Encapsulation

1. Encapsulation CMSC 202

2. Types of Programmers • Class programmers • Developers of new classes • Goal: Expose the minimum interface necessary to use a new class • Implementation (code) is “hidden” • Client programmers • Write code (the client program) that uses existing classes

3. Encapsulation • Combining data and operations into a single entity (class) • Providing appropriate access control • Achieved through information hiding/abstraction Pre-coded Class X Class X interface Client Program Pre-coded Class Y Class Y interface

4. The Value of Encapsulation • Client programmers do not need to know how the class is implemented, only how to use it. • Class implementation may be changed with no impact on code that uses the class. Client Program X Pre-coded Class X Class X interface Modified Class X Class X interface

5. Class Interface • Instance variables • name • what it represents • type • Methods • name • what it does (not how) • pre & post-conditions • parameters • return value public String make; // car make public String model; // car model public int year; // car year /* Resets a car’s make * Preconditions: New make is non-null * Postconditions: Car’s make is reset * Parameters: Car’s new make * Returns: Nothing */ public void getMake(String newMake )

6. Visibility Modifiers • Provide access control to class • instance variables • constants • methods • public – directly accessible by client code • private – accessible only by methods within the class (class scope) • Other – later Reference: http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html

7. Private Instance Variables Example public class Car { public String model; public String make; private int year; public void setYear(intnewYear){ year = newYear; } } public class CarDemo { public static void main(String[] args) { Car car = new Car(); car.make = “Toyota"; // OK car.model = “Prius"; // OK car.year = 2008; // compiler error car.setYear(2008); // OK } }

8. Private Instance Variables: Summary • Only accessible within the class • Are notdirectly accessibleby a client program • Hide implementationdetails, promoting encapsulation • Good programming practice: • Label all instance variables as non-public (typically, private). • Bottom line: • The class should have complete control over how/when/if the instance variables are changed or accessed. Reference: http://docs.oracle.com/javase/tutorial/java/javaOO/variables.html

9. Accessors and Mutators • Accessor method (getter) • Retrieves the value of a private instance variable • Conventions: • Start the method name with get. • Start edge case for booleans with is. • Mutator method (setter) • Changes the value of a private instance variable. • Convention: Start the method name with set. • Gives the client program indirect access to the instance variables.

10. Why Accessors and Mutators? • Accessors • The class implementer decides • which instance variables will have accessors, and • in what form they will be returned. • Mutators • validate the new value of the instance variable, and • decide whether or not to actually make the requested change.

11. Example Car Accessor and Mutator public class Car { private int year; // 4-digit year between 1000 and 9999 private String vin; // Vehicle identification number // Accessorto return the vin member public String getVin() { return vin; } // Mutatorto change the year member public booleansetYear(intnewYear) { if(newYear >= 1000 && newYear <= 9999) { year = newYear; return true; } return false; } // ... }

12. Accessor/Mutator Caution • In general, do not provide accessors and mutators for all private instance variables. • Bottom line: Limit the class interface.

13. Private Methods • Have class scope • Cannot be invoked by a client program • Used as “helper” methods to support top-down implementation of a public method. Reference: http://docs.oracle.com/javase/tutorial/java/javaOO/methods.html

14. Private Method Example public class Car { private int year; // 4-digit year between 1000 and 9999 // Helper method – class scope private booleanisValidYear(int year) { return(year >= 1000 && year <= 9999); } // Mutatorto change the year member public booleansetYear(intnewYear) { if(isValidYear(newYear)) { year = newYear; return true; } return false; } // ... }

15. Encapsulation Summary • Combine methods and data in a single class. • Use private instance variables for information hiding. • Minimize the class’s public interface. Keep it secret, keep it safe

16. Method Overloading

17. Method Names • Different classes can define a method with the same name. • Example: • Java can determine which method to call based on the type of the calling object. Cat myCat = new Cat(); Dog myDog = new Dog(); System.out.println(myCat); System.out.println(myDog);

18. Method Overloading • When two or more methods in the same class have the same name • Example: publicbooleansetStyle(String make, String model) publicbooleansetStyle(String make) Reference: http://docs.oracle.com/javase/tutorial/java/javaOO/methods.html

19. Car Class — Overloaded setStyle public class Car { // ... private String model; private String make; public booleansetStyle(String make, String model) { if(isValidMake(make) && isValidModel(model)) { this.make = make; this.model = model; return true; } return false; } public booleansetStyle(String make) { if(isValidMake(make)) { this.make = make; return true; } return false; } private booleanisValidMake(String make) { return make != null && !make.equals(""); } private booleanisValidModel(String model) { return model != null && !model.equals(""); } }

20. CarDemo Class public class CarDemo { public static void main(String[] args) { Car car = new Car(); car.setStyle("Mazda"); System.out.println(car); car.setStyle("Audi", "A8"); System.out.println(car); } } How does Java know which setStyle method to call?

21. Method Signature • A method is uniquely identified by its • name and • parameter list (types and order). • This is known as its signature. • Examples … public booleansetStyle(String make, String model) public booleansetStyle(int year, String make, String model) public booleansetStyle(String make, String model, String color) public booleansetStyle(int year, String color) public booleansetStyle(String make) Reference: http://docs.oracle.com/javase/tutorial/java/javaOO/methods.html

22. Return Type is Not Enough • Attempt to overload Car’s setStyle() method by using different return types: • This is NOT valid method overloading. • The method return value can be ignored. publicvoidsetStyle(String make) { /* code here */} publicbooleansetStyle(String model) { /* code here */ } car.setStyle("Mazda");

23. Common Problem • Type coercion can confuse the compiler: • So now, • The Java compiler can’t decide whether to • promote 7 to 7.0 and call the first version of calculateAverage, or • promote 5 to 5.0 and call the second. • Solution: Cast the arguments so that they match one of the signatures. //version 1 publicdoublecalculateAverage(int a, double b) { /* code */ } //version 2 publicdoublecalculateAverage(double a, int b) { /* code */ } MathUtils math = newMathUtils(); math.calculateAverage(5, 7); // two intparameters math.calculateAverage(5, (double)7); OR Math.calculateAverage((double)5, 7);