1. An Object-Oriented Approach to Programming Logic and Design Fourth Edition Chapter 6 Using Methods

2. Objectives In this chapter, you will learn about: The advantages of modularization Modularizing a program Declaring local and global variables and constants Methods that require parameters Methods that return a value Passing an array to a method Overloading methods Using predefined methods 2An Object-Oriented Approach to Programming Logic and Design

3. Understanding the Advantages of Modularization Modules – Smaller part of a larger task – Also called methods, subroutines, functions, or procedures Advantages of modularization – Provides abstraction – Simplifies the logic – Allows multiple programmers to work on a problem – Allows you to reuse work more easily 3An Object-Oriented Approach to Programming Logic and Design

4. Modularization Provides Abstraction Abstraction – Process of focusing on important properties while ignoring small details – Example: “do laundry” is an abstract task; it doesn’t list the small details like “pick up laundry basket,” etc. All computer programs provide some level of abstraction – No longer need to understand low-level computer circuitry instructions to write a computer program Methods provide a way to achieve abstraction 4An Object-Oriented Approach to Programming Logic and Design

5. Modularization Simplifies the Logic With modularization: – Individual methods are smaller and easier to follow – Errors are easier to find Methods – Execute a single, finite task – Smaller methods are less complex and therefore more reliable 5An Object-Oriented Approach to Programming Logic and Design

6. Multiple Programmers Can Work on a Problem Modularization – Allows division of work among programmers – Speeds up development process Used in the development of commercial software applications 6An Object-Oriented Approach to Programming Logic and Design

7. Modularization Allows You to Reuse Your Work Methods – Can be used more than once within a program – Can be reused to other programs Reusability – Feature of modular programs that allow methods to be reused Reliability – Feature of modular programs that indicates methods have been tested and proved to function correctly 7An Object-Oriented Approach to Programming Logic and Design

8. Modularizing a Program Application classes contain a main() method – May include other methods that main() can use Parts of a method – Header: contains method identifier and other information – Body: contains all statements in the method – Return statement: marks the end of the method To use a method, call the method using its name Flowchart conventions – Method call symbol: a rectangle with a bar across the top – Method name goes in the rectangle 8An Object-Oriented Approach to Programming Logic and Design

9. Figure 6-1 – Accepts customer name and balance due and produces a bill – Only uses main() method 9An Object-Oriented Approach to Programming Logic and Design Figure 6-1

10. Figure 6-2 – Shows modularized version of customer billing example – main() method calls displayAddressInfo() method – Method is reusable – main() method is easy to follow 10An Object-Oriented Approach to Programming Logic and Design

11. Modularizing a Program (cont’d) displayAddressInfo() method – Full name: BillingProgram.displayAddressInfo() – Only use full name when called in another class Statements contained in a method are said to be encapsulated Breaking a program into methods – Requires experience; no specific rules to follow Functional cohesion – Refers to the extent to which a method’s statements are focused on the single, specific method task 11An Object-Oriented Approach to Programming Logic and Design

12. Declaring Variables and Constants within Methods Any statements can be included in methods – Input – Processing – Output – Variable and constant declarations Variables and constants declared within a method – Usable only within the method – Considered local, in-scope, or visible – Go out-of-scope (cease to exist) when method ends – Figure 6-3 on next slide shows declarations with a method 12An Object-Oriented Approach to Programming Logic and Design

13. 13An Object-Oriented Approach to Programming Logic and Design Figure 6-3 –Three named constants are declared within the method Figure 6-3

14. Declaring Variables and Constants within Methods (cont’d) Variables and constants declared within main() – Local to main: Cannot be used in method Global variables – Accessible to the entire class When methods must share data – Pass data into and return data from method When calling a method, you must know: – Name of the method – What type of information to send to the method, if any – What type of return data to expect from the method 14An Object-Oriented Approach to Programming Logic and Design

15. Creating Methods that Require Parameters Argument – Data item passed into a method from a calling program Parameter – Method receives the argument into a parameter Argument and Parameter – Closely related terms – Calling method sends an argument to a called method – Called method accepts the value as its parameter 15An Object-Oriented Approach to Programming Logic and Design

