COMP 14 I O and Boolean Expressions May 24 2000 Nick Vallidis Announcements Anyone tried the assignment yet Review What are the 2 parts of a program What are the 2 types of data in Java What is a data type What are the 4 primitive data types we ll be using in this class What is a method Review part 2 What is a string literal How do we concatenate things onto a string How do we put newline and in a string What are these things called Today Input Output I O Expressions Boolean Expressions Input Output How do we get data into the program from the keyboard How do we display data to the user Output You ve already seen the basics of this one System out println Beefcake What about displaying data We talked yesterday about concatenation int level 3 System out println You are on level level double price 2 75 System out println Please pay price Input Input is a little more complicated We are not using the Keyboard class described in the book Let s take a look at the current assignment Part of program P1 BufferedReader stdin new BufferedReader new InputStreamReader System in int age Holds the age of the person String name Holds name Ask the user s name System out println Hello System out print What is your name name stdin readLine Part of program P1 This is some setup code We create an object stdin You don t have to understand the details of this BufferedReader stdin new BufferedReader new InputStreamReader System in int age Holds the age of the person String name Holds name Ask the user s name System out println Hello System out print What is your name name stdin readLine Part of program P1 BufferedReader stdin new BufferedReader new InputStreamReader System in int age Holds the age of the person String name Holds name Ask the user s name System out println Hello System out print What is your name name stdin readLine Here we actually read in what the user types Another piece of program P1 Ask the user s age System out print Please enter your age doesn t have to be true age Integer parseInt stdin readLine And here we read it in and then convert it to an integer Input Summary Put this at beginning on one line BufferedReader stdin new BufferedReader new InputStreamReader System in To read in a String variable stdin readLine To read in an int variable Integer parseInt stdin readLine To read in a double variable new Double stdin readLine doubleValue Expressions the revenge We talked about and Just like with arithmetic there is an order 18 5 1 is 4 not 3 and have higher precedence than and Using parenthesis There are many levels of precedence see pg 69 in book You can force whatever order you want using parenthesis 18 5 1 is 4 BUT 18 5 1 is 3 Mixing data types Avoid this if you can Guidelines only mix int and double the result should be double double total shoePrice 54 99 int numShoes 4 total shoePrice numShoes Should be a double Methods can be in expressions If a method returns a value then it can be in an expression too age Integer parseInt stdin readLine 21 This brings us to a special kind of method Class Methods Normally you have to instantiate an object of a class before you can use its methods But not if the method has the word static in front of it Where have you seen this before Class or static methods How about main public class Simple public static void main String args System out println Hello Aside method data types public class Simple What s this public static void main String args System out println Hello Aside method data types public class Simple It s a data type It tells you whether the method returns a value if it can be used in an expression public static void main String args System out println Hello This can be very useful The Math class contains many methods that are useful in expressions double result calculate the absolute value result Math abs 17 2 calculate the sin of pi radians result Math sin 3 1416 Boolean expressions These are expressions that result in a value of the data type boolean They almost always result from the use of equality operators relational operators or logical operators Equality operators These are and means equal to means not equal to They go between two expressions 2 2 evaluates to true 2 5 evaluates to false 15 7 evaluates to true 14 14 evaluates to false Equality Operators Of course you can also use variables and more complex expressions int myAnswer 5 int trueAnswer 7 myAnswer trueAnswer evaluates to false int myRaise 0 int yourRaise 100000 myRaise yourRaise evaluates to true Relational Operators These are and means less than means less than or equal to means greater than means greater than or equal to They work the same way as the equality operators Last bunch of boolean operators Logical operators and means NOT means logical AND means logical OR Truth tables a false true a false false true true b false true false true a true false a b false false false true a b false true true true Logical operators in action Much like relational operators but each side has to be a boolean expression 3 5 6 7 evaluates to true 2 5 evaluates to false 3 3 2 2 evaluates to true Examples Homework Read 3 1 3 2 3 4 Don t forget that P1 is due tomorrow