Defining Classes 1

1. Defining Classes 1 This slide set was compiled from the Absolute Java textbook and the instructor’s own class materials. SWE 510: Object Oriented Programming in Java

2. Outline • Introduction/Overview • Constructors • Information Hiding • Overloading • Recursion • Homework!

3. Outline • Introduction/Overview • Constructors • Information Hiding • Overloading • Recursion • Homework!

4. b 20 a 10 object1 object2 data data 10 20 method( ) method( ) Primitive Data Types versusAbstract Data Types (Classes) • Primitive data types • a single piece of data • byte, char, short, int, long, float, double, boolean • Variables declared before their use: int a = 10, b = 20; • Classes • a collection of multiple pieces of data + methods • Objects defined before their use (“instantiation”) • Including the same methods • Including the same set of data, but • Maintaining different values in each piece of data MyClass object1 = new MyClass( ); MyClass object2 = new MyClass( ); object1.data = 10; object2.data = 20; SWE 510: Object Oriented Programming in Java

5. Primitive Data Types versus Classes—comparison • “Primitive” data type (String is not primitive in java, but to some extent we can treat it like one) • String can be used like a primitive data type String myString; myString = “test string”; String myString = “test string”; String myString = someOtherString; • Abstract data type (class) • The String class is much more powerful than a primitive type String myString = new String (“test string”); myString.length(); myString.valueOf(); myString.equals(someOtherString); myString.compareTo(someOtherString); myString.split(regularExpression); SWE 510: Object Oriented Programming in Java

6. Java Doc • Google • Use Google to locate the particular class or language feature of interest • Dive Straight in • Go straight to the Java documentation • http://download.oracle.com/javase/6/docs/ • http://download.oracle.com/javase/6/docs/api/ SWE 510: Object Oriented Programming in Java

7. The Contents of a Class Definition • A class definition specifies the data items and methods that all of its objects will have • These data items and methods are sometimes called members of the object • Data items are called fields or instance variables • Instance variable declarations and method definitions can be placed in any order within the class definition • Methods define an object’s behavior

8. Members, Instance Variables, and Methods public class Course { public String department; // SWE, IAS, CS, etc public int number; // course number public char section; // A, B, C ... public int enrollment; // the current enrollment public int limit; // max number of students public double textPrice1; // primary textbook public double textPrice2; // secondary textbook public double courseFee; // laboratory fee public int availableSpace( ) { return limit - enrollment; } public double capacity( ) { return ( double )enrollment / limit; } public double totalExpenditure( ) { return textPrice1 + textPrice2 + courseFee; } } Instance variables: (data members) Each object has its own values in these variables. members Methods: Each object has the same methods (actions, computations). SWE 510: Object Oriented Programming in Java

9. Object Instantiation with new • Declare a reference variable (a box that contains a reference to a new instance.) ClassName object; • object has no reference yet, (= null). • Create a new instance from a given class. object = new ClassName( ); • object has a reference to a new instance. • Declare a reference variable and initialize it with a reference to a new instance created from a given class ClassName object = new ClassName( ); SWE 510: Object Oriented Programming in Java

10. public class Course { public String department; // CSS, IAS, BUS, NRS, EDU public int number; // course number public char section; // A, B, C ... public double textPrice1; // primary textbook public double textPrice2; // secondary textbook public double courseFee; // laboratory fee public double totalExpenditure( ) { return textPrice1 + textPrice2 + courseFee; } } Multiple pieces of data Method myCourse2 myCourse1 Class Instantiation--Example public class CourseDemo { public static void main( String[] args ) { Course myCourse1 = new Course( ); Course myCourse2 = new Course( ); myCourse1.department = "CSS"; myCourse1.number = 161; myCourse1.section = 'A'; myCourse1.textPrice1 = 111.75; myCourse1.textPrice2 = 37.95; myCourse1.courseFee = 15; myCourse2.department = "CSS"; myCourse2.number = 451; myCourse2.section = 'A'; myCourse2.textPrice1 = 88.50; myCourse2.textPrice2 = 67.50; myCourse2.courseFee = 0; System.out.println( "myCourse1(" + myCourse1.department + myCourse1.number + myCourse1.section + ") needs $" + myCourse1.totalExpenditure( ) ); System.out.println( "myCourse2(" + myCourse2.department + myCourse2.number + myCourse2.section + ") needs $" + myCourse2.totalExpenditure( ) ); } } instantiated instantiated department: CSS number: 451 section: A textPrice1: $88.50 textPrice2: $67.50 courseFee: $0 totalExpenditure( ) department: CSS number: 161 section: A textPrice1: $111.75 textPrice2: $37.95 courseFee: $15 totalExpenditure( ) 164.7 = 117.75 + 37.95 + 15 156.0 = 88.50 +67.50 + 0 myCourse1(CSS161A) needs $164.7 myCourse2(CSS451A) needs $156.0 SWE 510: Object Oriented Programming in Java

