What is Computer Science?• The process of finding digital solutions to real world problems• Examples

– ACORNS: support tribal efforts to revitalize and restore culture and language

– Global Positioning: computer based computations that map places on the earth

– Weather Models: Predict climate change– Robotics: Create devices that can assist in every day life– Data Mining: Extract information from huge amounts of data– Web services: Create portals for interacting with web-sites– Security: Protecting computer systems from attack– Cognition: Create programs to help stroke victims

This class: teach foundational skills• Foundational Skills

– Computer programming consists of those skills that provide a foundation to getting a computer to do anything that we can imagine

– A program is those instructions needed to accomplish some useful task

– This class introduces you to the skills needed to design, implement, and make programs work

• Computer Scientists– Lifelong learning those aspects of real world problems so

solutions can be digitized– Communication skills to be able to relate to people in other

disciplines without technical background

Computers and memory• Computer memory is a

huge table of numbers• This table holds

program instructions and data

• Each spot in the table is called a location and is numbered

• A possible snapshot of memory is on the left

Location Contents

…

1000223 Instruction

1000224 Instruction

…

1001353 Instruction

1002345 Data

…

1002499 Data

…

Note: Computers also have locations within the processing unit (the ‘brain’)called registers. These are very fast, but not actually part of memory.

Computer instructions• Computer instructions tell the computer what to do

• Each instruction is simply a numeric value that the computer processing unit (CPU) understands

• Computers execute instructions one at a time in order

• A program is a group of instructions that performs some useful task

• An application is a group of programs that work together to mimic some real world task

• A small sample of possible machine instructions:– Load the contents of a memory location into a CPU register– Add the contents of two registers together– Store a register back into a memory location– Input from the keyboard into memory– Output from memory to the disk

How computers work

1. The CPU loads the next instruction from the program counter (PC) into the CPU

2. Execute the instruction3. Increment the PC4. Go back to step 1

Note: A branch is an instruction that tells the CPU to change the PC to another location that is far away.

How do we write programs?• Machine Language: We need to memorize all of the numeric numbers for

every possible instruction. Not fun!!

• Assembly Language: We need to know all of the computer’s instructions, but using names instead of numbers. An assembler program converts the names we use to machine language so it can run. Still hard, and only runs on a single type of computer (platform) !

• High level: We write instructions that are more English like. A compiler program converts to machine language so the program can run.

• Scripting Language: We write instructions into a text file. A program interprets the instruction as it processes the text file.

• Markup Language: Scatter commands throughout a text file

• Declarative Special Purpose: Instructions are an easy-to-use syntax. A program interprets these instructions and executes them.

• Natural Language: We speak to the computer in our native languages (like in Star Trek).

Compilers and Interpreters• Compiler (Example: Translate a book to French once)

– Translate the program’s instructions machine code before it executes– Advantage: executes fast– Disadvantage: Must recompile for each computer platform, hard to debug

• Interpreter( Example: A translator interprets as a speech is given)– Interprets the instructions as the program runs– Advantage: easier to create programs and debug– Disadvantage: slow

• Hybrid– Compile instruction partially, and interpret or compile the rest of the way

(just-in-time technology) as the program runs– Advantage: Don’t have to recompile for each computer platform, just-in-

time technology achieves speeds almost as fast as a compiled language– Disadvantage: Still somewhat slower than compiled languages

Programming languages• High Level: C, C++, Fortran, Cobol• Scripting Languages: php, JavaScript, asp, perl, awk• Markup Languages: HTML, XML, postscript• Declarative: SQL• Hybrid: Java, Visual Basic• Natural Language: None yet

Note: The syntax (grammatical rules) is similar for C, C++, php, JavaScript, and Perl. The kinds of instructions of many of the others is vey similar.Note: Byte code defines the format of a partially Java compiled program which a Java Virtual Machine (JVM) understands.

Note: Why Java? Answer: Java is at this time the most used programming language in the world, followed closely by C and C++. Visual Basic is 4th.

More on memory• Computer memory is measured in bytes (ex: 2gigabytes [2gb])• A bit is an electronic circuit

______/ ______ (off = 0)______________ (on = 1)

• A byte is eight bits strung together

• Questions:– How many unique numbers can a byte hold?– If half of the numbers are negative, what is the possible range?

