Week 1 - Friday What did we talk about last time? Our first Java - - PowerPoint PPT Presentation

Week 1 - Friday

 What did we talk about last time?  Our first Java program  Hardware

 Storage for all the data and instructions on your computer  Modern computers store everything as binary digits (bits)

which have a value of 0 or 1. 1 byte = 8 bits 1 kilobyte (kb) = 210 bytes 1 megabyte (mb) = 220 bytes 1 gigabyte (gb) = 230 bytes 1 terabyte (tb) = 240 bytes

Cache

• Actually on the CPU
• Fast and expensive

RAM

• Primary memory for a desktop computer
• Pretty fast and relatively expensive

Flash Drive

• Faster than hard drives
• Seen on USB drives but SSDs are becoming common too

Hard Drive

• Secondary memory for a desktop computer
• Slow and cheap

Optical Drive

• Secondary memory that can usually only be written once
• Very slow and very cheap

Monitor

• Common visual output device

Speakers

• Common audio output device

Mouse

• Common input device

Keyboard

• Common input device

 Now that we've (sort of) defined computers, what is computer science?  The study of information, computation, and solving problems with

programs

 Subfields:

• Theoretical computer science
• Programming languages and compilers
• Operating systems and networking
• Graphics
• Numerical computing
• Information storage, retrieval, and security
• Architecture and hardware
• Artificial intelligence and machine learning

 Computers are stupid, but fast  Programming is the process of giving them very detailed

instructions about what to do

 Usually, programming is done in a rigid, formalized language,

like Java

 English is insufficient:

• E.g., "Computer! Solve my relationship problems!"
• Writing a program to solve your relationship problems in Java would

require you to be more detailed and explicit

 Computer science is built out of layers

(like a burrito)

 No one can understand everything  People tend to focus on a particular level

User Application

Operating System Hardware

We will program here

 The process of giving computers very detailed instructions

about what to do

 How do we do that exactly?  First, we need a programming language like Java  How do we turn a set of instructions written so that a human

can read them into a set of instructions that a computer can read?

 Magic, of course!

 There are many different programming languages:

• Java
• C/C++
• ML
• …thousands more

 Each has different advantages in different situations

 We can classify languages as high or low level  High level languages allow you to give more abstract

commands that are more like human thought processes or mathematics

 Low level languages are closer to the computer world and give

explicit instructions for the hardware to follow

Python Java C++ C Assembly Language Machine Code

Low High

 We use a program called a compiler to turn a high level

language into a low level language

 Usually, the low level language is machine code  With Java it's a little more complex

Computer! Solve a problem; 010101010 010100101 001110010

Execute

Source Code Machine Code Hardware

 Java is more complicated  Java runs on a virtual machine, called the JVM  Java is compiled to an intermediate stage called bytecode,

which is platform independent

 Then, the JVM runs a just-in-time compiler whenever you run

a Java program, to turn the bytecode into platform dependent machine code

class A { Problem p; p.solve(); } 101110101 101011010 110010011

JVM

010101010 010100101 001110010

Java Source Code Machine Code Hardware Java Bytecode

• 1. Write a program in Java
• 2. Compile the program into bytecode
• 3. Run the bytecode using the JVM (which automatically

compiles the bytecode to machine code)

Often goes through phases similar to the following:

• 1. Understand the problem
• 2. Plan a solution to the problem
• 3. Implement the solution in a programming language
• 4. Test the solution
• 5. Maintain the solution and do bug fixes

Factor of 10 rule!

 You have heard people talking about all the 1's and 0's inside

• f a computer

 What does that all really mean?  Using semiconductor physics, we can make a tiny little piece

• f a microchip be in one of two states, say, OFF and ON, like a

switch

 If we say that OFF is 0 and ON is 1, then, by using a lot of

these switches, we can represent a lot of 1's and 0's

 What do we do with those 1's and 0's?  To begin with, we represent numbers  How many of you have heard of base 10?  How many of you have heard of base 2?  What's the definition of a number system with a given base?

 Our normal number system is base 10  This means that our digits are: 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9  Base 10 means that you need 2 digits to represent ten, namely

1 and 0

 Each place in the number as you move left corresponds to an

increase by a factor of 10

3,482,931

Ones Millions Hundreds Thousands Tens Hundred thousands Ten thousands

 The binary number system is base 2  This means that its digits are: 0 and 1  Base 2 means that you need 2 digits to represent two, namely

1 and 0

 Each place in the number as you move left corresponds to an

increase by a factor of 2 instead of 10

11111100100

Ones 1024's Sixteens Thirty twos Eights Sixty fours Twos Fours 256's 128's 512's

11111100100 =

1∙210 + 1∙29 + 1∙28 + 1∙27 + 1∙26 + + 1∙25 + 0∙24 + 0∙23 + 1∙22 + 0∙21 + 0∙20 =

1024 + 512 + 256 + 128 + 64 + 32 + 4 =

2020

 You don't actually have to worry about doing binary

conversions when you're coding Java

 You should know this information because it explains how

Java is different from math

 In math, you can talk about an arbitrarily large number  In Java, each number is stored with a specific number of

binary digits

 There are limits on how big (or small) a given number can be

 We'll talk more about data representation

 Read Chapter 3