组件未按预期添加到框架中

Question

i was studying event handling and performed the following:

• Created a JFrame without any component in it

• i overridden the keyPressed() method in such a way that whenever a key is pressed from the keyboard,A button should appear in the frame(by using add() and then calling repaint()). Now the thing i want to ask is that at the time of key press from the keyboard,nothing was being added to the frame,however after pressing the key when i resized the frame WINDOW,the button came out from nowhere in the frame.... what's happening?

    import java.awt.*; import java.awt.event.*; import javax.swing.*; class MyFrame extends JFrame implements KeyListener { private JButton bt=new JButton(); MyFrame() { addKeyListener(this); } public void keyPressed(KeyEvent ke) { this.add(bt); repaint(); } public void keyTyped(KeyEvent ke) { } public void keyReleased(KeyEvent ke) { } } public class MyClass /*class containing the main method*/ { public static void main(String args[]) { MyFrame frm=new MyFrame(); frm.setVisible(true); frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); } } 

Answer 1

You need to call revalidate after adding a component to the JFrame

this.add(bt);
revalidate();
repaint();

revalidate revalidates the component hierarchy to account for any new components that may have been added.

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KeyListeners don't work well for Swing applications as KeyEvents require focus to work. This is why in Swing it is better to use Key Bindings which allow you to map an Action to a KeyStroke even when a component doesn't have focus.

Answer 2

,nothing was being added to the frame,

The component was added to the frame. The problem is it has a size of (0, 0) so there is nothing to paint

however after pressing the key when i resized the frame WINDOW,the button came out from nowwhere in the frame

The layout manager gets invoked and the component is given a size and location based on the rules of the layout manager. So now you see the component.

When you add a component to a visible GUI the code is:

panel.add(...);
panel.revalidate();
panel.repaint();