COMP 110 Introduction to Programming

1. COMP 110Introduction to Programming Mr. Joshua Stough October 3, 2007

2. Announcements

3. Test taking Common Mistakes in written tests • You're not in math class •  instead of <= • 2(i+5) instead of 2 * (i+5) • num2 instead of num*num • num^2 instead of num*num • Changing loop control variable for computation inside loop • Not testing your code

4. COMP 110 So Far... • Problem Solving • Mathematical Calculations • Output • User Input • File I/O • Selection (if, if-else, switch) • Loops (while, do...while, for)

5. COMP 110 Next... • Object-Oriented Design • Writing Methods • pieces of code that we give a name • Writing Classes • organize related pieces of information • Arrays • access multiple pieces of information with a single identifier

6. Object-Oriented Design Problem Statement Write a program to input the length and width of a rectangle and calculate and print the perimeter and area of the rectangle nouns - objects, data members verbs - actions, operations (methods)

7. class RectangleData Members and Operations class name data members Last Step: design and implement an algorithm for each operation operations (methods)

8. Classes and Objects A class (the concept) An object (the realization) length = 15, width = 3 Rectangle length = 20, width = 6 Multiple objects from the same class length = 15, width = 15

9. Anatomy of a Class A class contains data declarations and method declarations int width; int length; Data declarations Method declarations (operations)

10. Rectangle.javaSo Far public class Rectangle { // data members private int width; private int length; // methods // setLength // setWidth // getLength // ... // computeArea // print }

11. What is a Method? • Group of programming statements that we give a name • Statements usually work together to perform a specific task/operation • Every method belongs to a class

12. Why Use Methods? • To divide complex programs into manageable pieces • Abstraction • provide an operation to be performed on an object (ex: computeArea) • Code Re-use • write a small piece of code that can be used (called) multiple times (saves typing)

13. Methods • Pre-defined methods • provided by Java standard library • we've used these before • Math class (Math.pow, Math.sqrt, ...) • Integer class (Integer.parseInt, ...) • User-defined methods • you write these

14. String MethodsMethod Description (pgs. 109-110) the result can be stored in a boolean variable or used in a boolean expression (return type) boolean equals(String str) the method needs to be given another String (parameter) method name int length() method specification, not syntax of use String toLowerCase() char charAt(int index)

15. Using String Methods • list of String methods: Chapter 3 • Steps: • create a String object • put data into the object • use the dot operator to "call" the method String line; line = keyboard.readLine(); String greeting = "Hello"; int len = line.length(); greeting = greeting.toUpperCase(); char ch = greeting.charAt(3);

16. Two Types of Methods • Value-returning methods • "return", "evaluate to", "result in" some value • like mathematical functions • only returns a single value • Void methods • performs operations but returns no value

17. Value-Returning Methods • Think mathematical function f(x) = 2x + 5 f(x, y) = 2x + y f(1) = 7 f(1, 5) = 7 f(2) = 9 f(2, 3) = 7 f(3) = 11 f(3, 1) = 7 • Can have multiple arguments (parameters) • Only one result of the function

18. Using MethodsWhat You Need To Know • Name of the method • Number of parameters • Data type of each parameter • Data type of value computed (returned) by the method • The code required to accomplish the task * Only need to know 1-4 for predefined methods

19. Value-Returning MethodsUses • Save the value for future calculation • Use the value in a calculation • Print the value So, often used in expressions: x = Math.pow (y, 2); z = a + Math.pow (y, 2) + x; System.out.println (Math.pow (y, 2));

20. Control Flow • Program control flow • execution always begins with the first statement in the method main • other methods execute only when called • Method control flow • when a method is invoked, the flow of control jumps to the method and the computer executes its code • when complete, the flow of control returns to the place where the method was called and the computer continues executing code

21. Method Declaration • Specifies the code that will be executed when the method is invoked (or called) • Located inside a class definition • Contains • method header • method body

22. Method Header A method declaration begins with a method header public static int countCharInWord (char ch, String word) visibility modifiers formal parameter list method name return type The parameter list specifies the type and name of each parameter The name of a parameter in the method declaration is called a formal argument

23. 2 Return Type • Indicates the type of value that the method evaluates to (or, sends back to the calling location) • primitive data type • class name • void • reserved word indicating that nothing is returned • When a value is returned, the method call is replaced with the returned value int number = countCharInWord ('e', "Heels");

24. Method Body The method header is followed by the method body public static int countCharInWord (char ch, String word) { int count = 0; for (int i = 0; i<word.length(); i++) { if (word.charAt(i) == ch) { count++; } } return count; } chandword are local data They are created each time the method is called, and are destroyed when it finishes executing The return expression must be consistent with the return type

25. Parameters Each time a method is called, the actual parameters in the call are copied into the formal parameters int num = countCharInWord ('e', "Heels"); public static int countCharInWord (char ch, String word) { int count = 0; for (int i = 0; i<word.length(); i++) { if (word.charAt(i) == ch) { count++; } } return count; }

26. The return Statement • Tells the computer to "return" back to where the call was originally made. • Specifies the value that will be returned return expression; • The data type of expression must match the method's return type • Methods with return types of void usually don't (but can) have a return statement • Value-returning methods musthave at least one return statement

27. public static double larger (double x, double y) { if(x >= y) return x; else return y; } Using return public static double larger (double x, double y) { double max; if(x >= y) max = x; else max = y; return max; } These are equivalent methods.

28. Void Methods • Does not return a value • Has a return type of void • Similar in structure to value-returning methods • Call to method is always a stand-alone statement • Can use return statement to exit method early

29. The main Method The main method looks just like all other methods public static void main (String[] args) modifiers return type method name parameter list

30. Code Re-Use • Methods can also be used to save typing • If you have a section of code that does one task and it needs to be done multiple times, you can write it as a method • Example: you want to print the following heading to start your program *********************************** *********************************** ************ Blackjack ************ *********************************** ***********************************

31. public class MyProgram • { • public static void main (String[] args) • { • printStars(); • printStars(); • System.out.print ("************"); • System.out.print (" Blackjack "); • System.out.println ("************"); • printStars(); • printStars(); • } • public static void printStars() • { • System.out.print ("***************"); • System.out.print ("***************"); • System.out.println ("*****"); • } • }

32. printStars What if we wanted to change the number of stars printed on the line? give the number of stars as a parameter

33. public class MyProgram { public static void main (String[] args) { printStars(35); printStars(35); System.out.print ("************"); System.out.print (" Blackjack "); System.out.println ("************"); printStars(35); printStars(35); } public static void printStars(int numStars) { for (int i = 0; i<numStars; i++) { System.out.print ("*"); } System.out.println(); } }

34. printStars What if we wanted to change the character printed (something besides stars?) give the character as a parameter • change the name of the method (because it may not print stars) --> printChars

35. public class MyProgram { public static void main (String[] args) { printChars(35, '#'); printChars(35, '#'); System.out.print ("###############"); System.out.print (" Blackjack "); System.out.println ("###############"); printChars(35, '#'); printChars(35, '#'); } public static void printChars(int num, char ch) { for (int i = 0; i<num; i++) { System.out.print (ch); } System.out.println(); } }

36. Parameters • Formal parameters • variable declarations in the method header • automatic local variables for the method • Actual parameters • actual values that are passed to the method • can be variables, literals, or expressions printStars (35); printStars (30 + 5); int num = 35; printStars (num);

37. Next Time in COMP 110 • More methods • more parameter passing • method overloading • practice writing methods • Reading: Ch 7