Statements, Comments & Simple Arithmetic

4 Statements, Comments & Simple Arithmetic

Previously • What’s Java? • Some history • Characteristics • Components • Type of programs • Development Application Library (y) Library (y) Library (y) JVM OS Applet Servlet Storage Storage Library (y) Library (y) Library (y) Web Server Browser applet OS

Overview Hello World! Reserved words & Identifiers Code Blocks Methods Statements Style & Comments Simple Arithmetic & Operators

classHiThere { /** * This is the entry point for an application. * @param astrArgs the command line arguments. */ publicstaticvoidmain(String[] astrArgs) { // Starting of the main method System.out.println("Hello World!"); }// main() }// end class HiThere Finally - a Java Program!

Reserved words & Identifiers Reserved words or keywords are part of the language class used to define a class public access keyword - public, private, protected and ‘default’ static type related to belonging void special return type Java is case sensitive class HiThere { public static void main(String[] astrArgs) { System.out.println("Hello World!"); }// main() }// end class HiThere

Reserved words & Identifiers • Identifier • HiThere is an identifier • This is a name we make up to identify a component of the program (in this case a class) • Must be a single word (no blank characters) • Must start with a letter, ‘_’, ‘£’ or ‘$’ • Following characters can also be numbers • Cannot contain any white space

Reserved words & Identifiers • Identifiers • The name of a classes always start in uppercase classHiThere • The name of methods always start in lowercase public static void main(String[] astrArgs) • The name of variables always start in lowercase public static void main(String[] astrArgs) • The name of static final variables (‘constants’) always in uppercase privatestaticfinalintSEC_PER_MINUTE=60;

Reserved words & Identifiers • Identifiers • A format to build identifiers • Camel case – no word separator space, underscore • Variable names prefix with their type (lowercase) public static void main(String[] astrArgs) array array of Strings array of Strings meaning word

Code Blocks class HiThere { public static void main(String[] astrArgs) { System.out.println("Hello World!"); }// main() }// end class HiThere • Braces ( i.e. { } ) delimit an isolated block of code • All programs have several (usually many) blocks • Braces must be balanced • Braces are often nested

Methods class HiThere { public static void main(String[] astrArgs) { System.out.println("Hello World!"); }// main() }// end class HiThere • Methods contain blocks of functional code • Methods are named by an identifier • This is a method called main - applications execute their main method on starting (application entry point) • NB the syntax of main must be exactly as shown

Statements class HiThere { public static void main(String[] astrArgs) { System.out.println("Hello World!"); }// main() }// end class HiThere • The program contains a single statement • Statements are terminated by a semi-colon • println • This statement calls a library method called System.out.println • Methods may be provided with an argument (data), which is contained in brackets

Statements class HiThere { public static void main(String[] astrArgs) { System.out.println("Hello World!"); }// main() }// end class HiThere • The argument of println is a string • A string is a sequence of characters • Java strings are delimited by double quotes

Style class HiThere { public static void main(String[] astrArgs) { System.out.println("Hello World!"); }// main() }// end class HiThere • Line breaks and indents are ignored by the compiler, but help greatly with readability • Use meaningful names indents

Comments class HiThere { /** * This is the entry point for an application. * @param astrArgs the command line arguments. */ public static void main(String[] astrArgs) { // Starting of the main method System.out.println("Hello World!"); }// main() }// end class HiThere } }

Comments • Comments are ignored by the compiler • Comments may be delimited by /* */ • For several lines of comment • A type of multiline comment is the documentation comments which start with /** and end with */ • Comments may be delimited by // to the end of the line • Single line comment

Arithmetic is accomplished by "numeric operators“ Binary Operators + addition - subtraction * multiplication / division %remainder Arithmetic intiNum = 0; iNum = 7+ 2; iNum = 7 – 2; iNum = 7 * 2; iNum = 7 / 2; iNum = 7 % 2; 9 5 14 3 1 3 1 • 2 • 3 • 1 3 1

Arithmetic • Arithmetic Unary Operators += addition -= subtract *= multiply /= divide %= remainder iNum+= 2; • iNum-= 2; • iNum*= 2; • iNum/= 2; • iNum%= 2; 9 5 14 3 1 intiNum = 7;

Arithmetic System.out.print("The answer is "); System.out.println(2 + 2); class Arithmetic { /** * Adds two to two and displays the result. * @paramastrArgs the command line arguments, not used. */ public static void main(String[] astrArgs) { System.out.print("The answer is "); System.out.println(2 + 2); }// main() }// end class Arithmetic The answer is 4

Operators • Equality and Relational Operators == equal to if(iNum == 1) { != not equal to if (iNum != 1) { > greater than if (iNum > 1) { >= greater than or equal to if (iNum >= 1) { < less than if (iNum < 1) { <= less than or equal if (iNum == 1) { intiNum = 7;

Operators • Assignment Operator = assignment • Conditional Operators && conditional-AND || conditional-OR • Type Comparison Operator instanceof compares an object to a specified type intiNum= 7; intiCount= 0; if (iNum == 7 && iCounter == 0) { true&&true true true&&false  false false&&true  false false&&false  false true||true true true||false  false false||true  true false||false  false if (iNum == 7 || iCounter == 0) {

Documentation • Statement and Blocks http://download.oracle.com/javase/tutorial/java/nutsandbolts/expressions.html • Operators http://download.oracle.com/javase/tutorial/java/nutsandbolts/operators.html Sun Certified Programmer for Java 5 - Study Guide

Eclipse Lets look at HelloWorld again!

Statements, Comments & Simple Arithmetic