1 1 Your First Java Program CS101 Introduction to Computer Science Slides adapted from Sedgewick and Wayne Copyright 2007 Parlez vous Java Group Exercise Form teams of 3 students CS101 Introduction to Computer Science Slides adapted from Sedgewick and Wayne Copyright 2007 2 Code Snippet 1 int a 20 int b 30 int t t a a b b t Swap the contents of variables a and b a 30 b 20 3 Code Snippet 2 int a 15 if a 2 0 System out println Yes else System out println No Checks whether a number is even or odd Modulo operator Calculates the reminder Checks for equality Program will print NO 4 Code Snippet 3 int n 7 int i 2 int flag 0 while i n if n i 0 System out println No flag 1 break i i 1 if flag 0 System out println Yes Checks whether n is prime Program will print YES 5 Create Compile Execute CS101 Introduction to Computer Science Slides adapted from Sedgewick and Wayne Copyright 2007 Create Compile Execute Create the program You express the computation you wish to perform algorithm using a language that YOU can understand The language should also be precise enough for the computer to understand High level language Java Java provides A set of constructs analogous to vocabulary A set of rules on how to use these constructs in a program analogous to grammar 7 A Rich Subset of the Java Language Built In Types System Math Library int double System out println Math sin Math cos long String System out print Math log Math exp char boolean System out printf Math sqrt Math pow Math min Math max Math abs Math PI Flow Control Parsing if else Integer parseInt for while Double parseDouble Boolean Punctuation Primitive Numeric Types true false String Arrays Objects a i class static length compareTo new public private charAt matches a length toString equals new main 8 How Does Your Program Run on the Hardware Hardware itself uses a very low level language called Machine Language to run the program Machine language consists of a set of very simple constructs called instructions that manipulate 0s and 1s Hardware also provides the means to store the 1s and 0s that are generated by a program as it runs a computation Need to convert the Java program to an equivalent machine language program that the hardware understands 9 Create Compile Execute Compile the program Compiler is itself a very sophisticated program Takes the Java program as input and converts it to the corresponding sequence of machine instructions Inspects the program you wrote to see whether you have followed the conventions of the language syntax correctly Compiler doesn t know what you are trying to compute though semantics Execute Computer runs the sequence of machine instructions produced by the compiler 10 Programming in Java Prints Hello World Everyone s first Java program public class HelloWorld public static void main String args System out println Hello World HelloWorld java 11 Using an Interactive Development Environment IDE to Create Compile Execute An IDE is a software tool that Includes a smart editor for your language Lets you compile all your Java from within the tool Lets you run the compiled program from within the tool Supports debugging Supports many other programmer needs especially for large programs Example IDEs for Java DrJava for beginners Eclipse powerful Important to know how Java programs are built and run without using IDEs 12 Dr Java Demo Where do I get this software Instructions for installing Java and Mac are available on Collab slides page 13 Programming in Java Programming in Java Create the program by typing it into a text editor or Dr Java editor and save it as HelloWorld java Compile it by typing at the command line javac HelloWorld java command line javac HelloWorld java or click the Compile button in your IDE This creates a Java bytecode file named HelloWorld class 14 Programming in Java Programming in Java Create the program by typing it into a text editor and save it as HelloWorld java Compile it by typing at the command line javac HelloWorld java Execute it by typing at the command line java HelloWorld command line javac HelloWorld java java HelloWorld Hello World or click the Run button in your IDE 15 Recap of Syntax Prints Hello World Everyone s first Java program public class HelloWorld public static void main String args System out println Hello World A Java program is created in a java file First line defines the class program and the name Name there must match name of java file Second line defines main method For now your program is set of statements in main 16 Recap of Syntax Prints Hello World Everyone s first Java program public class HelloWorld public static void main String args System out println Hello World A Java statement ends in a semi colon Sometimes statements are grouped inside and Curly brackets must nest properly Comments are indicated by or by How Java treats whitespace Indentation and spacing are important of human readers 17 Recap of Creating and Running Java Programs Java programs java files are compiled This creates a class file The class file contains machine instructions To run the program execute the class file 18 Variables Every data item used by a program is stored in a variable Variables also provide a way of communicating to the machine hardware that the data needs to be stored somewhere A variable has three parts 1 2 3 Name e g x Type e g int double String Value 19 Variables Variables are declared by the programmer E g int x How does a variable get a value Literal Can be initialized in the E g int x 10 Can be computed when running program Value the program is E g x y z y and z are integers too 20 Methods Methods have a name and invoke some operation or function on some data System out println Hello World Some methods return a value that can be used Math sqrt 100 Methods like these are part of Java s Standard Library 21 A More Complicated Example Program This is a comment at the top of class program Demo1 public class Demo1 public static void main String args program statements go inside method main System out println Hello world int x 3 System out println The value of variable x is x x x 3 System out println Now the value of variable x is x continued 22 More Complicated Example Program Continued double someValue Math sqrt x System out println The sqrt of x is someValue System out println The sqrt of 100 is Math sqrt 100 we re done 23 Next Class Built In Java Datatypes CS101 Introduction to Computer Science Slides adapted from Sedgewick and Wayne Copyright 2007 24
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