Java Methods

Methods Basics

Methods: The procedures or functions of a class.

• have access to the private variables of the class
• normally methods are public
• methods can have arguments/parameters
• a method can have multiple definitions under the same name as long as the argument number and type sequence is unique ("overloading" and "signature")
• methods are called or referenced using instancepath.method(arguments) notation

System.out.println(Math.sqrt(c*d+1));

Java is very rich with class definitions and complete sets of methods. 

Defining methods in classes shown well in the text.

Math class

abs(float|int|long)

ceil(float)  floor(float)

cos(float)  sin(float) tan(float)

exp(float)  log(float)  pow(float,float)

max(int|long|float, int|long|float)  min(int|long|float, int|long|float)

sqrt(float)

random()

Promotions

There are various numeric types.  Having the exact type match the parameter types can be tedious.  Promotions are provided:

Type casting

There are times the programmer must force a type to become another type.  This is called "type casting" or "coercion".

(type) valueExpression

dieValue = ((int) Math.random() * 6) + 1;

Identifier Scope and Duration

Identifiers have duration or lifetime.

• static - the identifier's lifetime exists once the class it is defined in is entered
• automatic - the identifier exists when the block/method it is defined is entered

Scope refers to the range of code that can access the identifier

• class scope - class variables and method names
• block scope - instance variables

Recursion

• Methods that call themselves

• Encouraged if the solution can be simplified

JApplet Methods

public void init() -- called once when the applet is start.  Meant for initialization code that must be run once and only once.

public void start()-- called after init() is completed.  This routine is rerun every time the user of the browser returns to the HTML page that holds the applet, after viewing another page.

public void paint (Graphics g) -- called after init() is completed and start() is started but not necessarily completed.  start() and paint(g) are run as separate threads.  Each time the applet is uncovered paint(g) is called.  This routine is called also by the repaint() routine.

public void stop() -- called when the user of the browser leaves the HTML page.

public void destroy() -- called when the browser is exited. (Note applets often remain resident and are not reloaded)

First example of event driven programming

This is the Craps.java program of Example 06-09.  Its associated html file is craps.html

Consider the following:

• implements ActionListener - a class of routines that handle events

• Container c = getContentPane();

• c.setLayout (new FlowLayout());

• c.add(GUIitem)

• JLabel("abc")

• JTextField(size)

• JButton("label")

• addActionListener(this)

• public void actionPerformed(ActionEvent e)