Intro to Computers and Java

Why Program?

• We rely on computers in our day-to-day lives
  • e.g., word processing, games, etc.
• Computer programmers implement programs that perform functions
• Programming is an art and a science.
  • Art: Careful design, mathematical procedures, UI, manuals, etc.
  • Science: Analytical correctness, strict syntax, experimentation (debugging)

Computer Systems: Hardware and Software

• Hardware: Tangible physical pieces of the computer
  • Major Parts:
    • CPU, Main Memory, Secondary Memory, I/O Devices
• Software: Intangible

CPU

• CPU as I/O Machine:
  • Instruction -> CPU (Arithmetic Logic Unit & Control Unit) -> Result
  • To process program info, the CPU performs the fetch, decode, execute cycle
    1. Fetch: a
    2. Decode: b
    3. Execute: c

Main Memory

• Aka: Random-access memory (RAM)
• Contains:
  • Sequence of instructions of the running programs
  • Data used by those programs
• Divided into units called bytes
• A byte consists of eight bits that may be either on or off
  • Bits form a pattern that represents a char or number
  • Smallest unit of measurement for a computer
  • Each byte in memory is assigned a unique number known as an address.
• RAM is volatile, stuff in RAM is loss on power-off.

Secondary Memory

• Can store info for longer periods of time (non-volatile)
• Common Sources:
  • HDD
  • SSD
  • External drive
  • USB drive
  • CD/DVD drive

Why Are SSDs Faster than HDDs?

• HDDs are essentially spinning metal platters while SSDs store and retrieve data using only flash-memory chips without any moving mechanical parts.

Input Devices

Any data the computer collects from the outside world

Output Devices

Software

Operating Systems

Set of programs that manages the computer’s hardware devices and controls their processes.

Most modern OS are multitasking (multitasking !== parallel processing), or time sharing (can do more than one task over a given time period)

Application Software

Specialized environment for the user to work in.

• e.g., spreadsheets, games, tax software, etc.

This is the type of software we’ll be working with.

Programming Languages

Special language used to write computer programs

An algorithm is a set of well-defined sequential steps to completing a task.

A computer program is a set of instructions that enable the computer to solve a problem or perform a task

• A program usually implements algorithms.

A computer needs the algorithm to be written in machine language, which is written using binary.

• Each CPU has its own machine language
  • e.g., Motorola 68000, Intel x86, ARM, etc.
• Programming languages translate human-friendly text into machine language.
  • The first programming language was Assembly.
  • High-level programming languages aren’t processor dependent
    • e.g., Java, C, BASIC, COBOL, Python, etc.

Procedural Programming

Older programming languages were procedural.

• A procedure is a set of programming language statements that, together, perform a specific task.
• Procedures typically operate on data items separate from the procedures
• Data items are commonly passed from one procedure to another
• Procedures are developed to operate on the program’s data
• Data tends to be global to the entire program

Object-Oriented Programming

• Centers on creating objects rather than procedures
• Objects are a melding of data and procedures that manipulate that data
  • They are self-contained units
• Data in an object are known as attributes
• Procedures in an object are known as methods.
• OOP combines data and behavior via encapsulation
  • Data hiding is an object’s ability to hide data from other objects in the program.
  • Only an object’s methods should be able to directly manipulate its attributes
  • Other objects can only manipulate other objects’ attributes via its methods
    • This indirect access is known as a programming interface.

Software Engineering

Encompasses the whole process of crafting computer software.

• Software engineers perform several tasks:
  • Designing, Writing, Testing, Debugging, Documenting, Modifying, Maintaining
• Software engineers develop:
  • Program specifications, diagrams of screen output, pseudocode, examples of expect I/O, etc.

Software engineers use special software designed for testing programs.

• More commercial software applications are large and complex, requiring a team of programs.
  • Program requirements are thoroughly analyzed and divided into subtasks that are handled by individual teams or individuals within a team

Java History

1991:

• Green Team formed at Sun Microsystems to speculate about important technological trends in the future.
  • Concluded that computers would merge with consumer appliances
• There was a need for a programming language that would run on various consumer devices with different processors
  • Java (first named Oak) was developed for this purpose
  • Programs written in Java were translated into an intermediate language known as byte code.
    • Another program would translate the byte code into machine language that could be executed by the processor of a consumer device.
• Java applications can be of multiple types:
  • Desktop, web, mobile
  • Standalone or client-server applications in multiple layers

What Is a Program Made Of?

Common conceptions:

• Keywords (reserved)
  • e.g., “int”, “void”, “main” in Java (note: keywords are lowercase and case-sensitive in Java)
• Operators
• Punctuation
• Programmer-defined names (identifiers)
• Syntax (strict syntactic rules)
  • e.g., semicolons are used to end Java statements

Lines vs Statements

• Statement: A complete Java instruction that causes the computer to perform an action
• Line: A single line of code.
  • Blank lines are used to make a program more readable.

Variables

• Data in a Java program is stored in RAM
• Variable names represent a location in memory
  • The Java Virtual Machine (JVM) decides where the value will be placed in memory
• Variables are created by the programmer who assigns it a programmer-defined identifier

The Compiler and JVM

• A Java programmer writes Java programming statements
  • These statements are known as source code
• A text editor is used to edit and save a Java source code file
• Source code file shave a .java file extension
• A compiler is a program that translated source code into an executable form
  • Syntax errors will be discovered during compilation
    • Syntax errors are mistakes that the programmer has made that violate the rules of the programming language
• Most compilers translate source code into executable files containing machine code
  • The Java compiler translates java source code into byte code of the JVM to execute

What is a Program Made Of? (cont.)

• Java byte code cannot be run by the CPU, it needs to be run by the JVM.
  • Byte code end with the .class extension
  • The JVM emulates a microprocessor
    • This is why Java is called an interpreted language.

Java is both a compiled and interpreted language!

• Source code compiled into byte code
• Byte code interpreted by the JVM

Portability

• Portable: A program may be written on one type of computer and run on a wide variety of computers with little or no modification.
• Java programs are highly portable and can run on any OS with a JVM

Java Editions

Different editions of the Java Development Kit:

• SE: Standard Edition
• EE: Enterprise Edition
• ME: Micro Edition

Quick Reference: Compiling and Running Java Code

1. Compile Source Code to Byte Code: javac <file.java>
   • javac is the java compiler
   • Tip: javac *.java to compile all Java files in the CWD
2. Run Byte Code: java <classname>

e.g., javac HelloWorld.java && java HelloWorld

The Programming Process

1. Clearly define what the program should do.
2. Visualize the program running on the computer.
3. Use design tools to create a model of the program.
4. Check the model for logical errors.
5. Compiled the code
6. Correct errors found during compilation (cycle between 5 and 6)
7. ???

Types of Errors

1. Compile Error: x = 2
2. Run-Time Error: x = 2/0;
3. Logical Error: grossPay = hours + payrate;