1. 1 Java Programming Using Methods, Classes, and Objects

2. 2 Topics:  Methods with no arguments,with no arguments, a single argument,a single argument, multiple argumentsmultiple arguments with return valueswith return values  Class concepts Create a classCreate a class Use instance methodsUse instance methods Declare objectsDeclare objects Organize classesOrganize classes Use constructorsUse constructors

3. 3 Method  A method is a series of statements that carry out a task  Any class can contain an unlimited number of methods

4. 4 Methods Methods must include:  A declaration  An opening curly brace  A body  A closing brace

5. 5 Methods Method declarations must contain:  Optional access modifiers  The return type for the method  The method name  An opening parenthesis  An optional list of method arguments  A closing parenthesis

6. 6 Access Modifiers Access modifiers for a method can be:  public- most often methods are given public access  private  protected  static

7. 7 Access Modifiers  public- Endowing a method with public access means any class can use it  static- Any method that can be used from anywhere within the class requires the keyword modifier static

8. 8 Creating Methods that Require a Single Argument  Arguments- Are communications to a method  Implementation hiding- Allows that the invoking program must know the name of the method and what type of information to send it, but the program does not need to know how the method works

9. 9 Creating Methods that Require a Single Argument The method declaration must include:  The type of the argument  A local name for the argument For example:For example: public void predictRaise(double moneyAmount)

10. 10 Calling a Method  Can use either argument: ConstantConstant VariableVariable  predictRaise(472.55)  predictRaise(mySalary)

11. 11 Creating Methods that Require Multiple Arguments  Methods can require more than one argument  Pass multiple arguments by: Listing the arguments in the call to the methodListing the arguments in the call to the method Separating them with commasSeparating them with commas  The declaration for a method that receives two or more arguments must list the type for each argument separately

12. 12 A Method’s Return Type  The return type is known as the method’s type For example: public static void nameAndAddress() This method is public and returns no valueThis method is public and returns no value public static String nameAndAddress() This method returns a string (with name and address)This method returns a string (with name and address)

13. 13 Return Statement  The return statement is the last statement in a method  Usually you use the returned value, but this is not required

14. 14 Overloading a Method Overloading:  Involves using one term to indicate diverse meanings  Writing multiple methods with the same name, but with different arguments  Overloading a Java method means you write multiple methods with a shared name

15. 15 Overloaded methods  Methods can be overloaded to support different types of data Overloaded methods have the same name and return type (the latter is often forgotten)Overloaded methods have the same name and return type (the latter is often forgotten) But the number and type of arguments can be changed to support different cases/scenariosBut the number and type of arguments can be changed to support different cases/scenarios For exampleFor example  float calculateRaise(float Salary, float Raise_Amount ); 2 nd argument represents raise in $ (float)2 nd argument represents raise in $ (float)  float calculateRaise(float Salary, int Percent_Amount); 2 nd argument represents raise in percent (integer)2 nd argument represents raise in percent (integer)  float calculateRaise(float Salary); No 2 nd argument, standard (constant raise)No 2 nd argument, standard (constant raise)

16. 16 Learning about Ambiguity  When you overload a method you run the risk of ambiguity An ambiguous situation is one in which the compiler cannot determine which method to useAn ambiguous situation is one in which the compiler cannot determine which method to use Examples?Examples?

17. 17 Overloading Example static float calculateRaise(float empSalary, float payRaise) { return empSalary + payRaise; } static float calculateRaise(float empSalary, int payRaise) { return empSalary * (1 + (float)payRaise/100); } public static void main(String[] arg) {// why need “f” after constant System.out.println("1: " + calculateRaise(100000.00f, 3000.00f)); System.out.println("1: " + calculateRaise(100000.00f, 3000.00f)); System.out.println("1: " + calculateRaise(100000.00f, 10)); System.out.println("1: " + calculateRaise(100000.00f, 10)); System.out.println("1: " + calculateRaise(100000, 10.0)); //doesn’t work System.out.println("1: " + calculateRaise(100000, 10.0)); //doesn’t work}

