Core java oop

 Decision Making  Object Oriented Fundamentals  Inner Classes  Exception Handling

In this chapter we will: discuss the use of decision making in computer programming describe the use of the Java if and switch statements

Ensures that a statement is executed only when a condition is true Conditions typically involve comparison of variables or quantities for equality or inequality Example: if (age >= 18) out.println(“You are eligible to vote.”);

An if statement may have an optional else clause that will only be executed when the condition is false Example: if (wages <= 57600) tax = 0.124 * wages; else tax = 0.124 * 57600;

 Used to accomplish multi-way branching based on the value of an integer selector variable  Example: switch (numberOfPassengers) { case 0: out.println(“The Harley”); break; case 1: out.println(“The Dune Buggy”); break; default: out.println(“The Humvee”); }

What is Access Specifier? A mechanism to set access levels for classes, attributes, variables, methods and constructors. The four access levels are: //In increasing order of accessibility • PRIVATE - Visible to the same class only. • DEFAULT - Visible to the same package. No modifiers are needed. • PROTECTED - Visible to the package and all subclasses. • PUBLIC - Visible to the world. ACCESS SPECIFIERS in JAVA public class Student { private String course; public void setCourse(String courseNew) { this.course = courseNew; } public String getCourse() { return this.course; } }

Within a method/constructor, this is a reference to the current object — the object whose method/constructor is being called. this KEYWORD in JAVA public class Point { public int x = 0; public int y = 0; //constructor public Point(int a, int b){ x = a; y = b; } } public class Point { public int x = 0; public int y = 0; //constructor public Point(int x, int y){ this.x = x; this.y = y; } }

Technical Definition: Encapsulation is the technique of making the fields in a class private and providing access to the fields via public methods. ENCAPSULATION If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. For this reason, encapsulation is also referred to as data hiding.

ENCAPSULATION - EXAMPLE 1. class Student{ 2. private String name; 3. public String getName(){ 4. return name; 5. } 6. public void setName(String newName){ 7. name = newName; 8. } 9. } 10.class Execute{ 11. public static void main(String args[]){ 12. String localName; 13. Student s1 = new Student(); 14. localName = s1.getName(); 15. } 16.} At line no14, we can not write localName = s1.name; The public methods are the access points to a class's private fields(attributes) from the outside class.

Technical Definition: Inheritance can be defined as the process where one object (or class) acquires the properties of another (object or class). With the use of inheritance the information is made manageable in a hierarchical order. INHERITANCE ANIMAL MAMMEL DOG REPTILE

INHERITANCE  In programming, inheritance is brought by using the keyword EXTENDS  In theory, it is identified using the keyword IS-A.  By using these keywords we can make one object acquire the properties of another object. This is how the extends keyword is used to achieve inheritance. public class Animal{ } public class Mammal extends Animal{ } public class Reptile extends Animal{ } public class Dog extends Mammal{ }

INHERITANCE Now, based on the above example, In Object Oriented terms, the following are true: • Animal is the superclass of Mammal class. • Animal is the superclass of Reptile class. • Mammal and Reptile are subclasses of Animal class. • Dog is the subclass of both Mammal and Animal classes. Alternatively, if we consider the IS-A relationship, we can say: • Mammal IS-A Animal • Reptile IS-A Animal • Dog IS-A Mammal Hence : Dog IS-A Animal as well

OVERRIDING The process of defining a method in child class with the same name & signature as that of a method already present in parent class.  If a class inherits a method from its super class, then there is a chance to override the method provided that it is not marked final.  Benefit: A subclass can implement a parent class method based on its requirement.  In object-oriented terms, overriding means to override the functionality of an existing method.

OVERRIDING: USING THE SUPER KEYWORD class Animal{ public void move(){ System.out.println("Animals can move"); } } class Dog extends Animal{ public void move(){ super.move(); // invokes the super class method System.out.println("Dogs can walk and run"); } } class TestDog{ public static void main(String args[]){ Animal b = new Dog(); // Animal reference but Dog object b.move(); //Runs the method in Dog class } } When invoking a superclass version of an overridden method the super keyword is used. This would produce the following result: Animals can move Dogs can walk and run

POLYMORPHISM CASE I: METHOD POLYMORPHISM We can have multiple methods with the same name in the same / inherited / extended class. There are three kinds of such polymorphism (methods): 1. Overloading: Two or more methods with different signatures, in the same class. 2. Overriding: Replacing a method of BASE class with another (having the same signature) in CHILD class. 3. Polymorphism by implementing Interfaces. POLYMORPHISM = 1 METHOD/OBJECT HAVING MANY FORMS/ROLES

POLYMORPHISM – METHOD OVERLOADING class Test { public static void main(String args[]) { myPrint(5); myPrint(5.0); } static void myPrint(int i) { System.out.println("int i = " + i); } static void myPrint(double d) { System.out.println("double d = " + d); } } OUTPUT: int i = 5 double d = 5.0 Please note that the signature of overloaded method is different.

