C SC 160 Chapter 2: Getting Started with Java (using classes and objects)
major resource: An Introduction to Object-Oriented Programming with Java, fourth edition, Wu, McGraw Hill, 2006

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Note: most of these topics are also covered in Dr. Pete's No-Nonsense Guide: Using Java Classes

A first program

• Ch2Sample1.java
• note: all Wu source code available in course repository
• main method creates and displays an empty JFrame
• worked through it line by line
• comments
• single line comments using //
• multi-line comments using /* to start and */ to end
• "javadoc" comments using /** to start and */ to end
• javadoc is utility program that reads Java source code and produces HTML documentation
• javadoc recognizes and formats contents of comments starting with /**
• the Java API web pages were produced by javadoc
• import and packages
• package is a group of related classes stored in the same folder
• import statement tells compiler you are going to use classes from specified package
• Example: to import all classes in package java.util, use import java.util.*;
• Example: to import only class Random from package java.util, use import java.util.Random;
• Package hierarchies are stored in corresponding folder hierarchies
• Example: class java.util.Random is stored in file Random.class, which is stored in folder util, which itself is stored in folder java
• naming conventions
• Class names traditionally start with upper-case letter
• Variable and method names traditionally start with lower-case letter
• Names should be descriptive but cannot contain blanks
• Multi-word names are traditionally composed by mixed case, e.g. timeOfDay
• Names for constants ("final"), traditionally are all upper case with underscore to compose words, e.g. DAYS_IN_WEEK
• declaring reference variable
• "reference variable" means the variable will later identify an object
• We say "reference" because objects, like puppies, intrinsically do not have names. Calling a puppy "Fido" does not affect the animal itself, it is just our way of referring to the animal.
• Declaration means introduce the identifier and associate it with a class name
• Format: ClassName identifier;
• At this point, it does not yet refer to any objects.
• Example: Random randu;
• creating object
• A class is just a template, to do useful work we must create objects
• Object creation occurs with the expression new ClassName(arguments), where ClassName is the desired class, and arguments is an optional list of values for initializing the object
• The thing that actually creates and initializes the object is called the constructor
• The result of the new ClassName(arguments) expression is a reference to the newly-created object
• This reference must be assigned or else it will be lost
• Normally, assign this new reference to the reference variable!
• Example: randu = new Random();, where randu was previously declared by Random randu;
• invoking method with arguments
• Once you have assigned the object reference to a variable, you use this variable to "send messages" to the object.
• The terminology I normally use instead of "send message", is "invoke method". They are equivalent.
• Format is: objectName.methodName(arguments)
• Again, arguments is an optional list of values needed by the method.
• The method may or may not respond with a result
• If it responds with a result, which we call its return value, the result should be assigned or else it will be lost (falls into the "bit bucket").
• In either case, the caller suspends execution until the method completes

using JOptionPane for input and output

• find JOptionPane in Java API documentation
• static methods: showConfirmDialog, showInputDialog, showMessageDialog
• A static method is a class method, invoked through the class name, e.g. Math.abs()
• Use showMessageDialog() method to display (output) values
• Use showInputDialog() method to get keyboard input
• We used null as the first argument to both methods, to indicate that the dialog should be centered on the screen
• showMessageDialog is one-way, it does not return a value
• showInputDialog() returns the value entered.
• We studied some example programs