Java Session. Day 2. Reference: Head First Java

Transcription

1 Java Session Day 2 shrishty_bcs11@nitc.ac.in Reference: Head First Java

2 Encapsulation This hides the data!! How do we do it? By simply using public private access modifiers. 1. Mark the instance variables private 2. Mark the getter and setter as private class GoodDog { private int size; public int getsize() { return size; // instance variable // Getter public void setsize(int s) { // Setter size = s; void bark() { // Method if(size > 60) { System.out.println( wooof! Woof! ); else if (size > 14) { System.out.println( Ruff! Ruff! ); else { System.out.println( Yip! Yip ); class TestDog { public static void main (String[] args) { GoodDog one = new GoodDog(); one.setsize(70); GoodDog two = new GoodDog(); two.setsize(8); System.out.println( Dog one: + one.getsize()); System.out.println( Dog two: + two.getsize());

3 one.bark(); two.bark(); Even though the methods here don't really add new functionality here, but you can change your mind later and make a method safer, faster and better. Remember: Any place where a particular value can be used, we use a method call that returns the type can be used. Instead of: int x = ; Use: int x = 3 + one.getsize(); What is the difference between instance variable and Local variable? 1. Instance variable are declared within a class but not within a method. class Monkey { private double height = 12.4; private String breed; 2. Local variables are declared within a method. Class AddThing { int a; int b = 12; public int add() { int total = a + b; return total; // this is a local variable 3. local variables MUST be initialized before use, but in case of instance variable it has a default value

4 The == operator & equals() method For primitive variables int a = 3; byte b = 3; if (a == b) { // true For non primitive variables, they are different But Integer i = new Integer(10); Integer j = i; if(i == j) {// this will return true Integer i = new Integer(10); Integer j = new Integer(10); if(i == j) {// This will give false Then how do we check if two objects are equivalent?? Note: Equivalent means that both the objects represent the same object. Using.equals() Integer i = new Integer(10); Integer j = new Integer(10); if(i.equals(j)) {// This returns true

5 Inheritance Lets have four classes, Novel{ TextBook { Diary{ NoteBook{ read(); read(); read(); read(); author(); author(); author(); author(); cost(); cost(); cost(); cost(); Is it not boring to write the same code, every time you make a new class. Can we abstract out the common features from all the classes and put it into a new class called Books?? Yes!! Books { > Novel // Novel inherits form books read(); > TextBook // TextBook inherits from books author(); > Diary // Diary Inherits from books cost(); > NoteBook // NoteBook inherits from books Books is the superclass and other four classes are called subclasses of Books. The subclasses inherits the methods and instance variables of the superclass. But what if we want to inherit all methods of a superclass but not one!! For NoteBook class the read() method works differently, we can't read an empty notebook!! Override that method, i.e. Write a new method with same name inside the subclass with the class specific properties. Superclass is more abstract and subclass is more specific. Instance variables are not overridden (they don't need to be). Five steps to remember: Look for the objects that have common attributes and behavior Design a class that have common state and behavior Decide a subclass that needs behaviors that are specific to that particularsubclass type.

6 Look for more opportunities to use abstraction, by finding two or more subclasses that might need common behavior. Finish the class hierarchy Lets build an animal kingdom Tiger, Lion, Dog, Cat, Fox, Hippopotamus What is common in all of them and How are they related?? They have five instance variable in common: picture food hunger boundaries location // Well they can have more They have four methods in common: makenoise() eat() sleep() roam() Now we have gotten all common instance variable and methods for animals we can build Animal class. Class Animal { // instance variables // Methods picture; makenoise() { food; eat() { hunger; sleep { boundaries; roam { location; But does each animal eat the same way or do all of them make the same noise?? No. We need to override the methods makenoise() and eat()

7 Can we find more subclasses? It seems Wolf and dog have same behavior and can be kept in separate class And Lion, tiger and cat can be subclass of another separate class Now we can finish the class hierarchy?!! Animal { picture; food; hunger; boundaries; location; > Canine // Canine inherits from Animal > Feline // Feline inherits from Animal > Hippo // Hippo inherits from Animal makenoise() eat(); sleep(); roam(); Hippo does not belong to any other subclasses of Animal Canine class has Dog and wolf as subclass. Feline class has Lion, Tiger and Cat as subclass But after overriding which method is called?? The lowest one wins!! Using IS A and HAS A If class B extends class A, then class B IS A class A. If class B, class C passes the IS A test for both B and A IS A works in only one direction Access Specifiers

8 public: public classes methods and fields are accessible to all. Protected: methods and fields can be accessed within same class to which the methods and fields belong, within its subclasses, within classes of same package, but not from anywhere else. default: methods and fields will be accessible inside the same package. private: methods and fields are accessible only within same class. Public: members are in inherited and private members are not inherited. Polymorphism Remember how objects were declared and assigned Declare the reference variable ( Book mybook = new Book()) Create an object (Book mybook = new Book()) Link the object and the reference (Book mybook = new Book()) Here reference and object type are the same but with polymorphism the reference and object type can be different. Animal mydog = new Dog(); The reference variable type is declared as Animal and the object is created as new Dog(). Example: Animal[] animals = new Animals[5]; animals[0] = new Dog(); animals[1] = new Cat(); animals[2] = new Wolf(); animals[3] = new Hippo(); animals[5] = new Lion(); for(int i = 0; i < animals.length; i++){ animal[i].eat(); animal[i].roam();

9 We can also have polymorphic arguments and return types class Vet { public void trythis(animal a){ // Do something with animal a a.makenoise(); Method Overloading class PetOwner { public void start() { Vet v = new Vet(); Dog d = new Dog(); Hippo h = new Hippo(); v.trythis(d); v.trythis(h); An overloaded method is just different method that happens to have same name. Example: public class Overloads { String uniqueid; public int addnums(int a, int b){ return a + b; public double addnums(double a, double b){ return a + b; public void setuniqueid(string theid) { uniqueid = theid; public void setuniqueid(int ssnumber) { String numstring = + ssnnumber; setuniqueid(numstring);

10 Example 2: Testing inheritance public class Rowboat extends Boat { public void rowtheboat() { System.out.print( Ta ta la ); public class Boat { private int length; public void setlength (int len) { length = len; public int getlength() { return length; public void move() { System.out.print( drift ); public class TestBoats { public static void main(string args[]){ Boat b1 = new Boat(); Sailboat b2 = new Sailboat(); Rowboat b3 = new Rowboat(); b2.setlength(32); b1.move(); b3.move(); b2.move(); public class Sailboat extends Boat { public void move() { System.out.print( hoist sail ) ;

11 OUTPUT: drift drift hoist sail