Answers to Questions and Exercises: User Interfaces That Swing - Java Tutorial 5.0 英文版

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Trail: Creating a GUI with JFC/Swing
Lesson: Getting Started with Swing

Answers to Questions and Exercises: User Interfaces That Swing

Questions

Use the API documentation to answer these questions.

Question 1: Show the code that creates a label displaying the following text, with the italics and boldface as shown in this screenshot:

Answer 1:
label = new JLabel("<html>The <em>last</em> word is <b>bold</b>.");
Question 2: Use the API documentation or online tutorial, if necessary, to answer the following questions:
Question 2a: Assume that you have a Swing label that tends to have more horizontal space than it needs to display its text. What code would you use to make the text within a label (Jlabel) be centered instead of left-aligned?
Answer 2a: Here are two ways:
      new JLabel(labelText, JLabel.CENTER);
or
      label.setHorizontalAlignment(JLabel.CENTER);
Question 2b: What method do you use to enable and disable such components as Swing buttons?
Answer 2b: The Component setEnabled method.
Question 2c: How do you add a component to the rightmost (east) cell in a container that uses BorderLayout?
Answer 2c: Here's one way:
        container.add(component, BorderLayout.EAST);
Question 3: Is the following code thread-safe? If so, why? If not, what can you do to make it thread-safe?
   JLabel label;

   Container createGUI() {
       ... 
	 //create a JPanel; add components to it, including label
	...
	return panel;
   }

   public static void main(String[] args) {
       JFrame f = new JFrame("A Frame");
       f.setContentPane(createGUI());
       f.pack();
       f.setVisible(true);
       String labelText = findTextFromSomewhere();
       label.setText(labelText);
   }
Answer 3: It's not thread-safe because the main thread updates the label after the label is visible. You can use SwingUtilities.invokeLater to make label.setText be executed on the event-dispatching thread.

Exercises

You'll need to work on these exercises in order, as they build upon each other. Hint: Your answer to question 2 will be useful.

Question 1: Write an application called SwingApp1 that has two buttons and one label, arranged as shown in the following screenshot:

Hint: You can use the content pane's default BorderLayout to manage the buttons and label.
Answer 1: See SwingApp1.
Question 2: Copy SwingApp1.java to SwingApp2.java, and modify SwingApp2 so that the Stop button is initially disabled. Implement and register one or two action listeners so that when the user clicks either button, the clicked button is disabled, the other button is enabled, and the label is updated appropriately. Your application should look like this:

Answer 2: See SwingApp2.
Question 3: Copy SwingApp2.java to SwingApp3.java, and modify SwingApp3 so that its Start button brings up a dialog that looks like this:

If the user chooses the Go ahead button, the Start button should do exactly what it did in SwingApp2. If the user does anything else, the application should update the label to report the cancellation and leave the buttons in their existing state.
Answer 3: See SwingApp3.

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