Encapsulation CMSC 202

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 2Version 9/12

Encapsulation Combining data and operations into a single entity (class) Providing appropriate access control Achieved through information hiding/abstraction 3 Client Program Pre-coded Class X Pre-coded Class Y Version 9/12 Class X interface Class Y interface

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. 4Version 9/12 Client Program Pre-coded Class X Class X interface Modified Class X Modified Class X Class X interface X

Class Interface Instance variables – name – what it represents – type Methods – name – what it does (not how) – pre & post-conditions – parameters – return value Version 9/125 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 )

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 6Version 9/12 Reference:

Private Instance Variables Example 7 public class Car { public String model; public String make; private int year; public void setYear(int newYear){ year = newYear; } Version 9/12 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 }

Private Instance Variables: Summary Only accessible within the class Are not directly accessible by a client program Hide implementation details, 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. 8Version 9/12 Reference:

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. 9Version 9/12

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. 10Version 9/12

Example Car Accessor and Mutator 11 public class Car { private int year; // 4-digit year between 1000 and 9999 private String vin; // Vehicle identification number // Accessor to return the vin member public String getVin() { return vin; } // Mutator to change the year member public boolean setYear(int newYear) { if(newYear >= 1000 && newYear <= 9999) { year = newYear; return true; } return false; } //... } Version 9/12

Accessor/Mutator Caution In general, do not provide accessors and mutators for all private instance variables. Bottom line: Limit the class interface. 12Version 9/12

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. 13Version 9/12 Reference:

Private Method Example 14 public class Car { private int year; // 4-digit year between 1000 and 9999 // Helper method – class scope private boolean isValidYear(int year) { return(year >= 1000 && year <= 9999); } // Mutator to change the year member public boolean setYear(int newYear) { if(isValidYear(newYear)) { year = newYear; return true; } return false; } //... } Version 9/12

Encapsulation Summary Combine methods and data in a single class. Use private instance variables for information hiding. Minimize the class’s public interface. 15 Keep it secret, keep it safe Version 9/12

Method Overloading

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. 17 Cat myCat = new Cat(); Dog myDog = new Dog(); System.out.println(myCat); System.out.println(myDog); Version 9/12

Method Overloading When two or more methods in the same class have the same name Example: 18Version 9/12 public boolean setStyle(String make, String model) public boolean setStyle(String make) Reference:

Car Class — Overloaded setStyle 19 public class Car { //... private String model; private String make; public boolean setStyle(String make, String model) { if(isValidMake(make) && isValidModel(model)) { this.make = make; this.model = model; return true; } return false; } public boolean setStyle(String make) { if(isValidMake(make)) { this.make = make; return true; } return false; } private boolean isValidMake(String make) { return make != null && !make.equals(""); } private boolean isValidModel(String model) { return model != null && !model.equals(""); } Version 9/12

CarDemo Class Version 9/1220 How does Java know which setStyle method to call? 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); }

Method Signature A method is uniquely identified by its – name and – parameter list (types and order). This is known as its signature. Examples … 21 public boolean setStyle(String make, String model) public boolean setStyle(int year, String make, String model) public boolean setStyle(String make, String model, String color) public boolean setStyle(int year, String color) public boolean setStyle(String make) Version 9/12 Reference:

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. 22 public void setStyle(String make) { /* code here */ } public boolean setStyle(String model) { /* code here */ } car.setStyle("Mazda"); Version 9/12

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. 23 //version 1 public double calculateAverage(int a, double b) { /* code */ } //version 2 public double calculateAverage(double a, int b) { /* code */ } MathUtils math = new MathUtils(); math.calculateAverage(5, 7);// two int parameters Version 9/12 math.calculateAverage(5, (double)7); OR Math.calculateAverage((double)5, 7);