Practical Example of Simple Java UI Component

In this article we'll discuss how to create a simple Java graphical component such as a javax.swing.JButton (without going into details of event handling) and include it within an application. Our component will display (paint) a flat rectangle of a set colour as illustrated below.

We'll be making use of two classes one representing the simple custom component and the other will be mounting the frame and including our component in it. All classes discussed here are saved under the package "com.albertattard.cgui". You can change this to your own package. In this article we'll only be exploring the creation of simple custom UI component. Many other related topics will be only mentioned here but not discussed. While you are encouraged to explore all the topics that are only mentioned (but not discussed) here, one must be cautious not to overwhelm him/herself in the process.

Custom UI Component

All Java Swing (don't worry if you don't know with Swing means but if you insist here is a link which may answer some of your questions: http://java.sun.com/docs/books/tutorial/uiswing/) graphical components inherit from the javax.swing.JComponent abstract class. Our component is no exception. So let's start by creating a simple class called: FlatRectComponent. This class will be extending the JComponent class mentioned earlier and will override the paintComponent() method as highlighted in the example below.

package com.albertattard.cgui;

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JComponent;

@SuppressWarnings("serial")
public class FlatRectComponent extends JComponent {
  @Override
  protected void paintComponent(Graphics g) {
    // Clear the previous screen
    super.paintComponent(g);
    // Set the painting colour to magenta
    g.setColor(Color.MAGENTA);
    // Draw a solid rectangle at coordinates x and y set to 
    // 10px with width and height set to 100px
    g.fillRect(10, 10, 100, 100);
  }
}

Why did we override the paintComponent() method? All UI programs are responsible from painting themselves on the screen. For example this article you're reading is an image displayed on your screen (unless you printed it on a piece of paper). The browser downloaded this article as HTML and transformed (or better rendered) it as an image. All other applications do the same. For example a button, is a simple set of paint instructions which makes it look real (as you see it on the screen). Also events, such as mouse hovering, may change the way the button or any other graphical component looks making the graphics interacting with the user inputs.

In the case of Java, all graphical components are responsible of painting themselves by overriding the paintComponent() method and paint on the provided java.awt.Graphics. In the above example, we're simply painting a flat rectangle in magenta. Then this method is called by some other class which displays it on the screen as you see it. You don't have to worry about how or what invokes this method. Just remember that you cannot call it yourself.

Alone this class is not of much use. But since it's inheriting from the JComponent class, we can include it with other Java Swing components such as javax.swing.JFrame.

Including it in a JFrame

Let's create a simple frame which will host our new component. For simplicity, we're not going to discuss Layout Managers. We're going to use the absolute layout where we specify the location and size for every component. An example of our frame class is shown below.

package com.albertattard.cgui;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;

@SuppressWarnings("serial")
public class FrameDemo extends JFrame {

  private static void createFrame() {
    JFrame frame = new FrameDemo();
    frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
    frame.setTitle("Custom UI Demo");
    frame.setSize(600, 400);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  }

  public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
      @Override
      public void run() {
        createFrame();
      }
    });
  }

  private FrameDemo() {
    // Set the layout of this frame to absolute layout
    setLayout(null);

    // Create the flat rectangle component and add it to 
    // the frame
    FlatRectComponent comp = new FlatRectComponent();
    comp.setBounds(0, 0, 200, 200);
    add(comp);
  }
}

The above class contains two static methods and a constructor. Let's analyse these further starting from the main() method. The main() method is the first method called by the JVM. This is from where the application starts. This method calls the other static method createFrame() by scheduling a job on the AWT event dispatching thread. This sounds as complex as rocket science but you have nothing to be afraid from. For now all you have to do is copy and paste it. The static method createFrame() creates an instance of our frame class, FrameDemo and set the frame properties, such as title, size and location.

