我的方法被调用，但它不画线

Question

下面是我绘制游泳池并同时计算其体积的代码。

有问题的方法是 paintSwimmingPool()，它在被 stateChanged() 侦听器方法调用时工作正常。但是，问题是我想在启动程序时绘制矩形。我添加了一些控制台消息来帮助调试，它告诉我该方法是从 createGUI() 方法正确调用的。但为什么 paintSwimmingPool() 不在面板上绘制任何东西？抱歉，我花了很多时间来解决这个问题，但无处可去。

提前感谢您的指点。

import java.awt.Color;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Frame;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JSlider;
import javax.swing.JTextField;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;

public class SwimingPoolCalculator implements ChangeListener {

    private JFrame frame;
    private JPanel primaryJPanel, drawingJPanel;
    private JLabel deepJLabel, shallowJLabel;
    private JSlider deepJSlider, shallJSlider;
    private JTextField resultTextField;

    final private int xPos = 20;
    final private int yPos = 20;
    final private int swimmingPoolLength = 200;
    final private int swimmingPoolWidth = 50;

    public static void main(String[] args) {

        SwimingPoolCalculator swimingPoolCalculator = new SwimingPoolCalculator();

        swimingPoolCalculator.createGUI();

    }

    private void createGUI(){

        frame = new JFrame("Swiming Pool Calculator");

        primaryJPanel = new JPanel();
        primaryJPanel.setPreferredSize(new Dimension(250, 500));

        drawingJPanel = new JPanel();
        drawingJPanel.setPreferredSize(new Dimension(250, 300));
        drawingJPanel.setBackground(Color.yellow);

        shallowJLabel = new JLabel("S End");
        deepJLabel = new JLabel("D End");

        shallJSlider = new JSlider(JSlider.HORIZONTAL, 10, 100, 10);
        deepJSlider = new JSlider(JSlider.HORIZONTAL, 50, 200, 100);

        shallJSlider.addChangeListener(this);
        deepJSlider.addChangeListener(this);

        resultTextField = new JTextField(15);

        primaryJPanel.add(drawingJPanel);
        primaryJPanel.add(shallowJLabel);
        primaryJPanel.add(shallJSlider);
        primaryJPanel.add(deepJLabel);
        primaryJPanel.add(deepJSlider);
        primaryJPanel.add(resultTextField);

        frame.getContentPane().add(primaryJPanel);
        frame.pack();
        frame.setVisible(true);

        // why the first call does not draw any line?
        paintSwimmingPool(deepJSlider.getValue(), shallJSlider.getValue());

    }

    private void paintSwimmingPool(int deepEnd, int shallowEnd){

        Graphics drawingBoard = drawingJPanel.getGraphics();

        drawingBoard.setColor(Color.yellow);
        drawingBoard.fillRect(0, 0, 250, 250);
        drawingBoard.setColor(Color.black);

        drawingBoard.drawLine(xPos, yPos, xPos + swimmingPoolLength, yPos);  // the top line
        drawingBoard.drawLine(xPos, yPos, xPos, yPos + deepEnd); // the left line
        drawingBoard.drawLine(xPos + swimmingPoolLength, yPos, xPos + swimmingPoolLength, yPos + shallowEnd); // the right line
        drawingBoard.drawLine(xPos, yPos + deepEnd, xPos + swimmingPoolLength, yPos + shallowEnd); // the bottom line

    }

    private void calculateVolume(double averageDepth){

        int swimmingPoolVolume = (int)(averageDepth * swimmingPoolLength * swimmingPoolWidth);

        resultTextField.setText("Volume: " + swimmingPoolVolume);

    }

    @Override
    public void stateChanged(ChangeEvent e) {

        int shallowEnd = shallJSlider.getValue();
        int deepEnd = deepJSlider.getValue();

        double averageDepth = (shallowEnd + deepEnd) / 2;

        paintSwimmingPool(deepEnd, shallowEnd);

        calculateVolume(averageDepth);

    }

}

Answer 1

Swing 可能在您绘制之后重新绘制 JPanel。 Swing 可以随时重绘组件，无论出于何种原因。因此，您不应使用 getGraphics.

相反，您需要创建自己的组件 class（扩展现有组件，例如 JPanel），并覆盖 paintComponent.

