Week 1 -Wednesday What did we talk about last time? Syllabus - PowerPoint PPT Presentation

Week 1 -Wednesday

 What did we talk about last time?  Syllabus  Course policies  Java basics

 In Java, every variable and every literal has a type  A type says what kind of data it is  In Python, you could just assign a value to a variable  In Java, every variable must be declared with its type before you use it: double x;  Java is also strongly typed, meaning that type errors will prevent your program from compiling int y = 4.0; //illegal, since 4.0 is a double

 Five basic types of data in Java let you get most things done  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)  There are also byte , short , and long versions of int , which use 1, 2, and 8 bytes, respectively

Type Kind of values Sample Literals -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 '&' "If you dis Dr. Dre" Sequences of 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; 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

Return type Name Job double sin( double theta ) Find the sine of angle theta double cos( double theta ) Find the cosine of angle theta double tan( double theta ) Find the tangent of angle theta double exp( double a ) Raise e to the power of a ( e a ) double log( double a ) Find the natural log of a double pow( double a, double b ) Raise a to the power of b ( a b ) long round( double a ) Round a to the nearest integer double random() Create a random number in [0, 1) double sqrt( double a ) Find the square root of a double toDegrees( double radians ) Convert radians to degrees double toRadians( double degrees ) Convert degrees to radians

 ! 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

 In some circumstances, Java doesn't check the whole expression:  (true || (some complicated expression) )  Ignores everything after || and gives back true  (false && (some complicated expression) )  Ignores everything after && and gives back false

 char values can be treated like an int int number; number = 'a'; // number contains 97  It can be more useful to get the offset from a starting point char letter = 'r'; int number; number = letter – 'a' + 1; //number is 18

 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? '\t'  tab: '\n'  newline:  The backslash is a message that a special command called an escape sequence is coming  These can be used in String literals as well:  "\t\t\t\nThey said, \"Wow!\""

 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"

 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

 The if -statement: int x = 4; if( x < 5 ) System.out.println("x is small!");  x is small will only print out if x is less than 5  In this case, we know that it is, but x could come from user input or a file or elsewhere

The if part Any boolean expression if( condition ) statement; Any single executable statement

 Any statement that evaluates to a boolean is legal  Examples:  x == y  true  Character.isDigit('r')  s.equals("Help me!") && (z < 4)

 The most common condition you will find 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  These are called relational operators

 You can use the == operator to compare any two things of the same type  Different numerical types can be compared as well ( 3 == 3.0 )  Be careful with double types, 0.33333333 is not equal to 0.33333332 int x = 3; if( x == 4 ) System.out.println("This doesn't print");

 Any place you could have used the == operator, you can use the != operator  If == gives true, the != operator will always give false, and vice versa  If you want to negate a condition, you can always use the ! as a not if( x != 4 ) is the same as if( !(x == 4) )

 Remember, a single equal sign ( = ) is the assignment operator (think of a left-pointing arrow)  A double equals ( == ) is a comparison operator int y = 10; if( y = 6 ) //compiler error! boolean b = false; if( b = false ) //no compiler error but wrong