如何在Java swing中的拆分面板中绘制

Question

Recently, I've met an issue that the paintComponent function is not invoked in the function, and I found that when I use splitpane function, it will disable the paint function, and gives error:

cannot add to layout: unknown constraint: null

I think the paint function may not be added to the right way, below is my code(partly):

Class: test

public class Test extends JFrame{

    public Test() throws IOException{
        //JFrame jf = new JFrame("my frame");
        this.add(new NewPanel(this));

        this.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
        this.setBounds(300,200,1050,600);
        this.setVisible (true);

    }

    public static void main (String[] args) throws IOException{

         Test test   = new Test ();
         test.setTitle("Hello");
          //frame.pack ();
     }
}

Class: NewPanel

public class NewPanel extends JPanel{
    public NewPanel(JFrame frame) throws IOException{
          JTabbedPane jTabbedpane = new JTabbedPane();
          JSplitPane splitPane    = new JSplitPane();
          JPanel p1               = new JPanel();

          p1.setLayout(null);
          p2.setLayout(new FlowLayout());

          splitPane.setOneTouchExpandable(true);
          splitPane.setContinuousLayout(true);
          //splitPane.setPreferredSize(new Dimension (250,500));
          splitPane.setOrientation(JSplitPane.VERTICAL_SPLIT);
          splitPane.setLeftComponent(p1);
          splitPane.setRightComponent (p2);
          splitPane.setDividerSize(3);
          splitPane.setDividerLocation(250);  //balance two panels width

          jTabbedpane.addTab("ABC",p2);
          jTabbedpane.addTab("AB",p3);
          jTabbedpane.addTab("AC",p4);
          jTabbedpane.addTab("BC",p5);

          frame.setContentPane(splitPane);
          frame.add(jTabbedpane);

          }
    }
     public void paintComponent(Graphics g){
        super.paint(g);
        g.setColor(Color.BLUE);
        g.drawLine(303, 90, 303, 200);
        g.drawLine(583, 90, 583, 200);
        g.drawLine(863, 90, 863, 200);
    }
}

When I comment frame.add(jTabbedpane) ,the line could be drawn in the panel, BUT it is only available in one panel, I cannot draw it into another split panel, I don't know why.. And when I uncomment frame.add(jTabbedpane) , it pops up the above mentioned error.

Answer 1

Your UI assembly doesn't make sense. You're calling 'setContentPane' to the splitpane, which is sort-of OK (but unusual), but then you're calling add() to the frame, which tries to then add something else to the contentPane (the JSplitPane). You should either add the JTabbedPane to the SplitPane before adding the splitPane to the JPanel, or set up your layout differently.

//These don't make sense together.
frame.setContentPane(splitPane);
frame.add(jTabbedpane);

Your second question about drawing the blue line is more complicated. You're doing a bunch of crazy stuff - you're creating a NewPanel and trying to add it to the JFrame , but then you're setting the contentPane of the JFrame to a different component later. You need to go through the Swing Tutorial and lay out your UI better.

Answer 2

I think the paint function may not be added to the right way,

 public void paintComponent(Graphics g){
    super.paint(g);

You are overriding paintComponent(...) , so why are you calling super.paint(...) ?

Start by reading the Swing Tutorial for Swing basics. All sections in the tutorial have working examples you can download and test.

So you might start with:

1. How to Use Split Panes - it will show you how to add a split pane to a frame
2. Performing Custom Painting - it will explain how painting works and show how to override the paintComponent(...) method.