Download Methods I

A Method to the Madness
Java's Work Horses
CS 102-02
Lecture 4-1
April 20, 1998
CS102-02
Lecture 4-1
What’s up?
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What’s a method?
Returning values from methods
Java’s built-in classes
Static members
April 20, 1998
CS102-02
Lecture 4-1
What’s a Method?
• Objects know how to do things
• A method is a set of instructions for
doing something
• A method shouldn’t do too much
– If the method’s code is more than half a
page, it’s probably doing too much
April 20, 1998
CS102-02
Lecture 4-1
“Methods, I Do Declare!”
• The syntax for declaring a method is:
Method
name
Parameter type
& name
public float Area(String shapeName)
Access
modifier
April 20, 1998
Return type
CS102-02
Lecture 4-1
Returning Values
• Calling a method returns a value
– Returned value could be a primitive type or
a reference type
– Objects and arrays are okay
• Don’t have to return a value
– Return type void
• Why write a method that doesn’t return
a value?
April 20, 1998
CS102-02
Lecture 4-1
Returning with return
• To return a value, use the syntax:
return expression;
• The return statement doesn’t have to be
the last statement in the method
• When a return is executed:
– Current method finishes executing
– Control jumps back to the line following the
method invocation
April 20, 1998
CS102-02
Lecture 4-1
Returning from Methods
// Do some stuff
sumOfDice = rollDice();
// jump into rollDice
return workSum;
// Do the next line
switch (sumOfDice ) {
April 20, 1998
CS102-02
Lecture 4-1
Returning an Expression
• Returning a value
return true;
• Returning an expression
return 9 + 12;
return n * factorial(n-1);
April 20, 1998
CS102-02
Lecture 4-1
Brown Paper Packages, Tied Up
in Strings
• Java lets you organize code into
classes and packages
• Packages are really just groups of
related classes
• Packages help prevent name collisions
among developers of similar classes
April 20, 1998
CS102-02
Lecture 4-1
Bundles of Java
• Java has special rules about declaring
classes to be part of a package
– Usually classes are stored in directories
which mimic the package name
– java.awt.datatransfer classes are
stored in a directory
java/awt/datatransfer/
• If you don’t declare a package, your
class is part of the default package
April 20, 1998
CS102-02
Lecture 4-1
Reuse, Reuse, Reuse
• Don’t write code someone else has
written already
• Java includes:
– The core language
– Libraries
April 20, 1998
CS102-02
Lecture 4-1
Java’s Built-in Classes
• The Applet Package (java.applet)
– The all-important Applet class
• The AWT Package (java.awt)
– Java’s Abstract Windowing Toolkit, a library
for building cross-platform graphical user
interfaces
– Includes sub-packages and classes for
events, images and data transfer
April 20, 1998
CS102-02
Lecture 4-1
More Built-in Packages
• Beans (java.beans)
– A component model for designing reusable
software.
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IO package (java.io)
Core Java classes (java.lang)
Networking (java.net)
Remote Method Invocation (RMI, in
java.rmi)
April 20, 1998
CS102-02
Lecture 4-1
Still More Packages
• SQL (java.sql)
– Classes for database access using SQL
• Text (java.text)
– Classes for manipulating text, especially
international text
• Utilities (java.util)
– Utility data structures, date, time and more
April 20, 1998
CS102-02
Lecture 4-1
Some Java Nitty-Gritty
• With data members, every object has its
own copy of data
– Since objects are also called instances,
data members are often called instance
variables
• All objects instantiated from a class
share the same methods
– Don’t need a copy for each object
April 20, 1998
CS102-02
Lecture 4-1
One Class, One Copy
• What if you want all objects of the same
class to share a variable?
• Use the static keyword
– When you declare a variable static, you’re
telling Java, “All objects of this class share
this variable’s value.”
April 20, 1998
CS102-02
Lecture 4-1
Using static
• Include the static modifier in the
declaration
• Suppose we wanted to track the total
number of employees created from an
Employee class
– Create a static vairable to track the total
– Add one to the total in the constructor
– Subtract one in the destructor
April 20, 1998
CS102-02
Lecture 4-1
Employee Example
public class Employee{
private String firstName;
private String lastName;
private static int employeeTotal;
public Employee(SSN socialSecurity) {
employeeTotal++;
:
}
}
April 20, 1998
CS102-02
Lecture 4-1