18. 18 Class Concepts In object-oriented programming:  Everything is an object  An object is an instantiation of a class, or a tangible example of a class  Every object is a member of a class Your desk is an object and is a member of the Desk classYour desk is an object and is a member of the Desk class These statements represent is-a relationshipsThese statements represent is-a relationships

19. 19 Class Concepts The concept of a class is useful because: The concept of a class is useful because: Objects inherit attributes from classesObjects inherit attributes from classes All objects have predictable attributes because they are members of certain classesAll objects have predictable attributes because they are members of certain classes You must: You must: Create the classes of objects from which objects will be instantiatedCreate the classes of objects from which objects will be instantiated Write other classes to use the objectsWrite other classes to use the objects Class Client or Class User-A program or class that instantiates objects of another prewritten class Class Client or Class User-A program or class that instantiates objects of another prewritten class

20. 20 Creating a Class You must:  Assign a name to the class  Determine what data and methods will be part of the class To begin, create a class header with three parts:  An optional access modifier  The keyword class  Any legal identifier you choose for the name of the class The define the class contents  Then add a open and close brace  Add the attributes (fields) and actions (methods) to the class

21. 21 Access modifiers for Classes Access modifiers include:  public This is the most used modifierThis is the most used modifier Most liberal form of accessMost liberal form of access Can be extended or used as the basis for other classesCan be extended or used as the basis for other classes  final- used only under special circumstances  abstract- used only under special circumstances

22. 22 Access modifiers for Fields and Methods  Private (default for fields)  Public (default for methods)  Static  Final

23. 23 Field Modifiers  Private No other classes can access a field’s valuesNo other classes can access a field’s values Only methods of the same class are allowed to set, get, or otherwise use private variablesOnly methods of the same class are allowed to set, get, or otherwise use private variables Highest level of securityHighest level of security Also called information hidingAlso called information hiding  Provides a means to control outside access to your data

24. 24 Using Instance Methods  Methods used with object instantiations are called instance methods You can call class methods without creating an instance of the class.You can call class methods without creating an instance of the class. Instance methods require an instantiated object.Instance methods require an instantiated object.

25. 25 Declaring Objects To declare an object:  Supply a type and an identifier  Allocate computer memory for the object Use the new operatorUse the new operator

26. 26 Organizing Classes  Most programmers place data fields in some logical order at the beginning of a class For example, use a unique identifier for each employeeFor example, use a unique identifier for each employee  empNum Last names and first names are organized togetherLast names and first names are organized together

27. 27 Using Constructor Methods  Constructor methods- Methods that establish an object Employee chauffer = new Employee();  Calls a method named Employee() that is provided by the Java compiler  Methods are commonly overloaded to provide various ways to initialize an object Employee chauffer = new Employee(“Mike Elm”); Employee chauffer = new Employee(“Mike Elm”, 2320);

28. 28  ADVANCED TOPICS

29. 29 Advanced Topics:  Understand blocks and scope  Learn about ambiguity  Send arguments to constructors  Overload constructors  Learn about the this reference  Work with constants  Use automatically imported, prewritten constants and methods  Use prewritten imported methods

30. 30 Understanding Blocks  Blocks-Within any class or method, the code between a pair of curly braces Outside block- The first block, begins immediately after the method declaration and ends at the end of the methodOutside block- The first block, begins immediately after the method declaration and ends at the end of the method Inside block- The second block, contained within the second pair of curly bracesInside block- The second block, contained within the second pair of curly braces The inside block is nested within the outside blockThe inside block is nested within the outside block

31. 31 Understanding Scope  The portion of a program within which you can reference a variable  A variable comes into existence, or comes into scope, when you declare it  A variable ceases to exist, or goes out of scope, at the end of the block in which it is declared