POLYMORPHISM – CONSTRUCTOR OVERLOADING This example displays the way of overloading a constructor and a method depending on type and number of parameters. class MyClass { int height; MyClass() { System.out.println("bricks"); height = 0; } MyClass(int i) { System.out.println("Building new House that is " + i + " feet tall"); height = i; } void info() { System.out.println("House is " + height + " feet tall"); } void info(String s) { System.out.println(s + ": House is “ + height + " feet tall"); } }

What is an Abstract class? Superclasses are created through the process called "generalization" Often, the superclass does not have a "meaning" or does not directly relate to a "thing" in the real world Abstract classes cannot be instantiated

Defining Abstract Methods Inheritance is declared using the "extends" keyword If inheritance is not defined, the class extends a class called Object public abstract class Transaction { public abstract int computeValue(); public class RetailSale extends Transaction { public int computeValue() { [...] Transaction - computeValue(): int RetailSale - computeValue(): int StockTrade - computeValue(): int public class StockTrade extends Transaction { public int computeValue() { [...] Note: no implementation

What is an Interface? • All methods defined in an interface are abstract. Interfaces can contain no implementation • Interfaces are declared using the "interface" keyword • Interfaces are more abstract than abstract classes • Interfaces are implemented by classes using the "implements" keyword.

Declaring an Interface public interface Steerable { public void turnLeft(int degrees); public void turnRight(int degrees); } In Steerable.java: public class Car extends Vehicle implements Steerable { public int turnLeft(int degrees) { [...] } public int turnRight(int degrees) { [...] } In Car.java: When a class "implements" an interface, the compiler ensures that it provides an implementation for all methods defined within the interface.

27  Inner classes are classes defined within other classes  The class that includes the inner class is called the outer class  There is no particular location where the definition of the inner class (or classes) must be place within the outer class  Placing it first or last, however, will guarantee that it is easy to find

28 public class Outer { private class Inner { // inner class instance variables // inner class methods } // end of inner class definition // outer class instance variables // outer class methods }

29  If an inner class is marked public, then it can be used outside of the outer class  In the case of a non-static inner class, it must be created using an object of the outer class BankAccount account = new BankAccount(); BankAccount.Money amount = account.new Money("41.99");

30  In the case of a static inner class, the procedure is similar to, but simpler than, that for nonstatic inner classes OuterClass.InnerClass innerObject = new OuterClass.InnerClass();  Note that all of the following are acceptable innerObject.nonstaticMethod(); innerObject.staticMethod(); OuterClass.InnerClass.staticMethod();

 A class can have as many inner classes as it needs.  Inner classes have access to each other’s private members as long as an object of the other inner class is used as the calling object. 31

32  It is legal to nest inner classes within inner classes  The rules are the same as before, but the names get longer  Given class A, which has public inner class B, which has public inner class C, then the following is valid: A aObject = new A(); A.B bObject = aObject.new B(); A.B.C cObject = bObject.new C();

33  If an object is to be created, but there is no need to name the object's class, then an anonymous class definition can be used  The class definition is embedded inside the expression with the new operator

Exceptions ➔ An "exceptional condition" that alters the normal program flow ➔ Derive from class Exception ➔ Exception is said to be "thrown" and an Exception Handler "catches" it ➔ Includes File Not Found, Network connection was lost, etc. Errors ➔ Represent unusual situations that are not caused by, and are external to, the application ➔ Application won't be able to recover from an Error, so these aren't required to handle ➔ Includes JVM running out of memory, hardware error, etc

Types of Exceptions ➔ Checked Exceptions ◆ Checked at compile time ◆ Must be either handled or specified using throws keyword ➔ Unchecked Exceptions ◆ Not checked at compile time ◆ Also called as Runtime Exceptions

Checked Exception Example ➔ import java.io.*; class Main { public static void main(String[] args) { FileReader file = new FileReader("a.txt"); BufferedReader fileInput = new BufferedReader(file); } } ➔ Compilation Error: Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - unreported exception java.io.FileNotFoundException; must be caught or declared to be thrown at Main.main(Main.java:5)

Runtime Exception Example ➔ class Main { public static void main(String args[]) { int x = 0; int y = 10; int z = y/x; } } ➔ Exception in thread "main" java.lang.ArithmeticException: / by zero at Main.main(Main.java:5) Java Result: 1

try-catch blocks ➔ try block : ◆ Used to enclose the code that might throw an exception. ◆ Must be used within the method. ◆ Java try block must be followed by either catch or finally block. ➔ catch block ◆ Java catch block is used to handle the Exception. It must be used after the try block only. ◆ You can use multiple catch block with a single try. Syntax of java try-catch try{ //code that may throw exception }catch(Exception_class_Name ref){ }

Example public class Testtrycatch{ public static void main(String args[]){ try{ int data=50/0; }catch(ArithmeticException e){ System.out.println(e); } System.out.println("Executing rest of the code..."); } } Output : Exception in thread main java.lang.ArithmeticException:/ by zero Executing rest of the code...

finally block ➔ Follows a try block. ➔ Always executes, whether or not an exception has occurred. ➔ Allows you to run any cleanup-type statements that you want to execute, no matter what happens in the protected code. ➔ Executes right before the return executes present in try block. Syntax of try-finally block: try{ // Protected Code that may throw exception }catch(Exception ex){ // Catch block may or may not execute }finally{ // The finally block always executes. }

Advantages of Exception Handling ➔ To maintain the normal flow of the application ➔ Separating Error-Handling Code from "Regular" Code ➔ Propagating Errors Up the Call Stack ➔ Grouping and Differentiating Error Types

Core java oop

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