Lecture Set 2 Starting Java This set has Java Concepts and Java Programming Basics CMSC 131 Spring 2007 Bonnie Dorr adapted from Rance Cleaveland This Course Intro to Procedural Programming using Java Why Java Popular modern language Used in web business telecom applications Developed in 1990s incorporates many features from earlier languages Object orientation Garbage collection Portability of object code CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 1 Portability of Object Code Object code is 2GL assembly 1GL machine code Last time we said that 2GL 1GL is architecture specific How can Java have portable object code Answer Java Virtual Machine JVM CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 2 Java Virtual Machine Java includes definition of Java bytecode fake machine code for Java Java compilers produce Java bytecode To run Java bytecode must have bytecode interpreter Java Virtual Machine on client machine JVM source code object code client java Java compiler class JVM client CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 3 Facts about JVMs For efficiency JVMs often compile bytecode into native machine code There are also native Java compilers these compile Java directly to machine code CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 4 example1a This is a very basic Java program to get things started Notice the difference between println and print The things inside the quotation marks are called String literals public class Example1a public static void main String args where the program starts System out println Hello World System out print Or maybe I should say System out println Goodbye World CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 5 Method Headers main is a method operation Operations require operands data to work on Operations return new data result Header gives information on form of operands result for methods For main Operand is collection of Strings Result is void unimportant More later on public static Every program must have exactly one main method where execution begins CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 6 Comments Comments explanations added by programmer Two styles to end of line Comments are essential for good programming CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 7 Objects Bundles of data instance variables and methods functions Created using classes as templates We ll learn more later this semester CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 8 Java Program Organization Class Structure around which all Java programs are based A typical Java program consists of many classes Each class resides in its own file whose name is based on the class s name The class is delimited by curly braces File name Example1 java public class Example1a contents of the class go here A class consist of data variables and operations methods CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 9 example1b Have you ever noticed that your dryer eats your socks This example illustrates variables and the assignment operator Note that each variable is declared just ONCE public class Example1b public static void main String args int numberOfSocks numberOfSocks 27 An odd number of socks Crazy System out print The number of socks at the beginning is System out println numberOfSocks System out println OK I m going to put them in the dryer int socksLostInDryer numberOfSocks 3 I lose about a third each time numberOfSocks numberOfSocks socksLostInDryer System out print The number of socks lost was System out println socksLostInDryer System out print The number of socks is now System out println numberOfSocks CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 10 Java Program Organization Methods Where most computation takes place Each method has a name a list of arguments enclosed in and body collection of statements in public static void main String args contents of the main method go here Variables Storage locations that program can operate on Variables can store data of different forms integers for example int secondsPerMinute 60 int minutesPerLecture 50 CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 11 Java Program Organization Statements Many different types Declarations specify variable types and optionally initialize int x y z three integer variables String s Howdy a character string variable boolean isValid true a boolean true false variable Assignments assign variables new values x 13 Method invocation call other methods System out println Print this message Control flow determine the order of statement execution These include if then else while do while for More later Built in Operators For manipulating values etc CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 12 example1c This example illustrates concatenation of strings and shows how Java automatically converts values into strings public class Example1c public static void main String args System out println My name is Fred int secondsPerMinute 60 int minutesPerLecture 50 int secondsPerLecture secondsPerMinute minutesPerLecture System out println There are secondsPerLecture seconds in a lecture CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 13 Built in Primitive Types Integers Reals Other Type name byte short int long float double char boolean Size bytes 1 2 4 8 4 8 2 1 CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 14 String Type Elements of String type are sequences of characters abc Call me Ishmael etc String type is not built in We will use it a lot Useful operation concatenation abc def abcdef CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 15 Example 2 Basic Types Demonstration of primitive types and also the String type Note that you can declare many different variables with one statement public class Example2 public static void main String args int i1 i2 i3 double f1 7 3 f2 9 4 boolean b1 b2 char c String s i1 7 i2 3 i3 i1 i2 5 2 f1 3 1415927 b1 true b2 f2 f1 c X s Hello there my friend System out println i3 i3 System out println b1 b1 System out println b2 b2 System out println c c System out println s s CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 16 Programming Errors Types of Errors Syntax Errors violates languages grammar compiler warns about these Eclipe puts red squiggles under the offending code Semantic Logic Errors program doesn t work properly run time errors crash or hang can be more subtle harder to find Debugging process of finding and fixing problems to
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