当前程序的目标是在窗口框架中的两个点之间创建不同厚度的线。到目前为止，非常好。
接下来，我希望程序能够识别用户已从JComboBox中进行选择。
这篇文章和代码已经更新。 （a）删除了itemStateChanged方法。它没有被执行，因此，不需要在程序中。 （b）每当修改任何对象时，都会更新actionPerformed方法以进行更新。 （c）颜色选择可能是有史以来最丑陋的开关/案例陈述之一。 （肯定有更好的办法）。 （d）我实施了Itamar Green关于JComboBox定义的建议。谢谢。
我不知道什么？ 注意：Java 8.111。 O / S：Windows 8.1。 IDE：Ecilpse Java EE 4.6.0
感谢所有对改进代码或问题的回应。
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class ThickPen extends JApplet implements ActionListener
{
private static final long serialVersionUID = 1L;
JLabel xStartLabel = new JLabel("X Start");
JLabel yStartLabel = new JLabel("Y Start");
JLabel xStopLabel = new JLabel("X Stop");
JLabel yStopLabel = new JLabel("Y Stop");
JLabel thickLabel = new JLabel("Thickness");
JComboBox<String> myColour;
String theColour = "black";
TextField xStartField = new TextField(4);
int xStart = 0;
TextField yStartField = new TextField(4);
int yStart = 0;
TextField xStopField = new TextField(4);
int xStop = 0;
TextField yStopField = new TextField(4);
int yStop = 0;
TextField thicknessField = new TextField(4);
int thick = 0;
String[] colourString = {"black","blue","cyan","darkgray","gray","green",
"lightGray","magenta","orange","pink","red","white","yellow"};
int theIndex = 0;
public void init()
{
setSize(550,500);
Container content = getContentPane();
setLayout(new FlowLayout());
xStartField.addActionListener(this);
yStartField.addActionListener(this);
xStopField.addActionListener(this);
yStopField.addActionListener(this);
thicknessField.addActionListener(this);
add(xStartLabel);
add(xStartField);
add(yStartLabel);
add(yStartField);
add(xStopLabel);
add(xStopField);
add(yStopLabel);
add(yStopField);
add(thickLabel);
add(thicknessField);
myColour = new JComboBox<String>(colourString);
// JComboBox<String> myColour = new JComboBox<String>(colourString);
myColour.setSelectedIndex(0); // start with black
myColour.addActionListener(this);
add(myColour);
content.setBackground(Color.white);
content.setForeground(Color.black);
}
public void paint(Graphics g)
{
super.paint(g);
Dimension d = getSize();
int fullWidth = d.width;
int fullHeight = d.height;
int deltaX = 0;
int deltaY = 0;
boolean xAxis = false;
System.out.println("So far Start x y: "+xStart+" "+yStart+" color: "+theColour);
if (xStart < 1 || xStart > fullWidth
|| yStart < 1 || yStart > fullHeight
|| xStop < 1 || xStop > fullWidth
|| yStop < 1 || yStop > fullHeight
|| thick < 1 || thick > fullHeight || thick > fullWidth) {
String outStr = "Start and stop numbers must be within this window frame";
String outStr2 = "Current width: "+fullWidth+" height: "+fullHeight;
g.setColor(Color.white);
g.fillRoundRect(d.width/4, d.height/4, 300, 40, 4, 4);
g.setColor(Color.red);
g.drawString(outStr, d.width/4+10, d.height/4+15);
g.drawString(outStr2, d.width/4+10, d.height/4+30);
g.drawString("The index: "+theIndex, d.width/4, 300);
} else {
g.drawString("", d.width/4, 260);
deltaX = Math.abs(xStart - xStop); // determine absolute delta of two Xs
deltaY = Math.abs(yStart - yStop); // determine absolute delta of two Ys
if (deltaX > deltaY) // make line thickness based on x axis if
xAxis = false; // the x axis has the most 'room'.
else // else, use the y axis.
xAxis = true;
pickAColour(g, theColour);
drawMyLine(g, xStart, yStart, xStop, yStop, thick, xAxis);
}
g.drawString("The index: "+theIndex, d.width/4, 300);
g.drawString("The color "+ theColour, d.width/4, 330);
}
public void drawMyLine(Graphics g, int xStart, int yStart,
int xStop, int yStop, int thick, boolean xAxis)
{
int count = 0;
while (count < thick)
{
g.drawLine(xStart, yStart, xStop, yStop);
count++;
if (xAxis) {
xStart++;
xStop++;
} else {
yStart++;
yStop++;
}
}
}
public void pickAColour(Graphics g, String theColour)
{
switch (theColour) {
case "black" :
g.setColor(Color.black);
break;
case "blue" :
g.setColor(Color.blue);
break;
case "cyan" :
g.setColor(Color.cyan);
break;
case "darkgray" :
g.setColor(Color.darkGray);
break;
case "gray" :
g.setColor(Color.gray);
break;
case "green" :
g.setColor(Color.green);
break;
case "lightGray" :
g.setColor(Color.lightGray);
break;
case "magenta" :
g.setColor(Color.magenta);
break;
case "orange" :
g.setColor(Color.orange);
break;
case "pink" :
g.setColor(Color.pink);
break;
case "red" :
g.setColor(Color.red);
break;
case "white" :
g.setColor(Color.white);
break;
case "yellow" :
g.setColor(Color.yellow);
break;
} // end of case statement
} // end of pickAColour
public void actionPerformed (ActionEvent ae)
{
Object source=ae.getSource();
// xStart
// if (source==xStartField)
// {
try {
xStart=Integer.parseInt(
xStartField.getText());
}
catch (NumberFormatException x) {
xStart= -1;
}
// }
// yStart
// else if (source==yStartField)
// {
try {
yStart=Integer.parseInt(
yStartField.getText());
}
catch (NumberFormatException x) {
yStart= -1;
}
// }
// xStop
// else if (source==xStopField)
// {
try {
xStop=Integer.parseInt(
xStopField.getText());
}
catch (NumberFormatException x) {
xStop= -1;
}
// }
// yStop
// else if (source==yStopField)
// {
try {
yStop=Integer.parseInt(
yStopField.getText());
}
catch (NumberFormatException x) {
yStop= -1;
}
// }
// thickness
// else if (source==thicknessField)
// {
try {
thick=Integer.parseInt(
thicknessField.getText());
}
catch (NumberFormatException x) {
thick= -1;
}
// } else {
if (source==myColour) {
JComboBox<String> cb = (JComboBox<String>)ae.getSource();
// String theColour = (String)cb.getSelectedItem(); ///can;
// Integer theIndex = (int)cb.getSelectedIndex();
theColour = (String)cb.getSelectedItem();
theIndex = (int)cb.getSelectedIndex();
}
// }
repaint();
} // end of ActionEvent
} // end of class
答案 0 :(得分：0)
简单地说这个
myColour = new JComboBox<String>(colourString);
而不是
JComboBox<String> myColour = new JComboBox<String>(colourString);
在您的init方法中。
问题是这里定义的myColour（在成员部分）
JComboBox<String> myColour;
和init()中创建的那个。您正在初始化Init中的那个而不是成员部分中的那个，所以当您尝试在myColour中使用actionPreformed时，Java正试图从没有对象的引用中调用方法。 / p>