.weightx = 0.5;
        mainContainer.add(someText, constraints);

        //
    }

}

I get the label and the textfield one next to the other in the center of the frame.
But I was expecting that they would show up in the top left corner since the gridx and gridy is 0 for the label.
Even if I set constraints.anchor = GridBagConstraints.FIRST_LINE_START; still the same result.
Am I wrong here?
From the SUN's post:

Specify the row and column at the upper left of the component. The leftmost column has address gridx=0 and the top row has address gridy=0.

Havildar

Answer 1

Add constraints.weighty = 1; to the JLabel constraints and constraints.anchor = GridBagConstraints.NORTHWEST; to the TextField constraints.

EDIT:

From Oracle's GridBagLayout guide:

Larger numbers indicate that the component's row or column should get more space. For each column, the weight is related to the highest weightx specified for a component within that column, with each multicolumn component's weight being split somehow between the columns the component is in. Similarly, each row's weight is related to the highest weighty specified for a component within that row. Extra space tends to go toward the rightmost column and bottom row.

Kobi

It works if I also set constraints.weighty=1 to the label as well and anchor to the label FIRST_LINE_START.But what does the value 1 in weighty actually mean? – Havildar

Also, in their code example, this line of code, with comment, comes up: c.weighty = 1.0; //request any extra vertical space – Kobi

For JButton5, I see it.But I do not think this is a good presentation.Anyway why 1 and not 0.8 for example. I do not understand how these numbers are selected – Havildar

In all honesty, I don't either. I use MiGLayout for everything...miglayout.com – Kobi

For the record: "If the resulting layout is smaller horizontally than the area it needs to fill, the extra space is distributed to each column in proportion to its weight. A column that has a weight of zero receives no extra space." The actual numerical values of weightx and weighty only matter in relation to one another. – Pritchard

Answer 2

You need to read further down in the Swing tutorial for the section on weightX/weightY where it states:

Unless you specify at least one non-zero value for weightx or weighty, all the components clump together in the center of their container.

You specified a weightX but not a weightY.

Edit, it's more complicated than I thought. It appears you also need to specify:

constraints.anchor = GridBagConstraints.FIRST_LINE_START;

for both components in addiation to the weighty.

Belden

The quote from SUN says weightx or weighty (so I did not expect it needs both).Anyway I tried weighty=1 for label and weighty=0.5 for text and weighty=0.5 for both but same result – Havildar

weightx if for the horizontal positioning. weighty is for the vertical positioning. I used 1.0 for both as well as FIRST_LINE_START for both and it worked for me. – Belden

Yes now it works.But what does the value 1 in weighty actually mean?I don't get it.Also it seems to me (by the need for anchor) that somehow the grid is already there and not constructed by placing the components on it.Right? – Havildar

GridBagLayout displays components at their preferred sizes. When there is extra space the weightX/Y values are a hint to the layout manager how to allocate this space. EON's tip worked because he added a second row of components and asked for that row to take all the extra space by using a weighty = 1.0. In your case you only have a single row, so you need to specify that one of the components takes all the available extra space. – Belden

Ok, but I used weighty only to the label.So the label gets the extra vertical space (in a way not visible to the user I guess).But why is this also affecting the textfield and also goes up? – Havildar

Plus one for quoting that bit about clumping together in the center, I somehow read over that part at first and was confused by a panel containing a single item that didn't keep to it's position. It's not every day a single component is used inside a panel. – Charinile

Answer 3

You can achieve this by using a trick, add a dummy component after your row and expand it to fill the vertical space. Also you can re-use the constraints, no need to create a new object:

EDIT: ok forget the trick :( The right way is as Deon Botha and BenCole said, I've updated my code using the anchor

Please DON'T accept this answer, accept either Deon's or Ben's

public class MainFrame extends JFrame { 
    public static void main(String args[]) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    MainFrame frame = new MainFrame();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

public MainFrame() {
    super();
    setBounds(100, 100, 500, 375);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    Container mainContainer = getContentPane();
    mainContainer.setLayout(new GridBagLayout());       

    JLabel someLabel = new JLabel("Label 1:");
    JTextField someText = new JTextField(30);

