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Lecture 2: Do you speak Java?

Lecture 2: Do you speak Java?. From Problem to Program. Last Lecture we looked at modeling with objects! Steps to solving a business problem Investigate current problem (Analysis) Transform this model to logical models (Design) Transform this model to software (implementation)

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Lecture 2: Do you speak Java?

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  1. Lecture 2: Do you speak Java?

  2. From Problem to Program • Last Lecture we looked at modeling with objects! • Steps to solving a business problem • Investigate current problem (Analysis) • Transform this model to logical models (Design) • Transform this model to software (implementation) • Need to translate model to something computers can understand (mathematical equations)

  3. Parlez Vous Binary? • 1001110000010101010000001111 • How do we translate our business problem into machine code? • Initially, a programmer writes a program in a particular programming language. This is called source code!!!!! • To execute the program however, the programmer must translate it into machine language……the language that the computer understands. How do we do this? • A COMPILER does this!!!!!

  4. Compiler • A compiler converts source code into machine level instructions • Almost every programming language comes with a compiler • In effect, the compiler is the language because it defines which instructions are acceptable

  5. Javac • The Java compiler (javac) is the component of the Java Developers kit (JDK) USED TO TRANSFORM Java source code files into bytecode (not machine code) • Javac phil.java • Bytecode is the compiled format for Java programs. Once a Java program has been converted to bytecode, it can be transferred across a network and executed by Java Virtual Machine

  6. Java Virtual Machine • Bytecode is not really machine code and cannot be used by a computer – it must be interpreted • The Java Virtual Machine (JVM) is an interpreter, it interprets byte code and can thus execute quickly • The Java interpreter is built into Java compatible web browsers such as Netscape navigator which enables applets to run

  7. Why Compile and Interpret? • Because byte code is used instead of machine code, a java program can be executed on any machine that has a Java interpreter • This makes java extremely portable……a program can be compiled on one machine and executed on any • Java used extensively on web sites • Java not only offers portability but identical behaviour on different systems • “WRITE ONCE, RUN EVERYWHERE”

  8. Compiling and interpreting • Java source code is written with a text editor and saved in plain ASCII text with file extension .java • These files are compiled using the javac compiler • E.g. javac Examplehelloworld.java • Executable bytecode class files have the extension .class and they represent a Java class in its usable form • The Java compiler generates exactly one .class file for each class you create. It is possible to define more than one class in a single source file, it is therefore possible for the compiler to generate multiple class files from a single source file.

  9. Java and the Web • Applet written and compiled in exact same way as Java application • Applets downloaded when user navigates to page • Browsers contain Java virtual machine and applet is executed on client machine

  10. Java Applets • Applets were originally small applications but there is no size restriction on Applets • As applets are downloaded, there is a risk that they may • Contain Bugs • Be written maliously • However, browsers are usually configured to prevent applets from accessing files on the machine on which they execute • Applets can access files on the server from which they were downloaded • Java applications will be covered in this course but applets will be revisited

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