Week 15 - Monday
What did we talk about last time? File I/O
20% multiple choice questions 20% short answer questions 60% programming Designed to take 2 hours (50% longer than the previous exams) But, you will have the full 3 hour time period More focus will be on the second half of the semester
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
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 Most commonly on USB keychains 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
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.
Monitor Common visual output device Speakers Common audio output device Mouse Common input device Keyboard Common input device
Computer! Solve a problem; Compile Execute Source Code Machine Code Hardware
class A { Problem p; p.solve(); } Compile JVM Execute Java Source Code Machine Code Hardware Java Bytecode
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!
The absolute smallest program possible, with a print statement public class Hello { public static void main(String[] args) { System.out.println("Hello, world!"); } 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: Each statement is an instruction to the computer They are printed in order, one by one System.out.println("Hello, world!"); System.out.println("Hello, galaxy!"); System.out.println("Goodbye, world!"); System.out.println("Hello, world!"); System.out.println("Hello, galaxy!"); System.out.println("Goodbye, world!");
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) is the same as: You should use whitespace effectively to make your code readable System.out.println("Hello, world!"); System.out. println( "Hello, world!"); System.out. println( "Hello, world!");
There are two kinds of comments (actually 3) Single line comments use // Multi-line comments start with a /* and end with a */ System.out.println("Hi!"); // this is a comment System.out.println("Hi!"); /* this is a multi-line comment */ System.out.println("Hi!"); /* this is a multi-line comment */
Binary representation Basic data types Using Scanner for input
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
Ones1024’s Sixteens Thirty twos Eights Sixty fours Twos Fours 256’s 128’s 512’s
We are going to focus 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 As you now know, String is a little different from the rest
TypeKind of valuesSample Literals int Integers double Floating-point Numbers e23 boolean Boolean values true false char Single characters 'A' 'Z' '&' String Sequences of characters "If you dis Dr. Dre" "10 Sequipedalians"
The int type is used to store integers (positive and negative whole numbers and zero) Examples: 54 00 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 = 1.28;
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 = false;
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 = 'L';
The String type is different from the other types in several ways The important thing to focus on is that it can hold a large number of char values A String literal is what we used in the Hello, World program A String variable is declared much like any other type String word = "Terrifying!";
There are three parts to using Scanner for input 1. Include the appropriate import statement so that your program knows what a Scanner object is 2. Create a specific Scanner object with a name you choose 3. Use the object you create to read in data
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: You can call it whatever you want, I chose to call it in Hopefully this code makes more sense now Scanner in = new Scanner(System.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; years = in.nextInt(); int years; years = in.nextInt();
Numerical operations Advanced math operations boolean operations char operations String operations Wrapper classes
+ 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 You can use parentheses to clarify or change the precedence Now a is 16 int a = 31; int b = 16; int c = 1; int d = 2; a = (((b + c) * d) – a / b) / d; int a = 31; int b = 16; int c = 1; int d = 2; a = (((b + c) * d) – a / b) / d;
You cannot directly store a double value into an int variable However, you can cast the double value to convert it into an int Casting tells the compiler that you want the loss of precision to happen You can always store an int into a double int a = 2.6;// fails! int a = (int)2.6;// succeeds! (a is 2)
The sin() method allows you to find the sine of an angle (in radians) This method is inside the Math class The answer that it gives back is of type double To use it, you might type the following: double value = Math.sin( 2.4 );
In Java, the conversion of a double into an int does not use rounding As in the case of integer division, the value is always rounded down You can think of this as using the floor function from math If you want to round normally, you can simply add 0.5 before the cast double x = 2.6; int a = (int)(x + 0.5);// rounds double x = 2.6; int a = (int)(x + 0.5);// rounds
Return typeNameJob doublesin(double theta) Find the sine of angle theta doublecos(double theta) Find the cosine of angle theta doubletan(double theta) Find the tangent of angle theta doubleexp(double a) Raise e to the power of a (e a ) doublelog(double a) Find the natural log of a doublepow(double a, double b) Raise a to the power of b (a b ) longround(double a) Round a to the nearest integer doublerandom() Create a random number in [0, 1) doublesqrt(double a) Find the square root of a
! NOT Flips value of operand from true to false or vice versa && AND true if both operands are true || OR true if either operand is true ^ XOR true if operands are different
char values can be treated like an int It can be more useful to get the offset from a starting point int number; number = 'a';// number contains 97 int number; number = 'a';// number contains 97 char letter = 'r'; int number; number = letter – 'a' + 1; //number is 18 char letter = 'r'; int number; number = letter – 'a' + 1; //number is 18
Remember that we use single quotes to designate a char literal: 'z' What if you want to use the apostrophe character ( ' )? apostrophe: '\'' What if you want to use characters that can't be printed, like tab or newline? tab: '\t' newline: '\n' The backslash is a message that a special command called an escape sequence is coming
The only operator that we will use directly with String s is the + (concatenation) operator This operator creates a new String that is the concatenation of the two source String s Concatenation can be used to insert the values of other types into String s as well String word; word = "tick" + "tock"; // word is "ticktock" String word; word = "tick" + "tock"; // word is "ticktock"
equals() Tests two String s to see if they are the same compareTo() Returns a negative number if the first String comes earlier in the alphabet, a positive number if the first String comes later in the alphabet, and 0 if they are the same length() Returns the length of the String charAt() Returns the character at a particular index inside the String substring() Returns a new String made up of the characters that start at the first index and go up to but do not include the second index
Each primitive data type in Java has a wrapper class Integer ▪ Allows String representations of integer values to be converted into int s Double ▪ Allows String representations of floating point values to be converted into double s Character ▪ Provides methods to test if a char value is a digit, is a letter, is lower case, is upper case ▪ Provides methods to change a char value to upper case or lower case
Making choices with if statements Basics Having if bodies with more than one line Using else blocks Nesting if statements Using switch statements
The if part Any boolean expression Any single executable statement if( condition ) statement;
Two different outcomes if( condition ) statement1; else statement2;
if( condition ) { statement1; statement2; … statementn; } A whole bunch of statements
if( condition1 ) { statement1; if( condition2 ) { if( condition3 ) statement2; … }
The most common condition you will find in an if is a comparison between two things In Java, that comparison can be: == equals != does not equal < less than <= less than or equal to > greater than >= greater than or equal to
switch( data ) { case value1: statements 1; case value2: statements 2; … case valuen: statements n; default: default statements; }
int data = 3; switch( data ) { case 3: System.out.println("Three"); case 4: System.out.println("Four"); break; case 5: System.out.println("Five"); } Both "Three" and "Four" are printed The break is optional The default is optional too
1. The data that you are performing your switch on must be either an int, a char, or a String 2. The value for each case must be a literal 3. Execution will jump to the case that matches 4. If no case matches, it will go to default 5. If there is no default, it will skip the whole switch block 6. Execution will continue until it hits a break
Review up to Exam 2
Bring a question to class Wednesday! Any question about any material in the course Keep working on Project 5 Due on Friday before midnight Study for Final Exam 2:30 - 5:30pm, Thursday, 12/10/2015 (CS121B) 11:00am - 2:00pm, Monday, 12/07/2015 (CS121C)