CS 225 Java Review

Java Applications A java application consists of one or more classes –Each class is in a separate file –Use the main class to start the application

main class Every application needs a main class (or driver) which contains a main method public class Driver { public static void main( String [] args) { }

Classes A class is a template for creating objects A class contains –Data declarations for representing the properties or attributes of an object created from the class –Method definitions for defining the behavior or operations of an object created from the class

Structure of a Java Class /** javadoc comment describing class */ public class ClassName { // Data declarations private DataType dataName; // method definitions /** javadoc comment describing method */ public returnType methodName( ) { }

Objects Objects are created (instantiated) from a class ClassName objectName = new ClassName(); There can be any number of objects created from a single class –Each object has its own data –A method has access to the data of the object is was called for

Java Statements Declarations Assignment Method calls Control Statements –if, switch –while, do, for

Declarations Use a declaration to create one or more variables varName1, varName2; Variables instance variables belong to an object static variables belong to a class local variables are declared inside a method

Data Types Primitive Types –integer types: byte, short, int, long –real types: float, double –character type: char –boolean has values true and false Reference Types –used for objects, arrays –declaration creates only the reference varaible –objects must be instantiated separately

Operators Arithmetic (numeric types only) : + - * / % Relational (numeric and char) : = > Relational (any type) : == != Logical (boolean): && || ! Assignment: = = et. al

Control Statements Sequence (block)- done in order { } Selection - select between different sequences if switch Repetition - do a sequence multiple times while do-while for

Methods Method definition public returnType methodName ( ) { } Method call objectName.methodName( )

Static Members Data and methods can be declared static –call static methods using the name of the class public static int objectCount –static methods need don't have access to object data –all objects share static data

Java API Java comes with an extensive collection of classes that are already written –Look at the documentation for these classes online Use a class from the Java library by importing it import java.packageName.ClassName;

Math class A collection of static methods and constants Math.PI Math.sqrt( double x) : double Math.sin( double x) : double Math.pow( double num, double power) : double … and many more

System Class Data and methods needed to interact with the OS System.in : InputStream for keyboard input System.out : PrintStream for console output System.exit( int exitCode) : stops the program

Handling Text String and StringBuffer can b used for text data –String is immutable StringTokenizer can be used to separate text into smaller pieces

Stream Classes for I/O Input –InputStream -> InputStreamReader - > BufferedReader (or Scanner ) –FileReader -> BufferedReader Output –OutputStream -> OutputStreamWriter -> PrintWriter –FileWriter -> PrintWriter

Arrays An array is an indexed collection of objects of the same type –arrays are objects To create an array: –declare it: dataType [] arrayName; –instantiate it: arrayName = new dataType[size] –assign or instantiate each element Get a particular element by index: arrayName[index]