Java Review

The environment

n A text editor is used to record the high level java program in ASCII code

n A java compiler translates it into bytecode (J-code)

n A bytecode interpreter translates it into the machine language for the specific machine it is running on

Applications and Applets

n Applets

n programs that can be embedded in HTML documents.

n HTML has tags specifically designed for Java applets

n Applications

n Traditional programs that can run stand-alone from the command line or a GUI

n In Java, a main method is included in a class to make it an application

So What!

n The platform independence of Java may be a huge marketing tool, but is actually of little use to people learning Object Oriented Programming and Abstract Data Types

n What is of use is the simplicity of the Java syntax and programming concepts

n Java is a "pure" Object Oriented Language

n encapsulation, inheritance, and polymorphism

n all code must be contained in a class

n no free functions (functions that do not belong to some class) like C++, although someone who wants to write messy Java code certainly can

n Is OO the best programming paradigm?

Data TYPES in Java

n When you program in Java, every variable belongs to one of two categories

n Primitive types

n Reference type (all objects)

n The space allotted to a primitive type holds its value

n The space allotted to a reference type holds the address where the object is stored in dynamic memory

n At compile time, no objects have been created so the address is null

n Objects are created as the program runs

Basic or Primitive types

Four integers type

byte -- 8 bit signed integral value (-128 to 127)

short -- 16-bit signed (-32,768 to 32,767)

int-- 32 bit signed (most commonly used)

long -- 64 bits

Floats

float – 32 bits (6 to 7 sig decimal digits , exponent to 38)

double – 64 bits (15 sig decimal digits, exponent to 308)

boolean

n Can only be true or false

char

n Stored in 16 bit ASCII code

n Java does not use 8-bit ASCII code

User written classes

n The class often has more than one constructor

n They should have accessors and mutators

n If an object of the class will ever be printed it should have a toString method

n The toString method is inherited from the Object class

n Most classes should override the inherited method

The Object class

n This is the common base or ancestor class from which all Java classes are derived

n All variables of reference type are instances of Object, including arrays

n Every method of Object is inherited, directly or indirectly by all other classes

n This means that the Object class provides a set of methods that all objects have

n To see all the methods contained in the Object class, (or any other) check the Java API

Constructors

EX: Person Jean = new Person( )

n This is a call to the Person constructor (a method)

n Person is the class name, Jean is the object name

n Constructor name is the same as the class name

n The constructor method creates a new instance of a specified class

n In Java, constructors are usually invoked (or called) through the new keyword

n Since a constructor is a method (i.e. function) it must be written just like other methods

Constructors can be overloaded

Person Bill = new Person(“Bill”, “Smith”, etc. )

= = and the equals method

n The = = operation just compares what is stored in the named storage locations

n For primitives, that is actual values

n For object, it is addresses

n The equals method is also inherited from the Object class

n It should be overridden so that it compares the data found at an address, not the addresses

Java API

n http://java.sun.com/j2se/1.5.0/docs/api/