How does Java applet use the item selected in a JComboBox

Question

The goal of the current program is to create a line, of varying thickness, between two points in a window frame. So far, so good.

Next, I would like the program to recognize that the user has made a selection from the JComboBox.

This post, and the code, have been updated. (a) the itemStateChanged method was removed. It was not executed, and so, did not need to be in the program. (b) the actionPerformed method was updated to update whenever any of the objects were modified. (c) the color choice was implemented in perhaps one of the ugliest switch/case statements ever. (there must be a better way). And (d) I implemented the suggestion from Itamar Green, regarding the definition of the JComboBox. Thank you.

What don't I know? Notes: Java 8.111. O/S: Windows 8.1. IDE: Ecilpse Java EE 4.6.0

All responses towards improving the code or the question are gratefully accepted.

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

Thank you for your response. I am closer to a solution. The change you described did not solve the problem. I am closer because the program doesn't crash. (a) removed all references to itemStateChanged. The method did not execute, and so, was removed. (b) I altered the actionPerformed substantially. I will attempt to edit the post to reflect the change.... — johniccp, Nov 04 '16 at 20:44

score 0 · Answer 1 · answered Nov 04 '16 at 13:49

Simply put this

myColour = new JComboBox<String>(colourString);

instead of this

JComboBox<String> myColour = new JComboBox<String>(colourString);

in your init method. The problem is the difference between the myColour defined here (in member section)

JComboBox<String> myColour;

and the one created in the init(). You are initializing the one in the Init but not the one in the member section, so when you are trying to use myColour in actionPreformed, Java is trying to call methods from a reference with no object.

How does Java applet use the item selected in a JComboBox

1 Answers1