Step 5
Overview
The changes for this step are mostly in the GraphicsApplet class.
There, you will write a helper method for creating slider controls,
add instance variables for x and y sliders, add the sliders to the control
panel, and add two getter methods for the slider values.
Finally, in the DrawPanel class, you will add two statements to the
updateDrawing() method to get the x and y values from the
controller.
The following five sections describe these changes in greater detail.
At the end of each section, you can recompile the affected class to check
for errors.
A Helper Method for Creating Slider Controls
This helper method is similar to the createMethodControl() and
createStringControl() helper methods.
One important difference is that it will eventually be used to create 8
sliders, so it needs parameters so that you can specify their differences.
Thus the form of the method is the following.
JSlider createSlider(String ttl, int min, int max, int strt) {
JSlider control = new JSlider(min, max, strt);
inititialization statements
}
The min, max, and strt parameters specify the
minimum, maximum, and starting slider values.
As you can see, they are just passed on to the JSlider constructor.
The ttl parameter is the title that will be placed on a border
around the slider.
For the initialization statements, you need statements to set
the preferred and maximum sizes of control to 160 by 40, set its
border with a titled border with title ttl, and add a listener.
The listener is a ChangeListener.
To use this class, you will need to add the following import.
import javax.swing.event.*;
A ChangeListener has a stateChanged() method with a ChangeEvent
parameter.
The only statement in it sends an updateDrawing() message to the
draw panel instance variable.
Instance Variables for X and Y Sliders
You need to add two JSlider instance variables, xControl and
yControl to the Graphics applet class.
They are are initialized by invoking the createSlider() helper
methods.
The declarations for these variables are similar to the ones for the
methodControl and stringControl variables.
One difference is that you need to specify four parameters for the helper
method: the title for the slider, its minimum and maximum values, and its
starting value.
Use the values from the following table.
| Variable | Name | Min Value | Max Value | Init Value
|
|---|
| xControl | X | 0 | 200 | 100
|
|---|
| yControl | Y | 0 | 200 | 100
|
|---|
The slider variable declarations should be placed before the declaration
for the drawPanel variable.
Add the Sliders to the Control Panel
The createControlPanel() helper method constructs the control
panel.
You need to add two more statements there to add in the xControl
and the yControl.
These statements are similar to the ones for the methodControl and
stringControl.
Getter Methods for the Slider Values
Now you need two methods, getX() and getY() that the draw
panel ca use to determine where to place its drawing.
These methods are similar to the getMethod() and
getString() methods.
One difference is that they have an int return value instead of a
String.
The other main difference is that they send a getValue() message
to the appropriate control instead of a getText() message.
Changes in the DrawPanel Class
In the DrawPanel class, you need to add two statements to the
updateDrawing() method to get the x and y values from the
controller.
These statements should invoke the getX() and getY()
getter methods that you just wrote, assigning their values to the
DrawPanel x and y instance variables.
The statements are similar to the ones you already have for the
string and method instance variables.
That completes this step.
After you have recompiled the classes, run your applet.
It should look like the following demonstration applet.
When you drag the slider knobs, the displayed text should move horizontally
for the X control, vertically for the Y control.
Demonstration Applet