Head First Java Chapter 12 summary

Udesh Kavinda

3 min readNov 17, 2021

Event & Graphics

Events

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

JFrame frame = new JFrame();

You can add widgets to the frame 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 (JFrame) 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’ and event based on user interaction.
The listener interface gives the event source a way to call you back, because the interface defines the method 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 add<EventType>Listener.
Implement the listener interface by implementing all of the interface event-handling methods. Put your event-handling code in the listener call-back method.

public void actionPerformed(ActionEvent e) {

button.setText("I have been Clicked");

}

The event object passed into the event-handler method carries information about the event, including the source of the event.

Graphics

You can draw 2D Graphics directly on to a widget.
You can draw a .gif or .jpeg directly on to a widget.
To draw your own graphics make a subclass of JPanel and override the painComponent() method.
The paintComponet() 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 screen.
Typical methods to call on a Graphics Object are:

g.setColor(Color.orange);

g.fillRect(20, 50, 100, 100);

The Object referenced by the Graphics parameter to paintComponent() is actually a instance of the Graphics2D class.
To invoke the Graphics2D methods, you must cast the parameter from a graphics object to a Graphics2D object.

Inner Class

An inner class is a class nested inside another class.

class MyOuterClass{

class MyInnerClass{

void go(){}}}

An inner class can use all the methods and variables of the outer class even the private ones.
The inner class gets to use those variables and methods just as if the methods and variables were declared within the inner class.

class MyOuterClass{

private int x;

class MyInnerClass{

void go(){

x = 42 //use 'x' if it were a variable of the inner class.}}}

An inner class instance must be tied to an outer class instance.

class MyOuter{

private int x; //The outer class has a private instance variable x

MyInner inner = new MyInner(); //Make an instance of the inner class,

public void doStuff(){

inner.go(); //call a method on he inner class.}

class MyInner{

void go(){

x = 45;}}}

Inner class is also useful when used as a subclass of something the outer class doesn’t extend.

The hierarchy of java swing API is given below.

Make a JFrame

JFrame jFrame=new JFrame();

Give a size to the frame

jFrame.setSize(200,200);//pixeljFrame.setVisible(true);

Make a jButton

JButton jButton=new JButton("click me");

Add the jButton to the JFrame

jFrame.getContentPane().add(jButton);