11. The this Parameter • All instance variables are understood to have <the calling object>. in front of them • If an explicit name for the calling object is needed, the keyword this can be used • myInstanceVariablealways means and is always interchangeable with this.myInstanceVariable

12. The this Parameter • thismust be used if a parameter or other local variable with the same name is used in the method • Otherwise, all instances of the variable name will be interpreted as local int someVariable = this.someVariable local instance

13. The this Parameter • The this parameter is a kind of hidden parameter • Even though it does not appear on the parameter list of a method, it is still a parameter • When a method is invoked, the calling object is automatically plugged in for this • A Constructor has a this Parameter

14. Outline • Introduction/Overview • Constructors • Information Hiding • Overloading • Recursion • Homework!

15. Constructors • A constructor is a special kind of method that is designed to initialize the instance variables for an object: Public ClassName(anyParameters){code} • A constructor must have the same name as the class • A constructor has no type returned, not even void • Constructors are typically overloaded

16. Constructors • A constructor is called when an object of the class is created using new: ClassName objectName = new ClassName(anyArgs); • The name of the constructor and its parenthesized list of arguments (if any) must follow the new operator • This is the only valid way to invoke a constructor: a constructor cannot be invoked like an ordinary method • If a constructor is invoked again (using new), the first object is discarded and an entirely new object is created • If you need to change the values of instance variables of the object, use mutator methods instead

17. You Can Invoke Another Method in a Constructor • The first action taken by a constructor is to create an object with instance variables • Therefore, it is legal to invoke another method within the definition of a constructor, since it has the newly created object as its calling object • For example, mutator methods can be used to set the values of the instance variables • It is even possible for one constructor to invoke another

18. Include a No-Argument Constructor • If you do not include any constructors in your class, Java will automatically create a default or no-argument constructor that takes no arguments, performs no initializations, but allows the object to be created • If you include even one constructor in your class, Java will not provide this default constructor • If you include any constructors in your class, be sure to provide your own no-argument constructor as well

19. Default Variable Initializations • Instance variables are automatically initialized in Java • boolean types are initialized to false • Other primitives are initialized to the zero of their type • Class types are initialized to null • However, it is a better practice to explicitly initialize instance variables in a constructor • Note: Local variables are not automatically initialized

20. Accessing Instance Variables • Declaring an instance variable in a class • public type instanceVariable; // accessible from any methods (main( )) • private type instanceVariable; // accessible from methods in the same class • Examples: public String department; public int number; public char section; public int enrollment; • Assigning a value to an instance variable of a given object • objectName.instanceVariable = expression; • Examples: myCourse1.department = "CSS"; myCourse1.number = 161; myCourse1.section = 'A'; myCourse1.enrollment = 24; • Reading the value of an instance of a given object • Operator objectName.instanceVariable operator • Example: System.out.println( "myCourse1 = " + myCourse1.department + myCourse1.number + ")" ); SWE 510: Object Oriented Programming in Java

21. Class Names and Files • Each class should be coded in a separate file whose name is the same as the class name + .java postfix. • Example • Source code Course.java CourseDemo.java • Compilation (from DOS/Linux command line.) javac Course.java javac CourseDemo.java javac CourseDemo.java javac Course.java • Compiled code Course.class CourseDemo.class • Execution (from DOS/Linux command line.) java CourseDemo Either order is fine. Compiling CourseDemo.java first automatically compiles Course.java, too. Start with the class name that includes main( ). SWE 510: Object Oriented Programming in Java

22. Information Hiding and Encapsulation • Information hiding is the practice of separating how to use a class from the details of its implementation • Abstraction is another term used to express the concept of discarding details in order to avoid information overload • Encapsulation means that the data and methods of a class are combined into a single unit (i.e., a class object), which hides the implementation details • Knowing the details is unnecessary because interaction with the object occurs via a well-defined and simple interface • In Java, hiding details is done by marking them private

23. Outline • Introduction/Overview • Constructors • Information Hiding • Overloading • Recursion • Homework!

24. A Couple of Important Acronyms: API and ADT • The API or application programming interface for a class is a description of how to use the class • A programmer need only read the API in order to use a well designed class • An ADT or abstract data typeis a data type that is written using good information-hiding techniques

