Java Tutorial

Java Tutorial Object-Oriented Programming Rajan Gupta Singsys Pte. Ltd.

Different Programming Paradigms Functional/procedural programming: program is a list of instructions to the computer Object-oriented programming program is composed of a collection objects that communicate with each other

Main Concepts  Object Class Inheritance Encapsulation

Objects identity – unique identification of an object attributes – data/state services – methods/operations − supported by the object − within objects responsibility to provide these services to other clients

Inheritance Class hierarchy Generalization and Specialization − subclass inherits attributes and services from its superclass − subclass may add new attributes and services − subclass may reuse the code in the superclass − subclasses provide specialized behaviors (overriding and dynamic binding) − partially define and implement common behaviors (abstract)

Encapsulation  Separation between internal state of the object and its external aspects  How ? − control access to members of the class − interface “type”

Why Java ? Portable Easy to learn [ Designed to be used on the Internet ]

Platform Dependent myprog.c C source code gcc myprog.exe machine code OS/Hardware Platform Independent myprog.java javac myprog.class bytecode Java source code JVM OS/Hardware

Primitive types • • • • • • • • int 4 bytes short 2 bytes Behaviors is long 8 bytes exactly as byte 1 byte in C++ float 4 bytes double 8 bytes char Unicode encoding (2 bytes) Note: boolean {true,false} Primitive type always begin with lower-case

Wrappers Java provides Objects which wrap primitive types and supply methods. Example: Integer n = new Integer(“4”); int m = n.intValue(); Read more about Integer in JDK Documentation

Hello World Hello.java class Hello { public static void main(String[] args) { System.out.println(“Hello World !!!”); } } C:javac Hello.java C:java Hello ( compilation creates Hello.class ) (Execution on the local JVM)

Arrays • Array is an object • Array size is fixed Animal[] arr; // nothing yet … arr = new Animal[4]; // only array of pointers for(int i=0 ; i < arr.length ; i++) { arr[i] = new Animal(); // now we have a complete array

Arrays - Multidimensional  In C++ a Animal arr[2][2] Is: • In Java Animal[][] arr= new Animal[2][2] What is the type of the object here ?

Static data Member data - Same data is used for all the instances (objects) of some Class. Assignment performed on the first access to the Class. Only one instance of ‘x’ exists in memoryb a Class A { public int y = 0; public static int x_ = 1; }; A a = new A(); A b = new A(); System.out.println(b.x_); a.x_ = 5; System.out.println(b.x_); A.x_ = 10; System.out.println(b.x_); Output: 1 5 10 0 y 0 y 1 A.x_

Static • Member function – Static member function can access only static members – Static member function can be called without an instance. Class TeaPot { private static int numOfTP = 0; private Color myColor_; public TeaPot(Color c) { myColor_ = c; numOfTP++; } public static int howManyTeaPots() { return numOfTP; } // error : public static Color getColor() { return myColor_; } }

String is an Object • Constant strings as in C, does not exist • The function call foo(“Hello”) creates a String object, containing “Hello”, and passes reference to it to foo. • There is no point in writing : String s = new String(“Hello”); • The String object is a constant. It can’t be changed using a reference to it.

Flow control Basically, it is exactly like c/c++. do/while if/else If(x==4) { // act1 } else { // act2 } int i=5; do { // act1 i--; } while(i!=0); for int j; for(int i=0;i<=9;i++) { j+=i; } switch char c=IN.getChar(); switch(c) { case ‘a’: case ‘b’: // act1 break; default: // act2 }

Access Control • public member (function/data) – Can be called/modified from outside. • protected – Can be called/modified from derived classes • private – Can be called/modified only from the current class • default ( if no access modifier stated ) – Usually referred to as “Friendly”. – Can be called/modified/instantiated from the same package.

Inheritance Base Derived class Base { Base(){} Base(int i) {} protected void foo() {…} } class Derived extends Base { Derived() {} protected void foo() {…} Derived(int i) { super(i); … super.foo(); } } As opposed to C++, it is possible to inherit only from ONE class. Pros avoids many potential problems and bugs. Cons might cause code replication

Inheritance (2) • In Java, all methods are virtual : class Base { void foo() { System.out.println(“Base”); } } class Derived extends Base { void foo() { System.out.println(“Derived”); } } public class Test { public static void main(String[] args) { Base b = new Derived(); b.foo(); // Derived.foo() will be activated } }

Abstract • abstract member function, means that the function does not have an implementation. • abstract class, is class that can not be instantiated. AbstractTest.java:6: class AbstractTest is an abstract class. It can't be instantiated. new AbstractTest(); ^ 1 error NOTE: An abstract class is not required to have an abstract method in it. But any class that has an abstract method in it or that does not provide an implementation for any abstract methods declared in its superclasses must be declared as an abstract class.

Interface Interfaces are useful for the following: • Capturing similarities among unrelated classes without artificially forcing a class relationship. • Declaring methods that one or more classes are expected to implement. • Revealing an object's programming interface without revealing its class.

Interface interface IChef { void cook(Food food); } interface BabyKicker { void kickTheBaby(Baby); } interface SouthParkCharacter { void curse(); } class Chef implements IChef, SouthParkCharacter { // overridden methods MUST be public // can you tell why ? public void curse() { … } public void cook(Food f) { … } } * access rights (Java forbids reducing of access rights)

Collections • Collection/container – object that groups multiple elements – used to store, retrieve, manipulate, communicate aggregate data • Iterator - object used for traversing a collection and selectively remove elements • Generics – implementation is parametric in the type of elements

Java Collection Framework • Goal: Implement reusable data-structures and functionality • Collection interfaces - manipulate collections independently of representation details • Collection implementations - reusable data structures List<String> list = new ArrayList<String>(c); • Algorithms - reusable functionality – computations on objects that implement collection interfaces – e.g., searching, sorting – polymorphic: the same method can be used on many different implementations of the appropriate collection interface

Collection Interface • Basic Operations – – – – – – int size(); boolean isEmpty(); boolean contains(Object element); boolean add(E element); boolean remove(Object element); Iterator iterator(); • Bulk Operations – – – – – boolean containsAll(Collection<?> c); boolean addAll(Collection<? extends E> c); boolean removeAll(Collection<?> c); boolean retainAll(Collection<?> c); void clear(); • Array Operations – Object[] toArray(); <T> T[] toArray(T[] a); }

Final • final member data Constant member • final member function The method can’t be overridden. • final class ‘Base’ is final, thus it can’t be extended final class Base { final int i=5; final void foo() { i=10; //what will the compiler say about this? } } class Derived extends Base { // Error // another foo ... void foo() { } (String class is final) }

final Derived.java:6: Can't subclass final classes: class Base class class Derived extends Base { ^ 1 error final class Base { final int i=5; final void foo() { i=10; } } class Derived extends Base { // Error // another foo ... void foo() { } }

Exception - What is it and why do I care? Definition: An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions. • Exception is an Object • Exception class must be descendent of Throwable.

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