有关详细信息，请参阅 this question。

Answer 2

不能这样画，正确的画法是

重写 JPanel subclass 中的 paintComponent() 方法并改为使用 Graphics 参数。
调用 repaint() 来触发重绘而不是您的 paintSwimmingPool() 方法
将参数，即 deepEnd 和 shallowEnd 移动到字段中，并从您的 paintComponent() 方法中引用它们 - 内部 class 我们能够 'see' 这些字段。理想情况下，这些将位于绘图面板和控制代码共享的单独模型中，但内部 class 完成了工作。
看看oracle tutorial on custom painting

完整的例子

public class SwimingPoolCalculator implements ChangeListener {

    private JFrame frame;
    private JPanel primaryJPanel, drawingJPanel;
    private JLabel deepJLabel, shallowJLabel;
    private JSlider deepJSlider, shallJSlider;
    private JTextField resultTextField;

    final private int xPos = 20;
    final private int yPos = 20;
    final private int swimmingPoolLength = 200;
    final private int swimmingPoolWidth = 50;
    private int deepEnd;   // <== moved to fields
    private int shallowEnd;  // <== moved to fields

    private class InnerDrawingPanel extends JPanel {
        @Override
        protected void paintComponent(Graphics drawingBoard) {
            drawingBoard.setColor(Color.yellow);   // <== drawing code now here
            drawingBoard.fillRect(0, 0, 250, 250);
            drawingBoard.setColor(Color.black);

            drawingBoard.drawLine(xPos, yPos, xPos + swimmingPoolLength, yPos);  // the top line
            drawingBoard.drawLine(xPos, yPos, xPos, yPos + deepEnd); // the left line
            drawingBoard.drawLine(xPos + swimmingPoolLength, yPos, xPos + swimmingPoolLength, yPos + shallowEnd); // the right line
            drawingBoard.drawLine(xPos, yPos + deepEnd, xPos + swimmingPoolLength, yPos + shallowEnd); // the bottom line
        }
    }

    public static void main(String[] args) {

        SwimingPoolCalculator swimingPoolCalculator = new SwimingPoolCalculator();

        swimingPoolCalculator.createGUI();

    }

    private void createGUI(){

        frame = new JFrame("Swiming Pool Calculator");

        primaryJPanel = new JPanel();
        primaryJPanel.setPreferredSize(new Dimension(250, 500));

        drawingJPanel = new InnerDrawingPanel();  // <== use inner class
        drawingJPanel.setPreferredSize(new Dimension(250, 300));
        drawingJPanel.setBackground(Color.yellow);

        shallowJLabel = new JLabel("S End");
        deepJLabel = new JLabel("D End");

        shallJSlider = new JSlider(JSlider.HORIZONTAL, 10, 100, 10);
        deepJSlider = new JSlider(JSlider.HORIZONTAL, 50, 200, 100);

        shallJSlider.addChangeListener(this);
        deepJSlider.addChangeListener(this);

        resultTextField = new JTextField(15);

        primaryJPanel.add(drawingJPanel);
        primaryJPanel.add(shallowJLabel);
        primaryJPanel.add(shallJSlider);
        primaryJPanel.add(deepJLabel);
        primaryJPanel.add(deepJSlider);
        primaryJPanel.add(resultTextField);

        frame.getContentPane().add(primaryJPanel);
        frame.pack();
        frame.setVisible(true);

        // why the first call does not draw any line?
        shallowEnd = deepJSlider.getValue(); // <== now update fields
        deepEnd = shallJSlider.getValue();
        drawingJPanel.repaint();        // <== trigger repaint

    }


    private void calculateVolume(double averageDepth){

        int swimmingPoolVolume = (int)(averageDepth * swimmingPoolLength * swimmingPoolWidth);

        resultTextField.setText("Volume: " + swimmingPoolVolume);

    }

    @Override
    public void stateChanged(ChangeEvent e) {

        shallowEnd = shallJSlider.getValue();  // <== update fields
        deepEnd = deepJSlider.getValue();

        double averageDepth = (shallowEnd + deepEnd) / 2;


        drawingJPanel.repaint();   // <== trigger repaint
        calculateVolume(averageDepth);

    }

}

我的方法被调用，但它不画线

My method is called, but it does not draw the lines

java

swing