The constructor of this class starts by setting the layout manager to absolute layout. Then it creates an instance of our components and set its size and location accordingly. The setBounds() method takes four parameters, the first two represent the location as x and y coordinate while the latter to represent the size. Finally it adds this component to its container (don't worry about containers for now - this is the fort time I'm asking to accept something as is without having to dig any further). This is a very important step. Many forget to add the components and then wonder why these don't appear in the frame.

How many instances of our component can we include in the frame? As many as you want. In practice there's a limit which is dependent on the amount of memory available to your application, but you don't have to worry about that neither as our application is fairly simple and consume very little memory. Let's amend the previous frame class and include another instance of the FlatRectComponent UI component. All we need to do is create another instance and adding this instance to the frame's container. We also need to set the bounds of the new component such that this don't conflict with the previous one as otherwise we may have a graphical issue. The following code fragment highlights the changes made to the frame class. Only the constructor was changed, the other methods are unchanged and thus not shown here. Also, note that a border was added to the components to visualise the component perimeter.

  private FrameDemo() {
    setLayout(null);

    // The first component
    FlatRectComponent comp1 = new FlatRectComponent();
    comp1.setBounds(0, 0, 200, 200);
    comp1.setBorder(
      BorderFactory.createLineBorder(Color.BLACK, 1));
    add(comp1);

    // The second component
    FlatRectComponent comp2 = new FlatRectComponent();
    comp2.setBounds(250, 50, 200, 200);
    comp2.setBorder(
      BorderFactory.createLineBorder(Color.BLACK, 1));
    add(comp2); 
  }

The program should now display two, identical, components as illustrated below.

Customising the Custom UI Component

As is, our component is not much of use as we cannot customise it. The painComponent() method make use of literals and constants and so our component will always look like that. In order to be able customise it we need to add some states to our custom UI component.

What are these? States in objects are instance fields, or as some call them: class variables. The instance fields are used to save the state of the object. The state of the object is all information about the object. For example our custom UI component displays a rectangle. This rectangle needs to have an x and y coordinates and a size. A rectangle cannot exist without these states (properties). Try to draw a rectangle on a piece of paper yourself without making use of the mentioned states. You'll notice that you cannot as, as soon as you draw it, you have provided the x, y and size states. In this case we need to capture these states in some instance fields. The same applies for the rectangle colour.

We can start by adding an instance of the java.awt.Rectangle class and save our rectangle states (size and location) there. Alternatively, we can create four integer variables which represent the x, y, width and height values for our rectangle. Even though it may sound more complex I suggest we go with the first option as this give us other functionality already available in the Rectangle class, which otherwise we have to program, such as the translate() method.

package com.albertattard.cgui;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import javax.swing.JComponent;

@SuppressWarnings("serial")
public class FlatRectComponent extends JComponent {

  private Rectangle rect = new Rectangle(10, 10, 100, 100); 

  @Override
  protected void paintComponent(Graphics g) {
    super.paintComponent(g);

    // Make use of the 2D version of the Graphics component
    Graphics2D g2D = (Graphics2D) g; 

    g2D.setColor(Color.MAGENTA);
    g2D.fill(rect); 
  }
}

In the above example we've changed the way the rectangle is painted. Before we used literals, while now we're painting an instance of Rectangle. Also, now we're using a java.awt.Graphics2D component instead of the Graphics component. We're not going to go into much detail about this here as otherwise we'll be ending up writing a book. But in a nutshell, the Graphics2D inherits from the Graphics and adds more functionality (methods) which we can use. The object invoking out paintComponent() method (which we agreed not to discuss here) will be passing an instance of java.awt.Graphics2D to the paintComponent() method. All we're doing is recasting it back to its original form. Don't worry about this for now. All you need to know is that you should cast your Graphics to Graphics2D.

