CSM-Java Programming-I Spring,2005

Fundamental Data Types Lesson - 2 CSM-Java Programming-I Spring,2005

final Keyword • Any number that is not completely self-explanatory should be declared as a named constant. • A final variable is a constant. Once its value has been set, it cannot be changed. • Use named constants to make your programs easier to read and maintain. • Eg: final int DAYS_PER_YEAR = 365; CSM-Java Programming-I Lesson-1

static Keyword • Java numbers are not objects, so you cannot invoke a method on a number. Eg: To calculate sqrt or power. • A static method does not operate on an object. • Eg: Math.sqrt(x); Math.round(y); CSM-Java Programming-I Lesson-1

Static Revisited • When a member is declared static, it can be accessed before any objects of its class are created. • Both methods and variables can be declared static. • A static member is a member that is only one per class, rather than one in every object created from that class. • A static method can access only static variables and static methods of the class. • There is no this reference because there is no specific object being operated on. • classname.staticmethod(); CSM-Java Programming-I Lesson-1

Inheritance • When a class B acquires the properties of another class A, then class B is said to have inherited class A. • Here, class A is a superclass and class B is a subclass. • A subclass inherits all the instance variables and methods defined by the superclass and adds its own, unique elsements. • The keyword extends is used to inherit a class. Eg: class subclass extends superclass { CSM-Java Programming-I Lesson-1

Inheritance • Every class in Java is an extended class, whether it is declared with an extends keyword or not. • If a class does not explicitly extend from another class, it implicitly extends the Object class. CSM-Java Programming-I Lesson-1

Example: Inheritance public class BankAccount { int acctNum; String name; public BankAccount(int bAcctNum, String name) { acctNum = bAcctNum; ……. } public double getBalance () { …… } } CSM-Java Programming-I Lesson-1

Example: Inheritance public class CheckingAccount extends BankAccount { double interest; public CheckingAccount(int aNum, String aName, double aInt) { super (aNum, aName); interest = aInt; } public double calculateInterest() { ……. }} CSM-Java Programming-I Lesson-1

super • Whenever a subclass class super(), it is calling the constructor of its immediate superclass. • super() must be the first statement executed inside a subclass constructor. • super.member always uses the superclass’s member (member could be a method or an instance variable) CSM-Java Programming-I Lesson-1

Example: super public class BankAccount { protected double getBalance() { return 1000.00; } } public class SavingsAccount extends BankAccount{ protected double getBalance() { return 1010.00; } CSM-Java Programming-I Lesson-1

super protected void printBalance() { BankAccount ba = this; //superclass variable //can reference //subclass object. System.out.println(super.getBalance()); System.out.println(this.getBalance()); System.out.println(ba.getBalance()); } } Output: 1000.00 1010.00 1010.00 CSM-Java Programming-I Lesson-1

Method Overriding • When a method in a subclass has the same name and type signature and a method in its superclass, then the method in the subclass is said to override the method in the subclass. • Only non-static methods can be overridden. • Both signature and return type must be the same as the superclass. • The throws clause of an overriding method can have fewer types listed than the method in the superclass, or more specific types or both. CSM-Java Programming-I Lesson-1

Example: Method Overriding public class Parent { public void hello() { System.out.println(“Hello from parent”); } } public class Child extends Parent { public void hello() { System.out.println(“Hello from Child”); } } CSM-Java Programming-I Lesson-1

Method Overriding • An extended class can change the access of a superclass’s methods, but only if it provides more access. • A method declared protected in the super class can be redeclared protected or public, but not private. • Fields cannot be overridden; they can only be hidden. • To access the hidden fields use the super keyword. CSM-Java Programming-I Lesson-1

final and Inheritance • A method declared final cannot be overridden • A class can also be declared final. Such a class cannot be extended. final class Security { // } • A final class’s methods are implicitly final. • static and private methods cannot be overridden. CSM-Java Programming-I Lesson-1

Abstract Classes • A superclass that only defines a generalized form that will be shared by all its subclasses, leaving the implementation details to its subclasses is said to an abstract class. • A concrete class has concrete methods, including implementations of any abstract methods inherited from its superclasses. • Any class with abstract methods should be declared abstract. CSM-Java Programming-I Lesson-1

Example: Abstract Class abstract class Shape { abstract double area(); public void display () { // concrete method // Do something } } class Square extends Shape { double area() { // Do something } CSM-Java Programming-I Lesson-1

Abstract Classes • An abstract class cannot have objects because it is not complete, but it can have references. • Static methods and constructors cannot be declared abstract. • Any subclass of an abstract class must either implement all the abstract methods in the superclass or be itself declared abstract. • A concrete method can be overriden to become abstract. • It is illegal to declare a class both final and abstract. CSM-Java Programming-I Lesson-1

CSM-Java Programming-I Spring,2005