25. Encapsulation

26. public and private Modifiers • The modifier public means that there are no restrictions on where an instance variable or method can be used • The modifier private means that an instance variable or method cannot be accessed by name outside of the class • It is considered good programming practice to make all instance variables private • Most methods are public, and thus provide controlled access to the object • Usually, methods are private only if used as helping methods for other methods in the class

27. public and private • Public • Methods and instance variables accessible from outside of their class • Private • Methods and instance variables accessible within their class public class Course css263.method1( ) public type method1( ) { } css263.method2( ) public type method2( ) { } css263.utility( ) private type utility( ) { } css263.department public String department; public int number; private int enrollment; private int gradeAverage; css263.number css263.enrollment SWE 510: Object Oriented Programming in Java

28. Encapsulating Data in Class • Which design is more secured from malicious attacks? public class Course public class Course public type method1( ) { } public type method1( ) { } css161.method1( ) css263.method1( ) public type method2( ) { } public type method2( ) { } css161.utility( ) public type utility( ) { } private type utility( ) { } css161.number = 263 public String department; public int number; public int enrollment; public int gradeAverage; private String department; private int number; private int enrollment; private int gradeAverage; SWE 510: Object Oriented Programming in Java

29. Accessor and Mutator Methods • Accessor methods allow the programmer to obtain the value of an object's instance variables • The data can be accessed but not changed • The name of an accessor method typically starts with the word get • Mutator methods allow the programmer to change the value of an object's instance variables in a controlled manner • Incoming data is typically tested and/or filtered • The name of a mutator method typically starts with the word set

30. Mutator Methods Can Return a Boolean Value • Some mutator methods issue an error message and end the program whenever they are given values that aren't sensible • An alternative approach is to have the mutator test the values, but to never have it end the program • Instead, have it return a boolean value, and have the calling program handle the cases where the changes do not make sense

31. Outline • Introduction/Overview • Constructors • Information Hiding • Overloading • Recursion • Homework!

32. Overloading • Overloading--two or more methods in the same class have the same method name

33. Overloading--Rules • All definitions of the method name must have different signatures • A signature consists of the name of a method together with its parameter list • Differing signatures must have different numbers and/or types of parameters • The signature does not include the type returned • Java does not permit methods with the same signature and different return types in the same class

34. Overloading—example • Familiar overloaded method System.out.println() System.out.println(boolean) System.out.println(char) System.out.println(char[]) System.out.println(double) System.out.println(float) System.out.println(int) System.out.println(long) System.out.println(java.lang.Object) System.out.println(java.lang.String) SWE 510: Object Oriented Programming in Java

35. Overloading and Automatic Type Conversion • If Java cannot find a method signature that exactly matches a method invocation, it will try to use automatic type conversion • The interaction of overloading and automatic type conversion can have unintended results • In some cases of overloading, because of automatic type conversion, a single method invocation can be resolved in multiple ways • Ambiguous method invocations will produce an error in Java

36. Outline • Introduction/Overview • Constructors • Information Hiding • Overloading • Recursion • Homework!

37. Outline • Introduction/Overview • Constructors • Information Hiding • Overloading • Recursion • Homework! • Odds and Ends

38. Nothing to return The same data type A variable, a constant, an expression, or an object More About Methods • Two kinds of methods • Methods that only perform an action public void methodName( ) { /* body */ } • Example publicvoidwriteOutput( ) { System.out.println(month + " " + day + ", " + year); } • Methods that compute and return a result pubic type methodName( ) { /* body */ return a_value_of_type; } • Example publicdouble totalExpenditure( ) { return textPrice1 + textPrice2 + courseFee; } SWE 510: Object Oriented Programming in Java

39. Method to perform an action void Method( ) Return is not necessary but may be added to end the method before all its code is ended Example public void writeMesssage( ) { System.out.println( “status” ); if ( ) { System.out.println( “nothing” ); return; } else if ( error == true ) System.out.print( “ab” ); System.out.println( “normal” ); } Method to perform an action type Method( ) Return is necessary to return a value to a calling method. Example public double totalExpenditure( ) { return textPrice1 + textPrice2 + courseFee; } return Statement SWE 510: Object Oriented Programming in Java

40. Any Method Can Be Used As a void Method • A method that returns a value can also perform an action • If you want the action performed, but do not need the returned value, you can invoke the method as if it were a void method, and the returned value will be discarded: objectName.returnedValueMethod(); SWE 510: Object Oriented Programming in Java

41. Methods That Return a Boolean Value • An invocation of a method that returns a value of type boolean returns either true or false • Therefore, it is common practice to use an invocation of such a method to control statements and loops where a Boolean expression is expected • if-else statements, while loops, etc.