【发布时间】：2016-01-08 10:37:43
【问题描述】：
我正在尝试创建用户可以在 Canvas 中绘制线条的应用程序。用户可以从下拉列表中选择方向并输入线的长度。第一行从 Canvas 的中心开始，下一行从上一行结束的地方开始，依此类推 - 用户可以一条一条地绘制多条线，并且所有线都连接在一起。
我有两个类 - TurtleApplet，它使用程序逻辑创建 GUI 和 Canvas：
public class TurtleApplet extends JApplet implements ActionListener
{
private JComboBox direction;
private JRadioButton activeButton, passiveButton;
private Button drawButton;
private ButtonGroup group;
private TextField pixels;
private Canvas canvas;
private JPanel panel;
private JPanel panelRadio;
private Button quitPr;
public void init()
{
//directions
String[] directionStrings = { "Right", "Left", "Up", "Down"};
direction = new JComboBox(directionStrings);
//direction.setSelectedIndex(4);
//Buttons
activeButton = new JRadioButton("Aktīvs");
passiveButton = new JRadioButton("Neaktīvs");
quitPr = new Button("Iziet");
//Canvas
canvas = new Canvas();
//canvas.setSize(600, 500);
//canvas.setBackground(Color.red);
canvas.setBorder(BorderFactory.createTitledBorder("Turtle drawing"));
//Panels
panel = new JPanel();
panelRadio =new JPanel();
panel.setLayout(new FlowLayout());
panelRadio.setLayout(new FlowLayout());
//actionListener
activeButton.addActionListener(this);
passiveButton.addActionListener(this);
activeButton.setSelected(true);
quitPr.addActionListener(this);
//Add radiobuttons
group = new ButtonGroup();
group.add(activeButton);
group.add(passiveButton);
//Add Buttons to panel
panelRadio.add(activeButton);
panelRadio.add(passiveButton);
//textfield
pixels = new TextField(12);
//Draw button
drawButton = new Button("Zīmēt");
drawButton.addActionListener(this);
direction.addActionListener(this);
panel.add(panelRadio);
panel.add(pixels);
panel.add(direction);
panel.add(drawButton);
panel.add(quitPr);
getContentPane().add(panel,"North");
getContentPane().add(canvas, "Center");
setSize(650,550);
}
public void actionPerformed( ActionEvent e)
{
if ( e.getSource() == activeButton ) {
drawButton.setVisible(true);
pixels.setEditable(true);
} else if (e.getSource() == passiveButton) {
drawButton.setVisible(false);
pixels.setEditable(false);
} else if(e.getSource() == quitPr){
System.exit(0);
}else if(e.getSource() == drawButton){
int y = Integer.parseInt(pixels.getText());
canvas.addPatt(direction.getSelectedIndex(), Integer.parseInt(pixels.getText()));
repaint();
}
//repaint();
}
}
public class Canvas extends JPanel {
private static final int RIGHT=0, LEFT=1, UP=2, DOWN=3;
public static final int WIDTH=600, HEIGHT=500;
private int direction = 0 ;
private int pixels;
//rivate List points;
public Polygon t = new Polygon();
//public Dimension d = getSize();
public int x = WIDTH/2;
public int y = HEIGHT/2;
public Canvas() {
setSize(WIDTH, HEIGHT);
}
public void addPatt(int pat, int lev) {
direction = pat;
pixels = lev;
}
public void paintComponent(Graphics g) {
switch (direction) {
case LEFT:
drawLineLeft(g, pixels);
break;
case RIGHT:
drawLineRight(g, pixels);
break;
case UP:
drawLineUp(g, pixels);
break;
case DOWN:
drawLineDown(g, pixels);
break;
}
}
private void drawLineLeft(Graphics g, int pix){
if(pix > 0){
g.drawLine(x, y, x-10*pix, y);//left
x =x -10*pix;
}
}
private void drawLineUp(Graphics g, int pix){
if(pix > 0){
g.drawLine(x, y, x, y-10*pix);//up
y = y-10*pix;
}
}
private void drawLineRight(Graphics g, int pix){
if(pix > 0){
g.drawLine(x, y, x+10*pix, y);//right
x = x+10*pix;
}
}
private void drawLineDown(Graphics g, int pix){
if(pix > 0){
g.drawLine(x, y, x, y+10*pix);// down
y = y+10*pix;
}
}
}
Applet 可以工作，但问题是在绘制新行时保存以前的行。当用户输入线条的方向和长度并按下按钮时，屏幕上会出现新的线条，但之前的线条会消失。我知道问题出在paintComponent 方法上，但我不知道如何准确更正我的代码以使所有行都可见。有人建议我将点坐标存储在数组中，然后通过在paintComponent 中循环遍历数组来绘制线条，但我不知道如何实现这一点。也许有更好的解决方案？
【问题讨论】：
-
正如我在您的last question 中所说，
case状态就像if-else，您只允许它画一条线。您需要维护一个List的“行”，每次调用paintComponent方法时都可以对其进行迭代 -
非常感谢您的帮助！我更新了我的代码，但是当我在 Canvas 类中声明
private List<Line> lines时，我得到了The type List is not generic; it cannot be parameterized with arguments <Line>。在 GUI 类中单击按钮时如何正确调用升级后的 addPatt 方法？再次感谢您！ -
为什么要编写小程序？如果是老师指定的，请参考Why CS teachers should stop teaching Java applets。
-
不要导入“java.awt.List”导入“java.util.List”
标签： java swing canvas paintcomponent japplet