JFrame pack() and requestFocus() is not working
-
21-12-2019 - |
Question
My main issue is with the following piece of code when setting up a JFrame
:
Why the panel doesn't show if I use the
pack()
and how to make it work?Why the first
requestFocusInWindow()
doesn't work and what the principle to use it?Why the default layout manager of
JPanel
doesn't work if I delete thesetLayout()
?
public class SoundGUI extends KeyAdapter{
public static void main(String[] args) {
SoundGUI sGUI = new SoundGUI();
sGUI.setUp();
}
public void setUp () {
JFrame frame = new JFrame ("Key test");
frame.setSize (1000, 300);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible (true);
Panel p = new Panel ();
p.setLayout(new BorderLayout());//why this sentence is necessary FlowLayout doesn't fill the container but rather lets components size to their preferredSizes.
p.addKeyListener (this);
p.requestFocusInWindow();//it's useless here
//requestFocus only works on focusable components that are displayed.
MyDrawPanel dp = new MyDrawPanel();
dp.setBackground(Color.darkGray);
JLabel test = new JLabel("a trial");
JButton t = new JButton("b");
dp.add(t);
dp.add (test);
p.add (dp);
frame.getContentPane().add(p);
p.requestFocusInWindow();
//frame.pack();//why it doesn't work
//frame.setVisible(true);
}
class MyDrawPanel extends JPanel {
public void paintComponent (Graphics g) {
Graphics2D g2 = (Graphics2D) g;
g2.setColor(Color.white);
for (int i = 0; i < 1000; i += 42) {
g2.fill3DRect(i,100 ,20 ,80 ,true);
}
g2.setColor(Color.black);
for (int i = 21; i < 1000; i += 42) {
g2.fill3DRect(i,100 ,20 ,80 ,true);
}
}
}
}
Solution
Suggestions:
- Call
setVisible(true)
after callingpack()
. Makes sense, doesn't it? - The BorderLayout will tell the MyDrawPanel to fill the p container, since the component is being added in a default way (meaning BorderLayout.CENTER), and that's how BorderLayout works.
- FlowLayout doesn't fill the container but rather lets components size to their preferredSizes.
- Don't mix Swing with AWT components. i.e., don't use Panels, but rather use JPanels.
- requestFocus only works on focusable components that are displayed.
- Better to use Key Bindings than KeyListeners.
- Better to avoid setting the sizes of anything if possible.
Based on your new code, your problem is due to you're calling setSize()
. Most layout managers don't respect this but rather the preferred size. If your drawing JPanel needs to be so big, then make it so. For example try:
class MyDrawPanel extends JPanel {
private static final int PREF_W = 1000;
private static final int PREF_H = 300;
public void paintComponent(Graphics g) {
super.paintComponent(g); //!! ******** don't forget this!!! *********
Graphics2D g2 = (Graphics2D) g;
g2.setColor(Color.white);
for (int i = 0; i < 1000; i += 42) {
g2.fill3DRect(i, 100, 20, 80, true);
}
g2.setColor(Color.black);
for (int i = 21; i < 1000; i += 42) {
g2.fill3DRect(i, 100, 20, 80, true);
}
}
// the getPReferredSize will make this JPanel preferentially be this size
@Override
public Dimension getPreferredSize() {
return new Dimension(PREF_W, PREF_H);
}
}
Also note that request focus does work if the component is focusable:
JPanel p = new JPanel(); //!! This should be a JPanel, not a Panel
p.setFocusable(true); //!! This is needed
p.setLayout(new BorderLayout());
p.addKeyListener(this);
p.requestFocusInWindow();
But also note that KeyListeners should be avoided. Use Key Bindings instead.
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