Introduction to Java: Class, Objects, and Methods

Introduction to Java

Java • Examples in textbook include line numbers. Feature which may be turned on or off in Eclipse. • Source code • Right click to bring up list • Select Preferences • Click on link to Text Editors • Check mark on Line Numbers • Click OK

Objects • An object has structure • Attributes (characteristics) • Behaviors or operations it can carry out (actions)

Class Definition • A Java program is written as a classdefinition • A Java application is a class that contains a main( ) method

Class Definition • ImportDeclaration • Statements to identify the predefined classes used in a Java program • Import declarations must appear before the class declaration • Most Java API’s begin with either “java” (core classes) or “javax” (optional classes) import javax.swing.JOptionPane; import java.io.*;

Class Definition • Class Definition has two parts: • Class Header • Class’s name; must be the same name as the .java file • Accessibility as public or private • Pedigree which specifies where it fits in the Java class hierarchy; default is Object class • Class Body • Enclosed within curly brackets { } • Contains instance variables to store data • Contains methods for the actions or behaviors

Class Definition • Identifier – name for a class, method, or variable • Must begin with a letter, A to Z, or a to z, an underscore (_) • May be followed by any number of letters, digits, 0 to 9, an underscore • May not use a Java keyword • Java is case sensitive taxrate vs. TaxRate

Class Definition • Data Types – two types in Java • Primitive Data Types boolean char byte short int long float double • Objects or Reference Variables– programmer created through a class definition • Stringis an object and therefore a data type

Class Definition • Variables – named storage location in memory where a value of a given data type may be stored

Class Definition • Method Definition • Code that carries out a specific task or behavior of the class • Method heading and body • Heading contains • the name • accessibility as public, private, or protected • type of data returned by the method • list of parameters

Class Definition • Method Definition • Body contains • Executable statements System.out.print(“Hello.”): • Local variable declarations • Computations or assignment statements sum = num1 + 5; • Calls to other methods • Return statements return sum;

Flow of Control: Methods

Input from Keyboard • Input is complicated task • Things can go wrong • User types a letter instead of a number • File could be missing • Java has three I/O streams at startup • System.in : refer to standard input device (keyboard) • System.out : refer to standard output device (display) • System.err

Input from Keyboard • java.util.Scanner class has a list of methods for data types • nextByte( ) • nextInt( ) • nextDouble( ) • next( ) • nextLine( ) Scanner input = new Scanner(System.in); double radius = input.nextDouble();

Scanner Class • Selects chunks of data from input stream • Stops at the delimiter • Default delimiter = white space • Space, tab, return, newline characters • What happens when type full name in program?

Output • In Java, any source or destination for I/O is considered a streamof bytes or characters • To perform output, we insert bytes into the stream • I/O is handled through methods that belong to classes contained in: • java.io package

Output • java.io.PrintStream class contains methods • print( ) • println( ) //prints a line feed after data • System.outandSystem.errcan be used to write things to the console • System.out.print(“Hello”);

Math

Assignment Statements • Value are placed into variables using assignment statement • Assignment operator = • Data type of variable on left must be compatible with data type on the right int x = 1; x = 1; x = x + 1;

Constants • A constant is a finalvariable associated with the class rather than with its instances • final– declare a variable that has a value that never changes; must be initialized; named using uppercase by convention public final int EATING = 0;

Object Instantiation • Define objects (classes) • To use the object it must be instantiatedor invoked • We will do this in the main( ) method • Object instantiation is an example of a reference variable Lab1 x; // reference variable declared x = new Lab1( ); //object instantiated x.user_interface( ); //method call

Naming Conventions • Choose descriptive names with straightforward meanings • First word lowercase and capitalize first letter of subsequent words showInputDialog • Capitalize first letter of class name ComputeArea

Proper Indentation & Spacing • Consistent indentation style makes programs clear & easy to read, debug, and maintain int i= 3+4 * 4; inti = 3 + 4 * 4; Bad style Good style

Block Style • Next-line style • End-of-line style – Java API standard • Both are acceptable; pick one and stick with it

Testing and Debugging • Syntax Errors • Error messages from the compiler • Fatal or Warnings • Run-Time Errors • Cause the program to terminate abnormally • Logic Errors • Mistakes in algorithm used to solve problem

Introduction to Java: Class, Objects, and Methods

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