Week 13 - Friday What did we talk about last time? Finished - PowerPoint PPT Presentation

Week 13 - Friday

 What did we talk about last time?  Finished inheritance example  Files

 Reading from a text file is almost ridiculously easy  We use Scanner , just like reading from the command line  We just have to create a new File object that gives the file we want to read from Scanner in = new Scanner(new File("input.txt"));  This code will read from some file called input.txt , as if someone were typing its contents into the command line

 The alternative to catching an exception is throwing it up to the next level, making it someone else's problem  Sure, your program will crash if no one deals with it, but at least your code will compile  We do this by putting a throws FileNotFoundException on the declaration of main() (or whatever method we're in) public static void main(String[] args) throws FileNotFoundException { Scanner in = new Scanner(new File("input.txt"));

 Java loves objects  If you want to write to a file, you've got to create a PrintWriter object, based on a FileOutputStream object (which takes the file name as a parameter) PrintWriter out = new PrintWriter(new FileOutputStream ("output.txt"));  Once you've got a PrintWriter , you can use it just like System.out

 Unlike the command line, you should really close files when you're done reading from them  If you forget to close a file you’re writing to, everything you write might not show up  And, for situations where you're accessing multiple files, it may be even more important to close them Scanner in = new Scanner(new File("input.txt")); PrintWriter out = new PrintWriter(new FileOutputStream ("output.txt")); //do stuff in.close(); out.close();

 Let's write a program that prompts the user for 1o int values and then writes them to a file called numbers.txt  Then, let's write another program that opens numbers.txt , reads all 10 numbers, sorts them, and prints them out in order

 Let's write a program that prints the first million prime numbers to a file

 Format:  Multiple choice questions (~20%)  Short answer questions (~20%)  Programming problems (~60%)  Online  No notes  Closed book  No calculator

 Designed to be 50% longer than previous exams  But you'll have 100% more time  Time: Wednesday, 12/02/2020, 8:00 - 10:00 a.m.  Online  If you are entitled to extra time, please contact me as soon as possible to arrange how that extra time will be scheduled

 History of computers  Hardware  Software development  Basic Java syntax  Output with System.out.print()

Mechanical Calculation Devices (2400BC onward) • Aid to human calculation • No stored program Mechanical Computers (1725 onward) • Punch card programming • Serious limitations Early Electronic Computers (1941 onward) • General purpose, stored program computers • Electronic, using vacuum tubes Microprocessors (1970's onward) • Succeeded transistors • Now billions of computations per second at a nanometer scale

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!

Source Machine Code Code Hardware 010101010 Computer! Solve a 010100101 Execute problem; 001110010

Java Source Java Machine Hardware Code Code Bytecode class A { 101110101 010101010 Problem p; 101011010 010100101 JVM p.solve(); 110010011 001110010 }

 The absolute smallest program possible, with a print statement public class Hello { public static void main(String[] args) { System.out.println("Hello, world!"); } }

 For example, instead of one print statement, we can have several: System.out.println("Hello, world!"); System.out.println("Hello, galaxy!"); System.out.println("Goodbye, world!");  Each statement is an instruction to the computer  They are printed in order, one by one

 Java is a case sensitive language  Class is not the same as class  System.out.println("Word!"); prints correctly  system.Out.Println("Word!"); does not compile

 Java generally ignores whitespace (tabs, newlines, and spaces) System.out.println("Hello, world!");  is the same as: System.out. println( "Hello, world!");  You should use whitespace effectively to make your code readable

 There are two kinds of comments (actually 3)  Single line comments use // System.out.println("Hi!"); // this is a comment  Multi-line comments start with a /* and end with a */ System.out.println("Hi!"); /* this is a multi-line comment */

 Binary representation  Basic data types  Using Scanner for input  Numerical operations

 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

Sixty fours 256’s Sixteens Fours 11111100100 1024’s Ones 512’s Twos 128’s Eights Thirty twos

 We're focusing on five basic types of data in Java  These are:  int For whole numbers  double For rational numbers  boolean For true or false values  char For single characters  String For words  String is a little different from the rest, since you can call methods on it (and for other reasons)

Type Sample Literals Kind of values -5 int 0 Integers 900031 3.14 Floating-point double -0.6 Numbers 6.02e23 true boolean Boolean values false 'A' char 'Z' Single characters '&' Sequences of "If you dis Dr. Dre" String "10 Sesquipedalians" characters

 The int type is used to store integers (positive and negative whole numbers and zero)  Examples:  54  -893992  0  Inside the computer, an int takes up 4 bytes of space, which is 32 bits (1's and 0's)

 You will use the int type very often  Sometimes, however, you need to represent numbers with a fractional part  The double type is well suited to this purpose  Declaration of a double variable is just like an int variable: double x;

 Numbers are great  But, sometimes you only need to keep track of whether or not something is true or false  This is what the boolean type is for  Hopefully you have more appreciation for boolean s now  Declaration of a boolean variable is like so: boolean value;

 Sometimes you need to deal with characters  This is what the char type is for  The char type only allows you to store a single character like '$' or 'q'  Declaration of a char variable is like so: char c;

 The String type is different from the other types in several ways  The important thing for you to focus on now is that it can hold a large number of char s, not just a single value  A String literal is what we used in the Hello, World program String word;

There are three parts to using Scanner for input Include the appropriate import statement so that your program 1. knows what a Scanner object is 2. Create a specific Scanner object with a name you choose Use the object you create to read in data 3.

 Lots of people have written all kinds of useful Java code  By importing that code, we can use it to help solve our problems  To import code, you type import and then the name of the package or class  To import Scanner , type the following at the top of your program (before the class !) import java.util.Scanner;

 Once you have imported the Scanner class, you have to create a Scanner object  To do so, declare a reference of type Scanner , and use the new keyword to create a new Scanner with System.in as a parameter like so: Scanner in = new Scanner(System.in);  You can call it whatever you want, I chose to call it in

 Now that you've got a Scanner object, you can use it to read some data  It has a method that will read in the next piece of data that user types in, but you have to know if that data is going to be an int , a double , or a String  Let's say the user is going to input her age (an int ) and you want to store it in an int variable called years  We'll use the nextInt() method to do so: int years = in.nextInt();

 Scanner has a lot of methods (ways to accomplish some tasks)  For now, we're only interested in three  These allow us to read the next int , the next double , and the next String , respectively: Scanner in = new Scanner(System.in); int number = in.nextInt(); double radius = in.nextDouble(); String word = in.next();

 + adds  - subtracts  * multiplies  / divides (integer division for int type and fractional parts for double type)  % finds the remainder

 Order of operations holds like in math int a = 31; int b = 16; int c = 1; int d = 2; a = (((b + c) * d) – a / b) / d;  You can use parentheses to clarify or change the precedence  Now a is 16