为什么我的JScrollPane不显示我的JPanel？

Question

I have a JPanel and I want it to be able to be scrolled, so I am adding it to a JScrollPane, but my JScrollPane doesn't show my Panel completely. just a very small corner of it is shown and it doesn't scrollit either. But when I only add the LPanel to my JFrame, it is shown correctly! Can someone tell me what am I doing wrong here?

Info about my code:

• LPanel is extended from JPanel and I have override the paint. but as I said, the paint works fine! it has it's own width and height that are 2^k.
• k is the number of rows in my panel.

This is my code:

public class Graphics extends JFrame {
  LPanel panel;
  int k;
  public Graphics(int k1) {
    setLayout(null);
    this.k = k1;
    panel = new LPanel(k);
    panel.setSize(panel.width * 30, panel.height * 30);
    new CoveringwithLs(k, panel);
    JScrollPane pane = new JScrollPane(panel);
    pane.setLocation(20, 20);
    if (k < 4) {
      pane.setSize(panel.width * 30, panel.height * 30);
      this.setSize(panel.width * 30 + 80, panel.height * 30 + 80);
    } else {
      pane.setSize(panel.width * 30, 600);
      this.setSize(panel.width * 30 + 80, 600);
    }
    getContentPane().add(pane);
    this.setDefaultCloseOperation(EXIT_ON_CLOSE);
    setVisible(true);
  }
  public static void main(String[] args) {
    new Graphics(3);
  }
}

Can anyone help me?

Answer 1

I cant add an example,

Of course you can. You already have. Now what you need to do is follow the advice given and use layout managers instead of a null layout.

LPanel is extended from JPanel and I have override the paint.

Custom painting is done by overriding the paintComponent() method, not the paint() method.

it has it's own width and height that are 2^k.

You need to override the getPreferredSize() method of your panel to return the dimension. Then the scrollpane will function correctly.

Answer 2

Have you added the scrolls to the ScrollPane. eg

 JScrollPane pane = new JScrollPane(panel, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);

Answer 3

You need to set the preferred size for your JPanel, otherwise the JScrollPane does not know how big is the component it needs to support with its scroll bars. setSize has a different purpose and in this cas is not enough.