Java basic understand OOP

University Of Gondar Edited 1 Basics of Object Oriented In Java by Habitamu Asimare

Object-Oriented Nomenclature Introduction to Object Oriented Programming2 “Class” means a category of things A class name can be used in Java as the type of a field or local variable or as the return type of a function (method) “Object” means a particular item that belongs to a class Also called an “instance” For example, consider the following line: String s1 = "Hello"; Here, String is the class, s1 is an instance variable of class String, whose value is the String object "Hello"

Example 1: Instance Variables (“Fields” or “Data Members”) Introduction to Object Oriented Programming3 class Ship1 { public double x, y, speed, direction; public String name; } public class Test1 { public static void main(String[] args) { Ship1 s1 = new Ship1(); s1.x = 0.0; s1.y = 0.0; s1.speed = 1.0; s1.direction = 0.0; // East s1.name = "Ship1"; System.out.println(s1.name + " is initially at (" + s1.x + "," + s1.y + ")."); s1.x = s1.x + s1.speed * Math.cos(s1.direction * Math.PI / 180.0); s1.y = s1.y + s1.speed * Math.sin(s1.direction * Math.PI / 180.0); System.out.println(s1.name + " has moved to (" + s1.x + "," + s1.y + ")."); } }

Instance Variables: Results Introduction to Object Oriented Programming4 Compiling and Running: javac Test1.java java Test1 Output: Ship1 is initially at (1,0). Ship2 has moved to (-1.41421,1.41421).

Example 1: Major Points Introduction to Object Oriented Programming5 Java naming convention Format of class definitions Creating classes with “new” Accessing fields with “variableName.fieldName”

Java Naming Conventions Introduction to Object Oriented Programming6 Leading uppercase letter in class name public class MyClass { ... } Leading lowercase letter in field, local variable, and method (function) names myField, myVar, myMethod

First Look at Java Classes Introduction to Object Oriented Programming7  The general form of a simple class is modifier class Classname { modifier data-type field1; modifier data-type field2; ... modifier data-type fieldN; modifier Return-Type methodName1(parameters) { //statements } ... modifier Return-Type methodName2(parameters) { //statements } }

Objects and References Introduction to Object Oriented Programming8 Once a class is defined, you can easily declare a variable (object reference) of the class Ship s1, s2; Point start; Color blue; Object references are initially null The null value is a distinct type in Java and should not be considered equal to zero A primitive data type cannot be cast to an object (use wrapper classes) The new operator is required to explicitly create the object that is referenced ClassName variableName = new ClassName();

Accessing Instance Variables Introduction to Object Oriented Programming9 Use a dot between the variable name and the field name, as follows: variableName.fieldName For example, Java has a built-in class called Point that has x and y fields Point p = new Point(2, 3); // Build a Point object int xSquared = p.x * p.x; // xSquared is 4 p.x = 7; One major exception applies to the “access fields through varName.fieldName” rule Methods can access fields of current object without varName This will be explained when methods (functions) are discussed

Example 2: Methods Introduction to Object Oriented Programming10 class Ship2 { public double x=0.0, y=0.0, speed=1.0, direction=0.0; public String name = "UnnamedShip"; private double degreesToRadians(double degrees) { return(degrees * Math.PI / 180.0); } public void move() { double angle = degreesToRadians(direction); x = x + speed * Math.cos(angle); y = y + speed * Math.sin(angle); } public void printLocation() { System.out.println(name + " is at (" + x + "," + y + ")."); } }

Methods (Continued) Introduction to Object Oriented Programming11 public class Test2 { public static void main(String[] args) { Ship2 s1 = new Ship2(); s1.name = "Ship1"; Ship2 s2 = new Ship2(); s2.direction = 135.0; // Northwest s2.speed = 2.0; s2.name = "Ship2"; s1.move(); s2.move(); s1.printLocation(); s2.printLocation(); } } Compiling and Running: javac Test2.java java Test2 Output: Ship1 is at (1,0). Ship2 is at (-1.41421,1.41421).

