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Compiling and the Java Virtual Machine (JVM)

Compiling and the Java Virtual Machine (JVM). Working with a real language. The syntax of Pseudocode is pretty loose visual validation encourages a permissive approach emphasized the importance of structure Compilers essentially require perfection.

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Compiling and the Java Virtual Machine (JVM)

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  1. Compiling and the Java Virtual Machine (JVM)

  2. Working with a real language • The syntax of Pseudocode is pretty loose • visual validation encourages a permissive approach • emphasized the importance of structure • Compilers essentially require perfection. • The procedure for writing your algorithms won’t change (much!) • Write a high level approach focusing on algorithm. • Write the code concentrating on details. • Check it by hand before compiling. • Because we're introducing a real language at the same time as we're introducing the object oriented paradigm we will require you to just write some code. • You may not understand it all immediately • Eventually we'll explain!

  3. The First Part • Initially we’ll use Java to write procedural code just to let you get the feel of a real language. • In fact, we’ll start by programming some of the same types of modules that we used in the procedural pseudocode earlier in the semester. • Then, as we introduce the Object Oriented paradigm we’ll use Java as it was intended.

  4. About Java

  5. Introduction to Java • What Java is: • A real professional programming language (which is good and bad...) • Portable across any hardware platform that has a JVM interpreter (more later) • An object-oriented language • What Java is not: • “The Ultimate Programming Language” • HTML or another web-content language • Only useful for web applets • Just Another Vacuous Acronym

  6. Real Languages • Real languages require the programmer to write code using a very specific set of rules, keywords and syntax. • Then this code is transformed into actual instructions that a specific machine can execute. [Often a complex process] • A number of strategies have been invented to accomplish this process • Assemblers • Compilers • Interpreters • A traditional problem has been the necessity to have different versions of a program for different machines.

  7. A New Idea • Java was specifically developed to be able to write code once and run it anywhere • How is this magic accomplished? • Using an intermediate language! Byte code. • The Byte code is interpreted (executed) using a special piece of software (a program) called the Java Virtual Machine (JVM)

  8. Compilation “Source Code” [.java] Java compiler Generic “Byte Code” [.class] Execute program OS-specific “Object Code” OS-specificJVM interpreter Need one of these for every different OS

  9. Structure of Java Programs • Initially we’ll write programs that fit in one file. • Create the file with an editor (e.g. emacs) • Compile with javac producing a .class file • Execute by running java and sending it the .class file • Our first (tiny) program will be roughly like a cross between an algorithm and a procedure. • Let’s take a look...

  10. Sample Application We create a file (using an editor) called “HelloWorld.java” public class HelloWorld { public static void main(String argv[]) { System.out.println(“Hello World!”); } } We compile by typing (at the OS prompt): javac HelloWorld.java Which produces HelloWorld.class Then we execute by typing: java HelloWorld Hello World!

  11. Demo >javac HelloWorld.java >java HelloWorld Hello World! >

  12. Quick Trix • The name of the file must match the name of the class EXACTLY!!! • File: Bubba.java • Contains: • Everything must be EXACTLY correct!!! Capitalization counts class Bubba {...

  13. Eventually... • Applications* (“normal” computer programs): • Each program consists of multiple files. • Each file contains a class. • Each class will contain modules which will resemble procedures and functions. • THE MODULES WILL BE CALLED METHODS • The .class file that you send to java must contain a method named main • It will actually look like this: public static void main(String argv[] ) { .. } • the JVM will use the file naming convention to find the other classes required by this main program. *As opposed to Applets

  14. Some basic syntax issues • Your TA is smarter than the java compiler • Lines need to terminate with a ; • Easier said than done • Braces will be used to indicate "blocks" of code • Which essentially act like a single line public class HelloWorld { public static void main(String argv[]) { System.out.println(“Hello World!”); } }

  15. Pseudocode if some_boolean then a <- a + 1 else a <- a + 2 b <- 7 endif Java if(some_boolean) a = a + 1; else { a = a + 2; b = 7; } A look ahead... Note ; placement Note: Indentation is used in both cases. Means nothing to compiler. = instead of <- (more later) Must have parens in Java

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