Swing : Displaying Text in a Component

we are going to learn how to add a component into a Frame. This component is going to be pretty straight forward that writes out some text onto the frame.

As a first step, we need to know how to add a component to the frame. The code to add a component to a frame looks like below:

Container contentPane = frame.getContentPane();

Component c = . . .;

contentPane.add(c);

When you add a component to a frame, you are actually adding the component to the frame’s content pane.

Or, you can use a short cut. As of Java SE 5.0, you can simply use the call

frame.add(c);

This will in-turn call the content pane’s add method.

In our case, we want to add a single component to the frame on which we will write out our message. To draw on a component, you define a class that extends JComponent and override the paintComponent method in that class.

The paintComponent method takes one parameter of type Graphics. A Graphics object remembers a collection of settings for drawing images and text, such as the font you set or the current color. All drawing in Java must go through a Graphics object. It has methods that draw patterns, images, and text.

Here’s how to make a component onto which you can draw/write stuff:

class MyComponent extends JComponent {
    public void paintComponent(Graphics g)    {
        //code for drawing
    }

}

Each time a window needs to be redrawn, no matter what the reason, the event handler notifies the component. This causes the paintComponent methods of all components to be executed.
Never call the paintComponent method yourself. It is called automatically whenever a part of your application needs to be redrawn, and you should not interfere with this automatic process.

Trivia:
What kind of actions triggers this automatic response? For example, painting occurs because the user increased the size of the window or minimized and then restored the window. If the user popped up another window and it covered an existing window and then made the overlaid window disappear, the application window that was covered is now corrupted and will need to be repainted. (The graphics system does not save the pixels underneath.) And, of course, when the window is displayed for the first time, it needs to process the code that specifies how and where it should draw the initial elements.

As you saw in the code fragment above, the paintComponent method takes a single parameter of type Graphics. Measurement on a Graphics object for screen display is done in pixels. The (0, 0) coordinate denotes the top-left corner of the component on whose surface you are drawing.

Displaying Text in our Component:

Displaying text is considered a special kind of drawing. The Graphics class has a drawString method that has the following syntax:

g.drawString(text, x, y)

In our case, we want to draw the string "Welcome to Java Swings Tutorial!!!" in our original window, roughly one-quarter of the way across and halfway down. Although we don’t yet know how to measure the size of the string, we’ll start the string at coordinates (75, 100). This means the first character in the string will start at a position 75 pixels to the right and 100 pixels down.

Thus, our paintComponent method will look like this:

class TestPaintComponent extends JComponent {

    public void paintComponent(Graphics g)    {
        g.drawString("Welcome to Java Swings Tutorial!!!", MESSAGE_X, MESSAGE_Y);
    }

    public static final int MESSAGE_X = 75;
    public static final int MESSAGE_Y = 100;

}

Let us now take a look at a fully functional class that will write out some text in our frame:

class TestFrame extends JFrame
{
    public TestFrame()
    {
        setTitle("Print Something");
        setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);

        // add panel to frame

        TestPaintComponent comp = new TestPaintComponent();
        add(comp);
    }

    public static final int DEFAULT_WIDTH = 300;
    public static final int DEFAULT_HEIGHT = 200;
}

/**
 * A Component that displays a message.
 */
class TestPaintComponent extends JComponent
{
    public void paintComponent(Graphics g)
    {
        g.drawString("Welcome to Java Swings Tutorial!!!", MESSAGE_X, MESSAGE_Y);
    }

    public static final int MESSAGE_X = 50;
    public static final int MESSAGE_Y = 75;
}