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Trail: Writing Applets
Lesson: Overview of Applets

Methods for Adding UI Components

The Simple applet's display code (implemented in its paint method) is flawed: It doesn't support scrolling. Once the text it displays reaches the end of the display rectangle, you can't see any new text. Here's an example of the problem:


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The simplest cure for this problem is to use a pre-made user interface (UI) component that has the right behavior.

Note: This page glosses over many details. To really learn about using UI components, go to Creating a User Interface (AWT Only).

Pre-Made UI Components

The AWT supplies the following UI components (the class that implements each component is listed in parentheses):

Methods for Using UI Components in Applets

Because the Applet class inherits from the AWT Container class, it's easy to add components to applets and to use layout managers to control the components' onscreen positions. Here are some of the Container methods an applet can use:
add
Adds the specified Component.
remove
Removes the specified Component.
setLayout
Sets the layout manager.

Adding a Non-Editable Text Field to the Simple Applet

To make the Simple applet use a scrolling, non-editable text field, we can use the TextField class. Here is the revised source code (in a .java source file). The changes are shown below. 
//Importing java.awt.Graphics is no longer necessary
//since this applet no longer implements the paint method.
. . .
import java.awt.TextField;

    public class ScrollingSimple extends Applet {

    //Instead of using a StringBuffer, use a TextField:
    TextField field;

    public void init() {
        //Create the text field and make it uneditable.
        field = new TextField();
        field.setEditable(false);

	//Set the layout manager so that the text field will be
	//as wide as possible.
        setLayout(new java.awt.GridLayout(1,0));

	//Add the text field to the applet.
        add(field);
        validate();

        addItem("initializing... ");
    }

    . . .

    void addItem(String newWord) {
        //This used to append the string to the StringBuffer;
        //now it appends it to the TextField.
        String t = field.getText();
        System.out.println(newWord);
        field.setText(t + newWord);
        repaint();
    }

    //The paint method is no longer necessary,
    //since the TextField repaints itself automatically.

The revised init method creates an uneditable text field (a TextField instance). It sets the applet's layout manager to one that makes the text field as wide as possible (you'll learn about layout managers in Laying Out Components within a Container (in the Writing Applets trail)) and then adds the text field to the applet.

After all this, the init method calls the validate method (which Applet inherits from Component). Invoking validate once after adding one or more Components to an applet is a bit of voodoo that ensures that the Components draw themselves onscreen. If you want to delve into the arcane reasons why validate works, read Details of the Component Architecture (in the Writing Applets trail).

Below is the resulting applet.

You can't run applets. Here's what you'd see if you could:


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