    GridBagConstraints constraints = new GridBagConstraints();
    constraints.anchor = GridBagConstraints.FIRST_LINE_START;

    constraints.gridx = 0;
    constraints.gridy = 0;    
    constraints.weightx = 1.0;
    mainContainer.add(someLabel, constraints);      

    constraints.gridx = 1;                       
    constraints.weightx = 1.0;
    constraints.weighty = 1.0;        
    mainContainer.add(someText, constraints);                       
}
}

Uzbek

But why do I need this dummy component? Should the components be placed in the upper left corner by the layout manager?What is the meaning of gridx=gridy=0 then? – Havildar

You're right, it 'should work' but the GridBayLayout is pure evil, well actually it's very good ... just tricky, I'm not the purist, if it works with a dummy label then why not use it? – Uzbek

Alternatively you could set constraints.weighty = 1.0; constraints.anchor = GridBagConstraints.FIRST_LINE_START; – Cheltenham

Answer 4

I may not be answering your question directly, but trust me you should do your trial and erros on the layouts with an IDE. I personally suggests Netbeans. There you can drag and drop and then take a look over the properties. At first you would have some default values given in the property inspector and hence less auto generated code for that. But then, once you start playing with the layouts, you can see the code and get a nice understanding of knowing what you do how you do.

Masthead

Answer 5

This worked for me:

public class NewJFrame extends javax.swing.JFrame {

    public NewJFrame() {
        initComponents();
    }

    @SuppressWarnings("unchecked")
    private void initComponents() {
        java.awt.GridBagConstraints gridBagConstraints;

        jPanel2 = new javax.swing.JPanel();
        jComboBox3 = new javax.swing.JComboBox();
        jComboBox4 = new javax.swing.JComboBox();
        jComboBox5 = new javax.swing.JComboBox();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setBackground(new java.awt.Color(255, 204, 51));
        setMinimumSize(new java.awt.Dimension(800, 600));
        getContentPane().setLayout(null);

        jPanel2.setLayout(new java.awt.GridBagLayout());

        jComboBox3.setModel(new javax.swing.DefaultComboBoxModel(new String[] { "Item 1", "Item 2", "Item 3", "Item 4" }));
        gridBagConstraints = new java.awt.GridBagConstraints();
        gridBagConstraints.gridx = 0;
        gridBagConstraints.gridy = 0;
        gridBagConstraints.fill = java.awt.GridBagConstraints.HORIZONTAL;
        gridBagConstraints.anchor = java.awt.GridBagConstraints.NORTHWEST;
        gridBagConstraints.weightx = 1.0;
        jPanel2.add(jComboBox3, gridBagConstraints);

        jComboBox4.setModel(new javax.swing.DefaultComboBoxModel(new String[] { "Item 1", "Item 2", "Item 3", "Item 4" }));
        gridBagConstraints = new java.awt.GridBagConstraints();
        gridBagConstraints.gridx = 0;
        gridBagConstraints.gridy = 1;
        gridBagConstraints.fill = java.awt.GridBagConstraints.HORIZONTAL;
        gridBagConstraints.anchor = java.awt.GridBagConstraints.NORTHWEST;
        gridBagConstraints.weightx = 1.0;
        jPanel2.add(jComboBox4, gridBagConstraints);

        jComboBox5.setModel(new javax.swing.DefaultComboBoxModel(new String[] { "Item 1", "Item 2", "Item 3", "Item 4" }));
        gridBagConstraints = new java.awt.GridBagConstraints();
        gridBagConstraints.gridx = 0;
        gridBagConstraints.gridy = 2;
        gridBagConstraints.fill = java.awt.GridBagConstraints.HORIZONTAL;
        gridBagConstraints.anchor = java.awt.GridBagConstraints.NORTHWEST;
        gridBagConstraints.weightx = 1.0;
        gridBagConstraints.weighty = 1.0;
        jPanel2.add(jComboBox5, gridBagConstraints);

        getContentPane().add(jPanel2);
        jPanel2.setBounds(30, 40, 150, 260);

        pack();
    }

    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new NewJFrame().setVisible(true);
            }
        });
    }

    private javax.swing.JComboBox jComboBox3;
    private javax.swing.JComboBox jComboBox4;
    private javax.swing.JComboBox jComboBox5;
    private javax.swing.JPanel jPanel2;
}

Piwowar