Example 2: Major Points Introduction to Object Oriented Programming12 Format of method definitions Methods that access local fields Calling methods Static methods Default values for fields public/private distinction

Defining Methods (Functions Inside Classes) Introduction to Object Oriented Programming13 Basic method declaration: public ReturnType methodName(type1 arg1, type2 arg2, ...) { ... return(something of ReturnType); } Exception to this format: if you declare the return type as void This special syntax that means “this method isn’t going to return a value – it is just going to do some side effect like printing on the screen” In such a case you do not need (in fact, are not permitted), a return statement that includes a value to be returned

Examples of Defining Methods Introduction to Object Oriented Programming14 Here are two examples: The first squares an integer The second returns the faster of two Ship objects, assuming that a class called Ship has been defined that has a field named speed // Example function call: // int val = square(7); public int square(int x) { return(x*x); } // Example function call: // Ship faster = fasterShip(someShip, someOtherShip); public Ship fasterShip(Ship ship1, Ship ship2) { if (ship1.speed > ship2.speed) { return(ship1); } else { return(ship2); } }

Exception to the “Field Access with Dots” Rule Introduction to Object Oriented Programming15 Normally access a field via: variableName.fieldName but an exception is when a method of a class wants to access fields of that same class In that case, omit the variable name and the dot For example, a move method within the Ship class might do: public void move() { x = x + speed * Math.cos(direction); ... } Here, x, speed, and direction are all fields within the class that the move method belongs to, so move can refer to the fields directly As we’ll see later, you still can use the variableName.fieldName approach, and Java invents a variable called this that can be used for that purpose

Static Methods Introduction to Object Oriented Programming16 Static functions are like global functions in other languages You can call a static method through the class name ClassName.functionName(arguments); For example, the Math class has a static method called cos that expects a double precision number as an argument So you can call Math.cos(3.5) without ever having any object (instance) of the Math class

Method Visibility Introduction to Object Oriented Programming17 public/private distinction A declaration of private means that “outside” methods can’t call it -- only methods within the same class can Thus, for example, the main method of the Test2 class could not have done double x = s1.degreesToRadians(2.2);  Attempting to do so would have resulted in an error at compile time Only say public for methods that you want to guarantee your class will make available to users You are free to change or eliminate private methods without telling users of your class about

Example 3: Constructors Introduction to Object Oriented Programming18 class Ship3 { public double x, y, speed, direction; public String name; public Ship3(double x, double y, double speed, double direction, String name) { this.x = x; // "this" differentiates instance vars this.y = y; // from local vars. this.speed = speed; this.direction = direction; this.name = name; } private double degreesToRadians(double degrees) { return(degrees * Math.PI / 180.0); } ...

Constructors (Continued) Introduction to Object Oriented Programming19 public void move() { double angle = degreesToRadians(direction); x = x + speed * Math.cos(angle); y = y + speed * Math.sin(angle); } public void printLocation() { System.out.println(name + " is at (" + x + "," + y + ")."); } } public class Test3 { public static void main(String[] args) { Ship3 s1 = new Ship3(0.0, 0.0, 1.0, 0.0, "Ship1"); Ship3 s2 = new Ship3(0.0, 0.0, 2.0, 135.0, "Ship2"); s1.move(); s2.move(); s1.printLocation(); s2.printLocation(); } }

Constructor Example: Results Introduction to Object Oriented Programming20 Compiling and Running: javac Test3.java java Test3 Output: Ship1 is at (1,0). Ship2 is at (-1.41421,1.41421).

Example 3: Major Points Introduction to Object Oriented Programming21 Format of constructor definitions The “this” reference Destructors (not!)

Constructors Introduction to Object Oriented Programming22 Constructors are special functions called when a class is created with new Constructors are especially useful for supplying values of fields Constructors are declared through: public ClassName(args) { ... } Notice that the constructor name must exactly match the class name Constructors have no return type (not even void), unlike a regular method Java automatically provides a zero-argument constructor if and only if the class doesn’t define it’s own constructor That’s why you could say Ship1 s1 = new Ship1(); in the first example, even though a constructor was never defined

The this Variable Introduction to Object Oriented Programming23  The this object reference can be used inside any non-static method to refer to the current object  The common uses of the this reference are: 1. To pass a reference to the current object as a parameter to other methods someMethod(this); 2. To resolve name conflicts  Using this permits the use of instance variables in methods that have local variables with the same name  Note that it is only necessary to say this.fieldName when you have a local variable and a class field with the same name; otherwise just use fieldName with no this

Destructors Introduction to Object Oriented Programming24 This Page Intentionally Left Blank

Summary Introduction to Object Oriented Programming25 Class names should start with uppercase; method names with lowercase Methods must define a return type or void if no result is returned If a method accepts no arguments, the arg-list in the method declaration is empty instead of void as in C Static methods do not require an instance of the class; static methods can be accessed through the class name The this reference in a class refers to the current object Class constructors do not declare a return type Java performs its own memory management and requires no destructors

Java basic understand OOP

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Java basic understand OOP