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CSC204 – Programming I

Lecture 2Intro to OOP with Java

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Topics Ways to compile and execute a program Java’s approach Considerations for software design Object and class [Running a simple Java program]

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Automation Starts from Programs What is a program?

How can the instructions defined in a program be executed? They need to be translated into machine code first

public class AutomatedProcess { public static void main(String[] args) { System.out.println("Step #1"); System.out.println("Step #2"); System.out.println("Step #3"); }}

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processor

A Simple Computer

012345

4194300419430141943024194303

diskkeyboard

display

printer

memory

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Compilation The program’s source code is given to a

program called a compiler. The compiler checks that the source code

is valid (obeys the rules of the language) and translates it to machine instructions for a particular CPU.

The compiled program (also know as executable) is stored in a file, and it can be run as many times as desired.

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Compilation & Execution

CompiledFile

(a.out)

Source Code

(xxx.c)

text filesaved on disk

editor

compile

edit compiler

Instructions inmachine languageCPU specificsaved on disk

Memory

CPU

load

Execution

Compilation

Edition

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Interpretation The program’s source code is given to a

program known as an interpreter. The interpreter executes the program

without first translating it to machine instructions.

The interpreter itself is normally a compiled program, so it can execute machine instructions corresponding to the source code.

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Interpretation & Execution

Internalfile

Source Code

(xxx.c)

text file

editor

interpret

edit interpreter

Instructions inmachine languageCPU specific

CPU

Interpretation & Execution

Memory

Edition

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Problems w/ Networked Computers Different machines are interconnected to each

other in a interconnected farm of machines Each of them may uses a different machine

language How can a program be executed on these

different machines?

System A System B

X

How can it run on System B?

Compiled program compiled on System A

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Java’s Approach - illustrated

System A System B System N...

JVM-A JVM-B JVM-N

Compiled Java byte code (not in any specific machine language)

Compiled once and runs everywhere

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Java’s Approach Java employs a combination of compilation and

interpretation. The Java compiler translates the original program

into bytecode instructions for a computer called the Java Virtual Machine.

The resulting bytecode program is then executed by an interpreter.

One advantage of Java’s approach is that programs don’t need a particular CPU or operating system.

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Bytecodeprogram

(xxx.class)

Source Code

(xxx.java)

text filesaved on disk

editor

compile

edit Compiler(javac)

Instructions inbytecodesaved on disk

CPU

load

Execution

Compilation

Bytecode and JVM

Memory

JVM

interpret

Edition

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Ways to Handle Complexity S/W can be considered as virtual machineries that

automate some information processes These systems consist of virtual s/w parts Ways to handle the complexity

Abstraction: consider only the essential aspect of a real world information process

Encapsulation: don’t tell us how to make the ice cubes, just let us know which button to press to get them

Modularity: I need a car with 4 wheels, two doors, ... Hierarchy:

Composition: each book has a number of chapters, each ... Taxonomy: a bird is an animal that can fly, an eagle is ...

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Abstraction

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Encapsulation

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Modularity

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Hierarchy: composition

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Hierarchy: taxonomy

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What is an Object? An object is a s/w entity that can do something

for other part of the s/w system (another object) An object is a runtime entity, it only exists when

the s/w system is in execution A class, or the template of objects of the same kind is

defined by a program (source code) A class file (executable) is created as a result of

compilation. It is loaded into memory when executed Objects can then be instantiated using the class

definition

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How Do Objects Work Each Other? we can refer to the object that does sth a server,

the one that requests the service a client From the client perspective, how can you get the

service (alarm)? Passing a message to the server (clock):clock, please set the alarm to 7 am sharp!Recipient service (or operation) additional info

clock.setAlarm(7); From the server’s perspective, it is responsible for

Maintain certain properties: such the due time at which the alarm will ring

Be able to perform certain operations: allowing clients to set alarm and starting the alarm at due time

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Steps for Java Programming Analysis Design Implementation

Edit your code Compile your code

Testing Execute your code (using test data) [Debug your code]

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A Quick Walkthrough Open your favorite text editor (e.g. NotePad) Copy the code snippet (on slide #3) into a blank

text file Save it as c:\myname\AutomatedProcess.java

Create a folder c:\myname as shown in class Open a DOS prompt Change directory to the folder in which you saved

the .java file Type javac AutomatedProcess.java to compile Type dir to check the bytecode executable you

just created: AutomatedProcess.class Type java AutomatedProcess to execute