32. 32  public class Test  {  public static void main(String[] args) throws Exception  {  while (true)  {  int Count = 10; …. …. Count++; // this is ok Count++; // this is ok  }  for(int i = 0; i < MAX_NUM; i++)  { Count++; // this will generate an error, why? Count++; // this will generate an error, why?  }  } // end of main  }//end of class Test

33. 33 Overriding a Method or Variable  If you declare a variable within a class, and use the same variable name within a method of the class, then the variable used inside the method takes precedence, or overrides, the first variable  In Java you can override variables and methods Variable overriding is confusing and should be avoidedVariable overriding is confusing and should be avoided Method overriding is useful and necessaryMethod overriding is useful and necessary

34. 34 Constructors  Java automatically provides a constructor method when you create a class  Programmers can write their own constructor methods  Programmers can also write constructors that receive arguments Such arguments are often used for initialization purposes when values of objects might varySuch arguments are often used for initialization purposes when values of objects might vary

35. 35 Overloading Constructors  If you create a class from which you instantiate objects, Java automatically provides a constructor  But, if you create your own constructor, the automatically created constructor no longer exists  As with other methods, you can overload constructors Overloading constructors provides a way to create objects with or without initial arguments, as neededOverloading constructors provides a way to create objects with or without initial arguments, as needed

36. 36 Using the this Reference  Classes can become large very quickly Each class can have many data fields and methodsEach class can have many data fields and methods  If you instantiate many objects of a class, the computer memory requirements can become substantial It is not necessary to store a separate copy of each variable and method for each instantiation of a classIt is not necessary to store a separate copy of each variable and method for each instantiation of a class  The compiler accesses the correct object’s data fields because you implicitly pass a this reference to class methods  Static methods, or class methods, do not have a this reference because they have no object associated with them

37. 37

38. 38 Class Variables  Class variables- Variables that are shared by every instantiation of a class  Class variables (and methods) are also refered to as “static” variables (or methods) because of the access modifier used to define them

39. 39 Working with Constants Constant variable:  A variable or data field that should not be changed during the execution of a program To prevent alteration, use the keyword finalTo prevent alteration, use the keyword final  Constant fields are written in all uppercase letters For example:For example:  COMPANY_ID

40. 40 Using Automatically Imported, Prewritten Constants and Methods  The creators of Java created nearly 500 classes For example:For example:  System, Character, Boolean, Byte, Short, Integer, Long, Float, and Double are classes  These classes are stored in a package, or a library of classes, which is a folder that provides a convenient grouping for classes

41. 41  java.lang – The package that is implicitly imported into every Java program and contains fundamental classes, or basic classes  Fundamental classes include: System, Character, Boolean, Byte, Short, Integer, Long, Float, and DoubleSystem, Character, Boolean, Byte, Short, Integer, Long, Float, and Double  Optional classes – Must be explicitly named Using Automatically Imported, Prewritten Constants and Methods

42. 42 Using Prewritten Imported Methods  To use any of the prewritten classes (other than java.lang): Use the entire path with the class nameUse the entire path with the class nameOR Import the classImport the classOR Import the package which contains the class you are usingImport the package which contains the class you are using

43. 43 Using Prewritten Imported Methods  To import an entire package of classes use the wildcard symbol *  For example: import java.util.*;import java.util.*; Represents all the classes in a packageRepresents all the classes in a package

44. 44 Access Specifiers http://www.uni-bonn.de/~manfear/javamodifiers.php

45. 45 Key Concepts Learned Today  Objects  Classes  Constructors  Overloaded constructors  Method declarations  Scope specifiers

46. 46 Next Week:  Key topics Advanced OOP conceptsAdvanced OOP concepts InheritanceInheritance AggregationAggregation EncapsulationEncapsulation  Read Chapter 9  Read Articles in eResource (Week 4 and 5) ArraysArrays ObjectsObjects  Review WS4 Programming assignment  Team project approval