調用我的方法，但它不繪制線條

Question

下面是我繪制游泳池的代碼，同時計算其體積。

有問題的方法是paintSwimmingPool（），它在被stateChanged（）監聽器方法調用時工作正常。 但是，問題是我想在啟動程序時繪制矩形。 我在一些控制台消息中添加了幫助調試，它告訴我從createGUI（）方法正確調用該方法。 但為什么paintSwimmingPool（）不會在面板上繪制任何東西？ 對不起，我花了很多時間來解決這個問題，但沒有找到。

提前感謝您的任何指示。

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

你不能這樣繪制，正確的技術是

• 覆蓋JPanel子類中的paintComponent（）方法，改為使用Graphics參數。
• 調用repaint（）來觸發重繪而不是paintSwimmingPool（）方法
• 將參數（例如deepEnd和shallowEnd）移動到字段中並從paintComponent（）方法引用這些參數 - 我們能夠“看到”這些字段的內部類。 理想情況下，這些將在繪圖面板和控制代碼共享的單獨模型中，但內部類可以完成工作。
• 看看自定義繪畫的oracle教程

完全有效的例子

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);

    }

}

Answer 2

在你上畫之后，Swing可能會重新粉刷JPanel。 Swing可以隨時重新繪制組件，無論出於何種原因。 因此，您不應該使用getGraphics 。

相反，您需要創建自己的組件類（擴展現有組件，如JPanel ），並覆蓋paintComponent 。

有關詳細信息，請參閱此問題 。