Lecture 5 Java Introduction CPE 401 / 601 Computer Network Systems slides are modified from Ricky Sethi

So what the heck is Java (besides coffee, that is)?  A programming language.  Syntax and constructs are very similar to C++  A virtual platform  Java Virtual Machine is a software “machine” or “hypothetical chip”  Since it’s “virtual”, it can be implemented on any hardware  Cross-platform distribution achieved via.class binary file of bytecodes (instead of machine-dependent machine code)  Write Once, Run Anywhere  A class library  Standard APIs for GUI, data storage, processing, I/O, and networking. Java Introduction 2

Getting Java Brewing… 1.Download the latest Java SDK from  The SDK is a command-line based set of tools 2.A Text Editor 3.Web-browser that’s java-enabled (optional) 4.Some introductory links/guides/tutorials:  Java1/compile.html   Java Introduction 3

Mechanics of Writing Java Programs  Create a Java source file.  Must have the.java extension and contain only one public class.  Compile the source file into a bytecode file.  The Java compiler, javac, takes your source file and translates its text into instructions that the Java Virtual Machine (Java VM) can understand. The compiler puts these instructions into a.class bytecode file.  Run the program contained in the bytecode file.  The Java VM is implemented by a Java interpreter, java. This interpreter takes your bytecode file and carries out the instructions by translating them into instructions that your computer can understand. Java Introduction 4

Putting it all together public class Hello { public static void main(String args[]) { System.out.println( “ Hello, world! ” ); } 1.Put in: Hello.java 2.Compile with: javac Hello.java  Creates Hello.class 3.Run with: java Hello Java Introduction 5

Applications vs. Applets  A Java application:  Is a standalone program  Is interpreted by the Java Virtual Machine and run using the java command  Contains a main() method.  A Java applet:  Runs within a Java-enabled Web browser  extends the Applet or JApplet class (Inheritance)  Contains an init() or a paint() method (or both).  To create an applet, you'll perform the same basic steps: 1. Create a Java source file ( NameOfProgram.java ) and an HTML file ( NameOfHTMLFile.html ) 2. Compile the source file ( NameOfProgram.class ) 3. Run the program (either using java NameOfProgram or appletviewer NameOfHTMLFile.html ) Java Introduction 6

Java notes for C++ programmers  Everything’s an object  Every object inherits from java.lang.Object  No code outside of the class definition!  No global variables (use static variables instead)  Single inheritance only  Instead, implement interfaces  All classes are defined in.java files  One top level public class per file The file has to have the same name as the public class!  Syntax is similar (control structures are very similar).  Primitive data types are similar  But a bool is not an int  To print to stdout, use System.out.println() Java Introduction 7

Why Java?  Network Programming in Java is very different than in C/C++  much more language support Networking is at the core of the language  well defined error handling  no global variables  no struct, union types, goto’s, enums, bitfields, typedefs  no pointers! (garbage collection)  Threads are part of the language.  some support for common application level protocols (HTTP). Java Introduction 8

Requisite First Program (Application Version) Put in HelloWorld.java: public class HelloWorld { public static void main(String args[]) { System.out.println("Hello World"); } Java Introduction 9

Compiling and Running HelloWorld.java javac HelloWorld.java java HelloWorld HelloWorld.class compile run bytecode source code Java Introduction 10

So what’s going on? The Java bytecode and interpreter at work!  Bytecode is an intermediate representation of the program (the class file)  Think of it as the machine-code for the Java Virtual Machine  The Java interpreter ( java ) starts up a new “Virtual Machine”  The VM starts executing the user’s class by running its main() method Java Introduction 11

Put in HelloWorld.java: import java.awt.Graphics; public class HelloWorld extends java.applet.Applet { public void paint(Graphics g) { g.drawString("Hello World“, 35, 15); } Put in test.html: Test the applet Test the applet Requisite First Program (Applet Version) Java Introduction 12

Java Language Basics  Data types same as in C++ (except bool )  bool,char,byte,short,int,long,float, double,string, etc.  Operators (same as C++)  Assignment: =, +=, -=, *=, …  Numeric: +, -, *, /, %, ++, --, …  Relational: ==. !=,, =, …  Boolean: &&, ||, !  Bitwise: &, |, ^, ~, >, …  Control Structures  more of what you expect: 1. conditional: if, if else, switch 2. loop: while, for, do 3. break and continue not an int! Java Introduction 13

Classes, References, & Packages  Classes and Objects  “All Java statements appear within methods, and all methods are defined within classes”.  Java classes are very similar to C++ classes (same concepts).  Instead of a “standard library”, Java provides a lot of Class implementations or packages  What are packages?  You can organize a bunch of classes and interfaces into a package (or library of classes) defines a namespace that contains all the classes.  Use the import keyword to include the packages you need import java.applet.*;  You need to use some java packages in your programs java.io (for Files, etc.), java.util (for Vectors, etc.)  References  No pointers  everything’s a reference!  classes, arrays Java Introduction 14

Exceptions  When a program carries out an illegal action, an exception is generated.  Terminology:  throw an exception: signal (in the method header) that some condition or error has occurred but we want to pass the buck and not deal with it.  catch an exception: deal with the error (or whatever) ourselves inside the function/method.  Catch it using a try/catch block (next slide).  In Java, exception handling is necessary  forced by the compiler  compilation errors!  Except for RunTimeExceptions Java Introduction 15