16. Creating Methods that Require Parameters (cont’d) Example: Modify displayAddressInfo() method to display a message preceding the name and address on the bill if the balance is over $1000 Four approaches to modifying the method – Rewrite the program to eliminate the displayAddressInfo() method and place all statements within main() – Retain the displayAddressInfo() method but make the balance variable global by declaring it outside any methods 16An Object-Oriented Approach to Programming Logic and Design

17. Creating Methods that Require a Single Parameter (cont’d) Four approaches to modifying the method (cont’d) – Retain the displayAddressInfo() method, add a local variable to displayAddressInfo(), and prompt the user for the balance again within the method – Store the variable that holds the balance in main() so it can be used to display the customer’s balance and pass the balance to the displayAddressInfo() method Best choice: see Figure 6-4 on the next slide 17An Object-Oriented Approach to Programming Logic and Design

18. 18An Object-Oriented Approach to Programming Logic and Design Figure 6-4 –Billing application that passes an argument to a method Figure 6-4

19. 19An Object-Oriented Approach to Programming Logic and Design Creating Methods that Require a Single Parameter (cont’d) Figure 6-5 – Shows sample output from billing application in Figure 6-4 – Prompts are added for customer data – Program is executed in a command-line environment Figure 6-5

20. Creating Methods that Require a Single Parameter (cont’d) Method that can receive a parameter must include the following information in header’s parentheses – Type of parameter – Local name for the parameter 20An Object-Oriented Approach to Programming Logic and Design

21. Passing Arguments by Value and by Reference Two ways to pass an argument into a method – Passed by value Copy of value is sent to the method Stored in a new memory location – Passed by reference Address of original variable is sent to the method A variable in the calling method can have the same identifier as the parameter in the method header – Passed variable: a temporary placeholder – Parameter is re-declared with each method execution 21An Object-Oriented Approach to Programming Logic and Design

22. 22An Object-Oriented Approach to Programming Logic and Design Figure 6-6 – Argument is passed by value Figure 6-7 – Shows sample output Figure 6-6 Figure 6-7

23. Passing Arguments by Value and by Reference (cont’d) Implementation hiding – Calling a method without understanding the details of how the method works Method’s parameter list – parameter types within parentheses in a method header Method’s signature – the method’s name and parameter list Calling method needs to know: – Method’s signature – What type of return data to expect 23An Object-Oriented Approach to Programming Logic and Design

24. Passing Arguments by Value and by Reference (cont’d) Passing by reference – Gives receiving method address of original variable – Procedure differs by programming language – Usually involves placing a special symbol or keyword in the method header’s parentheses – Implementation hiding is reduced 24An Object-Oriented Approach to Programming Logic and Design

25. Creating Methods That Require Multiple Parameters To create and use methods with multiple parameters by: – List arguments in method call separated by commas – List a data type and local identifier for each parameter in header’s parentheses separated by commas – Pass arguments in correct order – Figure 6-8 on the next slide shows a method that accepts two parameters 25An Object-Oriented Approach to Programming Logic and Design

26. 26An Object-Oriented Approach to Programming Logic and Design Figure 6-8 – Program calls computeTax() method that requires two parameters Figure 6-8

27. Creating Methods That Return a Value Variable declared within a method ceases to exist when method ends To retain a value, return the value to the calling method – Return type must match data type of value that is returned – Return types can be any data type Return Type – The data type of the value that the method sends back to the location where the call is made Method could also return nothing - a void method 27An Object-Oriented Approach to Programming Logic and Design

28. Figure 6-9 – Payroll program calls a method that returns a value 28An Object-Oriented Approach to Programming Logic and Design – Return type num precedes method name in header of getHoursWorked () – Numeric value is included in return statement

29. 29An Object-Oriented Approach to Programming Logic and Design Figure 6-9 – Pseudocode for Payroll program – Calls the method getHoursWorked() which returns a value that is stored in the variable hours Figure 6-9

30. Figure 6-10 – Sample of a program that uses a method’s return value without storing it 30An Object-Oriented Approach to Programming Logic and Design Figure 6-10

