使用JPanel.paintComponent（）绘制两个正方形-错误的尺寸

Question

I am trying to paint two squares of defined size, one located next to another in a row, using paintComponent() method of JPanel. Here what I tried so far:

1. BorderLayout

I tried to draw each square in a separate subclass of JPanel, and then add these JPanels to a JFrame.

Result : squares are squashed to the opposite sides of JPanel: height is as expected but width is minimal.

JFrame frame = new JFrame();

GreenPanel greenPanel = new GreenPanel();
frame.getContentPane().add(BorderLayout.WEST, greenPanel);
BluePanel bluePanel = new BluePanel();
frame.getContentPane().add(BorderLayout.EAST, bluePanel);

frame.setSize(500, 350);
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

class GreenPanel extends JPanel {         
    public void paintComponent(Graphics g) {
        g.setColor(Color.GREEN);
        g.fillRect(0, 0, 100, 100);
    }
}

class BluePanel extends JPanel {         
    public void paintComponent(Graphics g) {
        g.setColor(Color.BLUE);
        g.fillRect(0, 0, 100, 100);
    }
}

2. FlowLayout

I tried to add my "children" JPanels with painted squares to a "parent" JPane and then call JFrame.setContentPane(JPanel).

Result: squares are painted in top-center region as tiny squares.

JFrame frame = new JFrame();
JPanel outerPanel = new JPanel();
frame.setContentPane(outerPanel);

GreenPanel greenPanel = new GreenPanel();
outerPanel.add(greenPanel);
BluePanel bluePanel = new BluePanel();
outerPanel.add(bluePanel);

frame.setSize(500, 350);
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

class GreenPanel extends JPanel {         
    public void paintComponent(Graphics g) {
        g.setColor(Color.GREEN);
        g.fillRect(0, 0, 100, 100);
    }
}

class BluePanel extends JPanel {         
    public void paintComponent(Graphics g) {
        g.setColor(Color.BLUE);
        g.fillRect(0, 0, 100, 100);
    }
}

What am I doing wrong? Is there any way I can force layout managers to respect the size of the squares painted with paintComponent() ?

Answer 1

Make the following changes. See comments for explanations.

  JFrame frame = new JFrame();
  //add layout manager. You can achieve the desired layout 
  //with GridLayout, Box layout and others 
  frame.getContentPane().setLayout(new GridLayout(1,2));
  GreenPanel greenPanel = new GreenPanel();
  //set preferred size to the panel 
  greenPanel.setPreferredSize(new Dimension(100,100));
  frame.add(greenPanel);
  BluePanel bluePanel = new BluePanel();
  //set preferred size to the panel 
  bluePanel.setPreferredSize(new Dimension(100,100));
  frame.getContentPane().add(bluePanel);

  //let the frame adapt to the panels size 
  //frame.setSize(500, 350); 
  frame.validate();
  frame.pack();
  frame.setVisible(true);
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

Don't hesitate to ask for clarifications as needed.

(BTW overriding paintComponent(Graphics g) is not really needed. You could simply set preferred size and color to each JPanel )