| | | More Panels and GUI Components |
Now let's try to make our drawing program even better. It's inconvenient having to select a geometric type and then click in the window to create an object of that type. Why not just include buttons for the three kinds of objects.
The Swing class JButton represents buttons, and the constructor takes the button label as a parameter. JButton takes an ActionListener, just like JComboBox. Thus we must write:
button.addActionListener(this);
while the class must implement ActionListener. The method associated with ActionListener is
public void actionPerformed(ActionEvent evt) {...}
Unfortunately just adding three buttons to the bottom panel makes it so crowded that the four different items don't all show up. We can try to fix that by using the GridLayout to lay out the panel so that each item takes the same amount of space.
bottomPanel.setLayout(new GridLayout(1,4));
Unfortunately that makes them too crowded so the labels of the buttons aren't readable. See BadButtonPanelDrawing.
A better solution is to add a separate panel to hold the three buttons and to place that panel above the slot for the color JComboBox. See ButtonPanelDrawing.
Suppose we want to be able to select any color at all for the geometric objects. Thus we need to make a color out of three components, each of which is in the range from 0 to 255. We can accomplish this by adding three sliders to our window.
For simplicity, we create a simpler application that just has a filled rectangle filling the canvas that will be set to be the color set by the three sliders. JSliders are similar to JComboBoxes and JButtons. The constructor is written in the form:
redSlider = new JSlider(orientation,MIN_VALUE, MAX_VALUE, STARTING_VALUE);
where orientation is either JSlider.HORIZONTAL or JSlider.VERTICAL, and the next three integer values provide the smallest and largest values represented by the JSlider and the starting position of the JSlider (somewhere between the min and max values). JSliders use ChangeListeners, so we write
redSlider.addChangeListener(this);
the class implements ChangeListener, and the event-handling method is
public void stateChanged(ChangeEvent evt) {...}
See the ColorSlider example.
Notice that we can obtain the integer value represented by the slider by evaluating redSlider.getValue(), which always returns an int.
We have made this example more readable by adding labels on both sides of each slider. The label on the left indicates what the slider represents, while the label on the right indicates the current value of the slider. Both are created using JLabel. JLabels are normally centered, but we can change the justification by adding a second parameter of JLabel.RIGHT or JLabel.LEFT (the first parameter is the initial text showing on the label). The text on a label can be changed using the setText method of JLabel.
Question: What is the difference between a Text item and a JLabel? (Hint: the difference is where they can occur and how you specify their locations.)
| | | More Panels and GUI Components |