《Head First Java 2nd Edition》Chapter 12

25 Jan 2021

Reading time ~21 minutes

BULLET POINTS

To make a GUI,start with a window,usually a JFrame

JFrame frame = new JFrame();
You can add widgets (buttons,text fields,etc.) to the JFrame using

frame.getContentPane().add(button);
Unlike most other components,the JFrame doesn’t let you add to it directly,so you must add to the JFrame’s content pane
To make the window display,you must give it a size and tell it be visible

frame.setSize(300, 300);

frame.setVisible(true);
To know when the user clicks a button you need to listen for a GUI event
To listen for an event,you must register your interest with an event source. An event source is the thing that ‘fires’ an event based on user interaction
The listener interface gives the event source a way to call you back,because the interface defines the method(s) the event source will call when an event happens
To register for events with a source,call the source’s registration method. Registration methods always take the form of: **addListener**. To register for a button's ActionEvents,for example,call:

button.addActionListener(this);
Implement the listener interface by implementing all of the interface’s event-handing methods. Put your event-handing code in the listener call-back method. For ActionEvents,the method is:
```
public void actionPerformed(ActionEvent event){
	button.setText("you clicked!");
}
```
The event object passed into the event-handler method carries information about the event,including the source of the event
You can draw 2D graphics directly on to a widget
You can draw your own graphics,make a subclass of JPanel and override the paintComponent() method
The paintComponent() method is called by the GUI system. YOU NEVER CALL IT YOURSELF. The argument to paintComponent() is a Graphics object that gives you a surface to draw on,which will end up on the screen. You cannot construct that object yourself
Typical methods to call on a Graphics object are:

graphics.setColor(Color.blue);

g.fillRect(20, 50, 100, 120);
To draw a .jpg,construct an Image using:

Image image = new ImageIcon("catzilla.jpg").getImage();

and draw the imagine using:

g.drawImage(image, 3, 4, this);
The object referenced by the Graphics parameter to paintComponent() is actually an instance of the Graphics2D class. The Graphics 2D class has a variety of methods including:

fill3DRect(),draw3DRect(),rotate(),scale(),shear(),transform()
To invoke the Graphics2D methods,you must cast the parameter from a Graphics object to a Graphics2D object:

Graphics2D g2d = (Graphics2D)g;

Inner class

You can have one class nested inside another. It’s easy. Just make sure that the definition for the inner class is inside the curly braces of the outer class.