JPanel & components change position automatically

问题:

Hi I am working on swing application but I am facing one problem.

When I first run application JPanel is positioned at proper position where I decided to set with components inside. But problem occur when

I minimize & again maximize frame window jpanel automatically changes it's position.

Below images shows that difference

As we can see on second image components changes it's position automatically.

For this I written below code,

jpanel_addPurchase = new JPanel();
jpanel_addPurchase.setLayout(null);
jpanel_addPurchase.setBounds(400, 0, 500, 500);
jpanel_addPurchase.setBackground(Color.white);
JLabel lbl_title = new JLabel("Purchase Form");
lbl_title.setBounds(90, 20, 100, 100);
jpanel_addPurchase.add(lbl_title);

And added this panel in frame using,

setContentPane(getJPanel());

Where I am going wrong?

回答1:

I think the original (non-broken) layout looks quirky and would make it difficult for the end user to follow the rows and labels/fields. I suggest instead using a GroupLayout to right align the labels, and left align the fields that contain the values. Like this:

import java.awt.*;
import java.util.HashMap;
import java.util.Map;
import javax.swing.*;

class TwoColumnLayoutWithHeader {

    /**
     * Provides a JPanel with two columns (labels & fields) laid out using
     * GroupLayout. The arrays must be of equal size.
     *
     * Typical fields would be single line textual/input components such as
     * JTextField, JPasswordField, JFormattedTextField, JSpinner, JComboBox,
     * JCheckBox.. & the multi-line components wrapped in a JScrollPane -
     * JTextArea or (at a stretch) JList or JTable.
     *
     * @param labels The first column contains labels.
     * @param fields The last column contains fields.
     * @param addMnemonics Add mnemonic by next available letter in label text.
     * @return JComponent A JPanel with two columns of the components provided.
     */
    public static JComponent getTwoColumnLayout(
            JLabel[] labels,
            JComponent[] fields,
            boolean addMnemonics) {
        if (labels.length != fields.length) {
            String s = labels.length + " labels supplied for "
                    + fields.length + " fields!";
            throw new IllegalArgumentException(s);
        }
        JComponent panel = new JPanel();
        GroupLayout layout = new GroupLayout(panel);
        panel.setLayout(layout);
        // Turn on automatically adding gaps between components
        layout.setAutoCreateGaps(true);
        // Create a sequential group for the horizontal axis.
        GroupLayout.SequentialGroup hGroup = layout.createSequentialGroup();
        GroupLayout.Group yLabelGroup = layout.createParallelGroup(GroupLayout.Alignment.TRAILING);
        hGroup.addGroup(yLabelGroup);
        GroupLayout.Group yFieldGroup = layout.createParallelGroup();
        hGroup.addGroup(yFieldGroup);
        layout.setHorizontalGroup(hGroup);
        // Create a sequential group for the vertical axis.
        GroupLayout.SequentialGroup vGroup = layout.createSequentialGroup();
        layout.setVerticalGroup(vGroup);

        int p = GroupLayout.PREFERRED_SIZE;
        // add the components to the groups
        for (JLabel label : labels) {
            yLabelGroup.addComponent(label);
        }
        for (Component field : fields) {
            yFieldGroup.addComponent(field, p, p, p);
        }
        for (int ii = 0; ii < labels.length; ii++) {
            vGroup.addGroup(layout.createParallelGroup().
                    addComponent(labels[ii]).
                    addComponent(fields[ii], p, p, p));
        }

        if (addMnemonics) {
            addMnemonics(labels, fields);
        }

        return panel;
    }

    private final static void addMnemonics(
            JLabel[] labels,
            JComponent[] fields) {
        Map<Character, Object> m = new HashMap<Character, Object>();
        for (int ii = 0; ii < labels.length; ii++) {
            labels[ii].setLabelFor(fields[ii]);
            String lwr = labels[ii].getText().toLowerCase();
            for (int jj = 0; jj < lwr.length(); jj++) {
                char ch = lwr.charAt(jj);
                if (m.get(ch) == null && Character.isLetterOrDigit(ch)) {
                    m.put(ch, ch);
                    labels[ii].setDisplayedMnemonic(ch);
                    break;
                }
            }
        }
    }

    /**
     * Provides a JPanel with two columns (labels & fields) laid out using
     * GroupLayout. The arrays must be of equal size.
     *
     * @param labelStrings Strings that will be used for labels.
     * @param fields The corresponding fields.
     * @return JComponent A JPanel with two columns of the components provided.
     */
    public static JComponent getTwoColumnLayout(
            String[] labelStrings,
            JComponent[] fields) {
        JLabel[] labels = new JLabel[labelStrings.length];
        for (int ii = 0; ii < labels.length; ii++) {
            labels[ii] = new JLabel(labelStrings[ii]);
        }
        return getTwoColumnLayout(labels, fields);
    }

    /**
     * Provides a JPanel with two columns (labels & fields) laid out using
     * GroupLayout. The arrays must be of equal size.
     *
     * @param labels The first column contains labels.
     * @param fields The last column contains fields.
     * @return JComponent A JPanel with two columns of the components provided.
     */
    public static JComponent getTwoColumnLayout(
            JLabel[] labels,
            JComponent[] fields) {
        return getTwoColumnLayout(labels, fields, true);
    }

    public static String getProperty(String name) {
        return name + ": \t"
                + System.getProperty(name)
                + System.getProperty("line.separator");
    }

    public static void main(String[] args) {
        Runnable r = new Runnable() {

            @Override
            public void run() {
                JComponent[] components = {
                    new JTextField(15),
                    new JTextField(10),
                    new JTextField(8),
                    new JSpinner(new SpinnerNumberModel(1,0,10,1)),
                    new JSpinner(new SpinnerNumberModel(9.95,0d,100d,.01)),
                    new JSpinner(new SpinnerNumberModel(9.95,0d,1000d,.01)),
                    new JSpinner(new SpinnerNumberModel(9.95,0d,100d,.01)),
                    new JSpinner(new SpinnerNumberModel(9.95,0d,1000d,.01)),
                    new JSpinner(new SpinnerNumberModel(9.95,0d,100d,.01)),
                    new JSpinner(new SpinnerNumberModel(9.95,0d,1000d,.01))
                };

                String[] labels = {
                    "Product Name:",
                    "Product Unit Name:",
                    "Purchase Date:",
                    "Quantity:",
                    "Price Per Unit:",
                    "Total Price:",
                    "Discount:",
                    "Total:",
                    "VAT:",
                    "Grand Total:"
                };

                JComponent labelsAndFields = getTwoColumnLayout(labels,components);
                JComponent orderForm = new JPanel(new BorderLayout(5,5));
                orderForm.add(new JLabel("Purchase Form", SwingConstants.CENTER),
                    BorderLayout.PAGE_START);
                orderForm.add(labelsAndFields, BorderLayout.CENTER);

                JOptionPane.showMessageDialog(null, orderForm);
            }
        };
        // Swing GUIs should be created and updated on the EDT
        // http://docs.oracle.com/javase/tutorial/uiswing/concurrency/initial.html
        SwingUtilities.invokeLater(r);
    }
}