Lecture 3 Starting Java Last time 1 Programming languages 2 Eclipse and CVS Today 1 CVS and project submission 2 Basics of Java Programs 9 6 2006 CMSC 131 Fall 2006 Rance Cleaveland 2006 Univeristy of Maryland 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 Fall 2006 Rance Cleaveland 2006 University of Maryland 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 Fall 2006 Rance Cleaveland 2006 University of Maryland 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 Fall 2006 Rance Cleaveland 2006 University of Maryland 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 Fall 2006 Rance Cleaveland 2006 University of Maryland 4 A Sample Java Program Example1 java This is a very basic Java program to get things started public class Example1 Class declaration Comments main is where the program starts public static void main String args int secondsPerMinute 60 int minutesPerLecture 50 int totalSeconds secondsPerMinute minutesPerLecture System out println There are totalSeconds seconds in a lecture CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 5 Comments Class Comments explanations added by programmer Two styles to end of line Comments are essential for good programming Classes Classes are key components of Java programs Class a collection of data and associated operations data instance variables operations methods One method in this program main CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 6 Java Program cont Example1 java This is a very basic Java program to get things started public class Example1 Method header main is where the program starts public static void main String args int secondsPerMinute 60 int minutesPerLecture 50 int totalSeconds secondsPerMinute minutesPerLecture System out println There are totalSeconds seconds in a lecture Statements CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 7 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 Fall 2006 Rance Cleaveland 2006 University of Maryland 8 Statements Basic commands in a program Two kinds of statements in this program Variable declarations int secondsPerMinute 60 Introduce a named storage location secondsPerMinute Define form of data that can be stored int integer Optional Give initial value 60 Method invocation System out println There are Calls to methods in objects class instances Method is println in subobject out within object System CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 9 Objects Bundles of data instance variables and methods Created using classes as templates We ll learn more later this semester CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 10 Executing the Program This is a very basic Java program to get things started public class Example1 main is where the program starts public static void main String args int secondsPerMinute 60 int minutesPerLecture 50 int totalSeconds secondsPerMinute minutesPerLecture System out println There are totalSeconds seconds in a lecture Variable Value secondsPerMinute 60 minutesPerLecture 50 totalSeconds 3000 There are 3000 seconds in a lecture CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 11 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 Example1 contents of the class go here A class consist of data variables and operations methods CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 12 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 Fall 2006 Rance Cleaveland 2006 University of Maryland 13 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 Fall 2006 Rance Cleaveland 2006 University of Maryland 14 Types Two references to the form of values Method headers Variables Type the form of a data value in Java CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 15 Built in Primitive Types Integers Reals Other Type name byte short int long float double char boolean CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland Size bytes 1 2 4 8 4 8 2 1 16 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 Fall 2006 Rance Cleaveland 2006 University of Maryland 17 Example 2 Basic Types public class Example2 public static void main String args int i1 i2 i3 double f1 f2 boolean b1 b2 char c String s i1
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