128 64 32 16 8 4 2 1

1 1 1 0 1 0 0 1

0 0 1 1 1 1 0 0

0 1 0 0 1 1 0 0

0 1 0 1 0 1 0 1

Encodings

• Hexadecimal groups four bits together– Sixteen possible values: 0, 1, …, 9, A, B, C, D, E, F– Used to represent colors, two digits for red, blue and green

(Ex: FF0000 for Red).• Letters

– The letters of the keyboard are each given a numeric value.– Capital A: 65– Capital B: 66– Space: 32– The letter 1: 49

Definitions

• Class: A blueprint for making objects• Object: A collection of properties and methods• Method: A block of instructions called by name• Variable: A place in memory that can hold a value• Property: A variable that is in an object• Instance Variable: Synonym for property• Identifier: The name we give to a variable• Literal: A built-in constant (like 15.3 or -1).• Call a method: Cause the method’s instructions to execute• Instantiation: The creating of an object from a class• Static: A method or variable that goes with a class

Memorize now, understand later

More Definitions

• Expression: A series of operands and operators• Argument: An expression passed to a method• Parameter: An identifier in a method referring to an argument• Type: A category of data

– String: A sequence of letters– Integer: int, byte, long– Fractions: float, double– Boolean: true or false value

• Declaring: creating a variable so it can be used• Array: An indexed table of variables

Syntax

• Statement: A single program language instruction (sentence)• Semi-colon: Java statements MUST end with semi-colons• Braces: ({}) enclose a block of instructions• Comments (Ignored by Java)

– /* … */ multi-line– // the rest of a line

• Square braces ([]): used to declare and access arrays• Parenthesis: enclose expressions, arguments, or parameters• Quotes: enclose the characters of a string literal (like “abc”)• Case sensitive: abc is different than Abc (be careful!!)• Reserved words (always lower case): Part of the Java

language. Examples include: static, void, true, false, and class.

The grammar of a language

First Java program

public class HelloWorld{ public static void main(String args[])

{ System.out.println(“Hello World”);}

} // End of Hello World class

Note: Every Java program must have a main methodQuestions: Which are the reserved words?

There are lots of important concepts here to discuss

.java, .class, and .jar• Java Source Files

– Pure text file containing Java program statements– Notepad++ or Notepad are programs for entering pure text– Word can be used, but you must save as plain text– We will use an integrated development environment– Java source files should be saved with a .java extension

• Java byte code files– Compile java source file into byte code– Java byte code files have a .class extension– Execute java byte code using an Java Virtual Machine (JVM)

• Jar files– Compression that lumps lots of java byte code files together

Integrated Development Environment (IDE)

• Text book uses command line instructions (The hard way)1. Install Java, use Notepad++ to create a .java source file2. Click on start, run, then type cmd3. To compile: javac HelloWorld.java (create a .class bytecode file)4. To execute: java HelloWorld (execute the .class bytecode)

• We will use JGrasp (An easier way)1. Install Java, and then JGrasp2. Clcick on File, New, Java, and type the Java Program, save it

with a .java extension3. Click on green plus to compile and create a .class bytecode file4. Click on the red stick figure to execute

Other Free IDE’s: Java, Netbeans, Eclipse

Debugging

• History– Early computers consisted of mechanical levers and gears– A moth got stuck in one of these machines and it malfunctioned– They removed the moth (debugged), and everything was fine– That moth is in the Smithsonian to this day

• Why don’t programs work?– Syntax: The Java compiler will detect; fix and recompile– Management: Changes not saved, or saved in the wrong place– Logic: The instructions were not what you intended

• Hint: Make sure you understand before making random changes

Debugging is the process of getting a program to work

Review questions• What are some of the definitions that we discussed?• What is debugging?• What is bytecode?• What is the difference between compiling and interpreting?• What is a high level language?• What are .java, .class, and .jar files?• What is an IDE? State some examples.• How are the following symbols used in Java: ; {} [] () /* */ // “• What does case sensitive mean?• Describe bit, byte, binary and hexadecimal?• What is computer memory?