Try/Catch/Finally try { // code that can throw an exception } catch (ExceptionType1 e1) { // code to handle the exception } catch (ExceptionType2 e2) { // code to handle the exception } catch (Exception e) { // code to handle other exceptions } finally { // code to run after try or any catch } This block is always run Java Introduction 16

Exception Handling  Exceptions take care of handling errors  instead of returning an error, some method calls will throw an exception.  Can be dealt with at any point in the method invocation stack.  But if no method in the hierarchy handles it, results in an unchecked exception which generates a compiler error (unless it’s a RunTimeException)  Forces the programmer to be aware of what errors can occur and to deal with them. Java Introduction 17

Defining a Class  One top level public class per.java file.  Typically end up with many.java files for a single program with at least one containing a static public main() method (if they’re applications).  Class name must match the file name!  The compiler/interpreter use class names to figure out what the file name is.  Classes have these three features:  A constructor that’s used to allocate memory for the object, initiailize its elements, and return a reference to the object  Methods (function members)  Fields (data members) Java Introduction 18

A Sample Class public class Point { public Point(double x, double y) { this.x = x; this.y=y; } public double distanceFromOrigin(){ return Math.sqrt(x*x+y*y); } private double x,y; } Java Introduction 19

Objects and new  You can declare a variable that can hold an object: Point p;  But this doesn’t create the object! You have to use new : Point p = new Point(3.1,2.4);  new allocates memory and the garbage collector reclaims unused memory Java Introduction 20

Using Java objects  Just like C++:  object.method() or object.field  BUT, never like this (no pointers!)  object->method() or object->field  Event driven model:  Objects “register” to receive (and respond to) certain messages like button presses, mouse clicks, etc. (e.g., mouseUp(), mouseDown(), keyUp(), keyDown() ) Java Introduction 21

Strings are special  You can initialize Strings like this: String blah = "I am a literal ";  Or this ( + String operator): String foo = "I love " + “CET375";  Or this ( new operator): String foo = new String(“Yummy FooBars!”); Java Introduction 22

Arrays  Arrays are supported as a second kind of reference type (objects are the other reference type).  Although the way the language supports arrays is different than with C++, much of the syntax is compatible.  however, creating an array requires new  Index starts at 0.  Arrays can’t shrink or grow.  e.g., use Vector instead.  Each element is initialized.  Array bounds checking (no overflow!)  ArrayIndexOutOfBoundsException  Arrays have a.length  You can use array literals like C/C++ (no need for new keyword): Java Introduction 23

Array Examples Java Introduction 24 int x[] = new int[1000]; byte[] buff = new byte[256]; float[][] mvals = new float[10][10]; int[] values; int total=0; for (int i=0;i<value.length;i++) { total += values[i]; } String[] names = {“Joe”, “Sam”};

Java Introduction Reference Types  Objects and Arrays are reference types  Primitive types are stored as values  Reference type variables are stored as references (pointers that we can’t mess with) int x=3; int y=x; Point p = new Point(2.3,4.2); Point t = p; There are two copies of the value 3 in memory There is only one Point object in memory! 25

Passing arguments to methods  Primitive types: the method gets a copy of the value. Changes won’t show up in the caller  Pass by value  Reference types: the method gets a copy of the reference, the method accesses the same object  Pass by reference  There is no pass by pointers! Java Introduction 26

Comparing Reference Types  Comparison using == means:  “Are the references the same?” Do they refer to the same object?  Sometimes you just want to know if two objects/arrays are identical copies.  Use the.equals() method You need to write this for your own classes!  All objects and arrays are references! Java Introduction 27

Inheritance  Use the extends keyword to inherit from a super (or parent) class  No multiple inheritance  Use implements to implement multiple interfaces (abstract, virtual classes)  Use import instead of #include (not exactly the same but pretty close) to include packages (libraries) Java Introduction 28

Concurrent Multi-threaded Programming  Java is multithreaded!  Threads are easy to use.  Two ways to create new threads:  Extend java.lang.Thread Override “ run() ” method.  Implement Runnable interface Include a “ run() ” method in your class.  Usually, you’ll implement the Runnable interface  How to implement the Runnable interface:  Add a public void start() function: This is where you’ll initialize the thread and start() it  Add a public void stop() function: This is where you’ll set the boolean stopFlag to true  Add a public void run() function: This is where you’ll call repaint() to paint each new frame and handle any synchronized variables or methods 29

The synchronized Statement  Instead of mutex (a binary semaphore), use synchronized : synchronized ( object ) { // critical code here }  Also, declare a method as synchronized : synchronized int blah(String x) { // blah blah blah }  Can also use wait() and notify() to put threads on hold and wake them up again (e.g., to implement a pause or suspend feature)  Must be called within a synchronized block Java Introduction 30

Using Documentation Comments  Documentation comments are delimited by /** and */  javadoc automatically generates documentation  Copies the first sentence of each documentation comment to a summary table Write the first sentence with some care!  For each method/class, supply: followed by the parameter name and a short explanation followed by a description of the return value for author info, etc. Need to use “ javadoc –author ” for this… Java Introduction 31

javadoc  The Java Standard calls for every class, every method, every parameter, and every return value to have a comment  Write the method comments first!  If you can’t explain what a class or method does, you aren’t ready to implement it!  How to create HTML documentation:  Type: javadoc *.java in the directory containing your source code  This produces one HTML file for each class and an index.html file  Documenation is together with code! Must come immediately before the class, method, etc. Java Introduction 32