31. Creating Methods That Return a Value (cont’d) Methods can include multiple return statements 31An Object-Oriented Approach to Programming Logic and Design – Practice is not recommended – Every selection structure has one entry point and one exit point; multiple return statements violate this convention – Example: Figure 6-11 Figure 6-11

32. 32An Object-Oriented Approach to Programming Logic and Design Figure 6-12 – Shows best solution to returning one of several values Figure 6-12

33. Passing an Array to a Method Pass single array element to a method – Same manner as passing a variable or constant Pass entire array to a method – Use square brackets to indicate that method parameter is an array – Passed by reference – Changes made to the array within the method are permanent 33An Object-Oriented Approach to Programming Logic and Design

34. Figure 6-13 – Array of four numeric values is created – Array is sent to methods three times 34An Object-Oriented Approach to Programming Logic and Design Figure 6-13

35. 35An Object-Oriented Approach to Programming Logic and Design Figure 6-13 – Pseudocode for PassEntireArray program Figure 6-14 – Output for PassEntireArray program Figure 6-14

36. 36An Object-Oriented Approach to Programming Logic and Design Overloading Methods Overloading – Allows for diverse meanings for a single identifier Overloaded methods – Multiple methods with the same name but different parameter lists – Have different signatures Polymorphism – Ability of a method to act appropriately according to the context – Example of polymorphism: overloaded methods

37. Figure 6-15 37An Object-Oriented Approach to Programming Logic and Design Figure 6-15 – Two overloaded versions of the printBill() method – Array is sent to methods three times

38. Figure 6-16 38An Object-Oriented Approach to Programming Logic and Design Figure 6-16 – Two additional overloaded versions of the method printBill()

39. Avoiding Ambiguous Methods Ambiguous methods – Situation in which compiler cannot determine which method to use – Can occur with an overloaded method – Compiler decides which version of a method to use based on parameter data type – Methods with identical names, identical parameter lists and different return types are ambiguous Avoiding ambiguous methods – Provide different parameter lists for methods with same name 39An Object-Oriented Approach to Programming Logic and Design

40. 40An Object-Oriented Approach to Programming Logic and Design Figure 6-17 –Two versions of method printBill() –One version accepts customer balance and discount rate –Other version accepts customer balance and discount amount –Both versions require same data type Figure 6-17

41. 41An Object-Oriented Approach to Programming Logic and Design Figure 6-17 –Pseudocode for program that contains ambiguous method call –Contains two versions of method printBill()

42. Using Predefined Methods All modern programming languages contain predefined methods May originate from various sources – Built into a language – Created by other programmers on a team – Standard method written for a company Examples – Printing message on the screen – Mathematical methods such as taking square root – Selecting a random number 42An Object-Oriented Approach to Programming Logic and Design

43. Using Predefined Methods (cont’d) Methods written as a convenience – Already written, tested, and available Information needed when using predefined methods – Method’s name – The method’s required parameters Might be multiple overloaded versions – Method’s return type Do not need to know how method is implemented 43An Object-Oriented Approach to Programming Logic and Design

44. Summary Modularization – Process of converting a larger task into smaller methods – Provides abstraction – Simplifies logic – Makes dividing work among programmers easier – Allows reuse of methods When creating a method, include a header, a body, and a return statement To use a method, call the method by using the method’s name 44An Object-Oriented Approach to Programming Logic and Design

45. Summary (cont’d) Variables and constants declared in a method are local to the method When you pass data into a method, it is called an argument to the method When the method receives the data item, it is stored in a parameter Two ways of passing data: by value and by reference 45An Object-Oriented Approach to Programming Logic and Design

46. Summary (cont’d) Return type – Indicates the data type of the value that will be returned – Also known as the method’s type – Indicated in front of method name when method is defined – Method returns a value using a return type Arrays – Can pass single array element to a method Exact same manner variable or constant – Can pass an entire array – Always passed by reference 46An Object-Oriented Approach to Programming Logic and Design

47. Summary (cont’d) Overload a method – Write multiple methods Shared name, different parameter lists Compiler understands meaning based on arguments – Risk of creating ambiguous methods Compiler cannot determine method to use Predefined methods – Built into programming language – Saves programming time and effort 47An Object-Oriented Approach to Programming Logic and Design