So far we have not yet provided any means (methods) which allow the rectangle instance field, rect, to be changed. That is, there is no way how one can change the x, y and size values for this instance. Let's add four methods, which can be used to set the any of the rectangle states.

package com.albertattard.cgui;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;

import javax.swing.JComponent;

@SuppressWarnings("serial")
public class FlatRectComponent extends JComponent {

  private Rectangle rect = new Rectangle(10, 10, 100, 100);

  @Override
  protected void paintComponent(Graphics g) {
    super.paintComponent(g);

    Graphics2D g2D = (Graphics2D) g;

    g2D.setColor(Color.MAGENTA);
    g2D.fill(rect);
  }

  public void setHeight(int height) {
    rect.height = height;
    repaint();
  }

  public void setWidth(int width) {
    rect.width = width;
    repaint();
  }

  public void setX(int x) {
    rect.x = x;
    repaint();
  }

  public void setY(int y) {
    rect.y = y;
    repaint();
  }
}

Note that at the end of every setXXX() method we're calling the repaint() method. All we're doing here is instructing the frame's container (this is the guy which is invoking our paintComponent() method) to repaint this component. The frame's container will in turn call the paintComponent() method for this component and providing the appropriate information. It is important to do so after every change as otherwise your changes will not immediately appear. But these will appear only when the frame completely repaints all its components. Remember that you cannot call the paintComponent() yourself. All you can do is call the repaint() method.

So far we've only provided a means to customise our component. But we have not yet customized it. Let's do that. We can customise our component by calling the setXXX() from the frame class as highlighted bold below. The following code fragment only shows the constructor of the frame class as the other methods are unchanged.

  private FrameDemo() {
    setLayout(null);

    // The first component
    FlatRectComponent comp1 = new FlatRectComponent();
    comp1.setBounds(0, 0, 200, 200);
    comp1.setX(5);
    comp1.setY(5);
    comp1.setWidth(190);
    comp1.setHeight(190); 
    comp1.setBorder(
      BorderFactory.createLineBorder(Color.BLACK, 1));
    add(comp1);

    // The second component
    FlatRectComponent comp2 = new FlatRectComponent();
    comp2.setBounds(250, 50, 200, 200);
    comp2.setX(50);
    comp2.setY(50);
    comp2.setWidth(100);
    comp2.setHeight(100); 
    comp2.setBorder(
      BorderFactory.createLineBorder(Color.BLACK, 1));
    add(comp2);
  }

The above program should produce the following outcome.

Note that now the coloured rectangle size and location can be varied from the setter methods provided. This makes our component customizable. We can also change the paint colour by including an instance field representing the colour and a method in our custom UI component which changes the colour as illustrated below.

// imports not shown here 
public class FlatRectComponent extends JComponent {
  private Color rectColour = Color.MAGENTA; 

  // other field and methods are not show for brevity

  @Override
  protected void paintComponent(Graphics g) {
    super.paintComponent(g);

    Graphics2D g2D = (Graphics2D) g;

    g2D.setColor(rectColour); 
    g2D.fill(rect);
  }

  public void setRectangleColour(Color rectColour) {
    this.rectColour = rectColour;
    repaint();
  }
}

All we need to do now is call this method and change the colour accordingly as highlighted below.

  private FrameDemo() {
    setLayout(null);

    // The first component
    FlatRectComponent comp1 = new FlatRectComponent();
    comp1.setBounds(0, 0, 200, 200);
    comp1.setX(5);
    comp1.setY(5);
    comp1.setWidth(190);
    comp1.setHeight(190);
    comp1.setRectangleColour(Color.GREEN); 
    comp1.setBorder(
      BorderFactory.createLineBorder(Color.BLACK, 1));
    add(comp1);

    // The second component
    FlatRectComponent comp2 = new FlatRectComponent();
    comp2.setBounds(250, 50, 200, 200);
    comp2.setX(50);
    comp2.setY(50);
    comp2.setWidth(100);
    comp2.setHeight(100);
    comp2.setRectangleColour(Color.CYAN); 
    comp2.setBorder(
      BorderFactory.createLineBorder(Color.BLACK, 1));
    add(comp2);
  }

The following screenshot shows the final version of our program.

Conclusion

In this article we've saw how to create a simple custom Java UI component and display a simple flat rectangle. We've also added methods which can be used to customise this component.