Methods that return values

1. Methods that return values suggested reading: 3.2

2. Java's Math class • Java has a class called Math that has several useful static methods to perform mathematical calculations.

3. Methods that "return" • The Math methods are called by writing:Math. <method name> (<parameters> ); • The methods of the Math class do not print their results to the console. • Instead, a call to one of these methods can be used as an expression or part of an expression. • Each method evaluates to produce (or return) a numeric result. • The result can be printed or used in a larger expression. • Example: double squareRoot = Math.sqrt(121.0); System.out.println(squareRoot); // 11.0 int absoluteValue = Math.abs(-50); System.out.println(absoluteValue); // 50 System.out.println(Math.min(3, 7) + 2); // 5

4. Math method questions • Evaluate the following expressions: • Math.abs(-1.23) • Math.pow(3, 2) • Math.pow(10, -2) • Math.sqrt(121.0) - Math.sqrt(256.0) • Math.round(Math.PI) + Math.round(Math.E) • Math.ceil(6.022) + Math.floor(15.9994) • Math.abs(Math.min(-3, -5)) • Math.max and Math.min can be used to bound numbers. Consider an int variable named age. • What statement would replace negative ages with 0? • What statement would cap the maximum age to 40?

5. Methods that return values • Declaring a method that returns a value: public static <type><name> ( <parameters> ) { <statements> ; } • Returning a value from a method: return <expression> ; • Example: // Returns the given number cubed (to the third power). public static int cube(int number) { return number * number * number; }

6. Example returning methods // Converts Fahrenheit to Celsius. public static double fToC(double degreesF) { return 5.0 / 9.0 * (degreesF - 32); } // Rounds the given number to the given number of decimal places. // Example: round(3.14159265, 4) returns 3.1416. public static double round(double value, int places) { double pow = Math.pow(10, places); value = value * pow; // upscale the number value = Math.round(value); // round to nearest whole number value = value / pow; // downscale the number return value; }

7. Return questions • Write a method named area that accepts a circle's radius as a parameter and returns the area of the circle. • You may wish to use the constant Math.PI in your solution. • Write a method named distanceFromOrigin that accepts x and y coordinates as parameters and returns the distance between that (x, y) point and the origin. • Write a method named attendance that accepts a number of lectures attended by a student, and returns how many points a student receives for attendance. The student receives 2 points for each of the first 5 lectures and 1 point for each subsequent lecture.

8. How to comment: methods 2 • If your method accepts parameters and/or returns a value, write a brief description of what the parameters are used for and what kind of value will be returned. • In your comments, you can also write your assumptions about the values of the parameters. • You may wish to give examples of what values your method returns for various input parameter values. • Example: // This method returns the factorial of the given integer n. // The factorial is the product of all integers up to that number. // Assumes that the parameter value is non-negative. // Example: factorial(5) returns 1 * 2 * 3 * 4 * 5 = 120. public static int factorial(int n) { ... }

9. Using Objects (in brief) suggested reading: 3.3

10. Objects • So far, we have seen: • methods, which represent behavior • variables, which represent data (categorized by types) • In Java and other "object-oriented" programming languages, it is possible to create new types that are combinations of the existing types. • Such types are called object types or reference types. • In this chapter we will learn how to use certain objects that exist in Java. • In Chapter 8 we will learn to create our own types of objects.

11. Objects and classes • object: An entity that contains data and behavior. • There are variables inside the object, representing its data. • There are methods inside the object, representing its behavior. • class: • A program, or... • A type of objects. • Examples: • The class named String represents objects that hold text characters. • The class named Point represents objects that hold (x, y) data. • The class named Scanner represents objects that read information from the keyboard, files, and other sources.

12. Constructing objects • construct: To create a new object. • Objects are constructed with the new keyword. • Most objects must be constructed before they can be used. • Constructing objects, general syntax: <type><name> = new <type>(<parameters>); • Examples: Point p = new Point(7, -4); DrawingPanel window = new DrawingPanel(300, 200); Color orange = new Color(255, 128, 0); • Classes' names are usually uppercase (e.g. Point, Color). • Strings are also objects but are constructed without new : String name = "Kelly T. Heffner";

13. Calling methods of objects • Objects contain methods that can be called by your program. • A String's methods manipulate or process the String in useful ways. • When we call an object's method, we are sending a message to it. • We don't just write the method's name; we also have to specify which object we are talking to. • Calling a method of an object, general syntax: <variable> . <method name> ( <parameters> ) • The results will be different from one object to another. • Examples: String name = "Kelly T. Heffner"; System.out.println(name.length()); // 16 Point p1 = new Point(3, 4); Point p2 = new Point(0, 0); System.out.println(p1.distance(p2)); // 5.0

14. Interactive Programs Using Scanner Objects suggested reading: 3.4

15. Interactive programs • We have written several programs that print output to the console. • It is also possible to read text input from the console. • The user running the program types the input into the console. • We can capture the input and use it as data in our program. • A program that processes input from the user is called an interactive program. • Interactive programs can be challenging: • Computers and users think in very different ways. • Users tend to misbehave.

16. Input and System.in • When we print text output to the console, we communicate with an object named System.out . • We call the println (or print) method of System.out to print a message to the console. • The object that holds the user's console input is named System.in . • System.in is not intended to be used directly. • Instead, we will use a second object of a type named Scanner to help read data from System.in. • We will construct a Scanner object and pass System.in as a parameter to it. • We can then ask the Scanner to read various kinds of input from the console.

17. Scanner • Constructing a Scanner object to read console input: Scanner <name> = new Scanner(System.in); • Example: Scanner console = new Scanner(System.in); • Java class libraries: The objects available to you as part of Java. • These objects are organized into groups named packages. • To use the objects from a package, you must include an import statement at the top of your program. • To use Scanner, put this at the start of your program: import java.util.*;

18. Scanner methods • Methods of Scanner that we will use in this chapter: • Each of these methods causes your program to pause until the user has typed input and pressed Enter, then it returnsthe typed value to your program. • token: A unit of user input, as read by the Scanner. • Tokens are separated by whitespace (spaces, tabs, new lines). • How many tokens appear on the following line of input? 23 John Smith 42.0 "Hello world"

19. Example Scanner usage import java.util.*; // so that I can use Scanner public class ReadSomeInput { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.print("What is your first name? "); String name = console.next(); System.out.print("And how old are you? "); int age = console.nextInt(); System.out.println(name + " is " + age); System.out.println("That's quite old!"); } } • Output (user input underlined): What is your first name? Marty How old are you? 14 Marty is 14 That's quite old!

20. Scanners as parameters • If multiple methods read user input from the keyboard, you should declare a Scanner in main and pass it to each of them as a parameter. • In this way, all of the methods can share the same Scanner object to read keyboard input. public static void main(String[] args) { Scanner console = new Scanner(System.in); int sum = readSum3(console); System.out.println("The sum is " + sum); } public static int readSum3(Scanner console) { System.out.print("Type 3 numbers: "); int num1 = console.nextInt(); int num2 = console.nextInt(); int num3 = console.nextInt(); return num1 + num2 + num3; }

21. Scanner question A person's body mass index (BMI) is computed by the following formula: • Write a program that produces the following output: This program reads in data for two people and computes their body mass index (BMI) and weight status. Enter next person's information: height (in inches)? 62.5 weight (in pounds)? 130.5 Enter next person's information: height (in inches)? 58.5 weight (in pounds)? 90 Person #1 body mass index = 23.485824 Person #2 body mass index = 18.487836949375414 Difference = 4.997987050624587