OO Fundamentals in Java

Core OO Concepts in Java

encapsulation principles and keywords remain the same as C++

• classes

• attributes

• methods

• constructors

• public

• private

• composition

Classes

• Use the class keyword to define

• Create in a .java file matching the name of the class

• Almost always declare as public

• There is no header file 

Attributes

• Create as variables inside the class

• Almost always declare as private

Methods

• Create inside the class

• May be declared public or private

• Implementation also goes inside the class definition

Constructors

• Create as a standard method inside the class 

• Name must match the name of the class 

• No return type defined

• Multiple constructors are permitted, provided the parameters are different 

public

• Access modifier that states the following method/variable is accessible to all code

• Add the public keyword explicitly before each method/variable 

private

• Access modifier that states the following method/variable is accessible only to code inside the class

• Add the private keyword explicitly before each method/variable

Composition

Classes may hold attributes of class type, not just primitive types…

Creating Object Instances - new

• Just like C++, once defined, classes are types and can be realized into object instances

  • By yourself, or by other programmers you give your class file to

  • A realized class is called an object instance and is treated just like any variable 

• In Java object instances are built from a class by using the new keyword:

Car c = new Car("White");

Java Object Creation

Car c = new Car("White");

Car c is a reference to Car object

Java Object Referencing

Java objects are automatically garbage collected (deleted) when they have no references…

Car c = new Car();
Car x = new Car();

Value Types and Reference Types

it is the data itself you are accessing/modifying or passing into a function

use pure lower case

E.g. int, char, float, double, boolean…

Reference Type

use object references to objects

whenever they are used, it is the reference that you are accessing/modifying

should use camelCase

E.g. Car, String, FluffyElephant, Minion, CatDog…

The this keyword in Java

• a special keyword that returns an object reference to the object instance which the current method is executing in

• can be used to unambiguously refer to methods and variables:

• which can be particularly useful in constructors

• but also anywhere we want to be explicit

• or anywhere we want to pass the current object as a parameter

• (C++ also has this, but it's a pointer)

The Java Class Library

a vast set of pre-written classes 

e.g. String, System, Math

API Documentation

• Programmers document their classes, so other programmers can learn how to use them in their applications

  • Java uses a standard called JavaDoc

    • All Java programs use this, so they all have documentation in the same common format

    • This makes it very easy to learn about other people classes which promotes reuse

JavaDoc

• Documentation is embedded inside your code using comment blocks

  • Remember Java can use /* */ comments like C

• JavaDoc uses /** **/ to identify a JavaDoc comment

  • Additional ‘*’ characters on following lines just make it look pretty

• JavaDoc is used to document all public classes, methods and constructors 

• Comment block goes immediately before the item it is documenting

/*Car class represents all drivable vehicles with 4 wheels
the class is also capable of modelling the outward appearance, car model and the number of miles driven*/
public class Car{
}

• Keywords allow structured documentation of methods, return values and required parameters

  • @param documents the meaning of the parameter called ‘name’ 

  • @return documents the meaning of a value returned from a method 

  • Many more: https://www.oracle.com/technical-resources/articles/java/javadoc-tool.html

/**
*creates a new Car with the given characteristics.
*@param col = the colour of the car being created.
*@param miles = the number of miles the car has driven.
**/
public Car(String col, int miles) {
	colour = col;
	milesDriven = miles;
}

• JavaDoc can then generate written documentation automatically

  • The semi-structured comments and the Java code are processed to build web pages documenting your classes

  • These can then be posted online let other programmers know how to use your classes

• Simply run the javadoc tool from the command line to generate the web pages

  • javadoc –d doc *.java 

  • The “-d” parameter specifies the directory (folder) to store the web pages in

  • After running javadoc, just double click ‘index.html’ in this folder

  • Recommend always specifying a folder – Javadoc produces a lot of html

The String Class Example

public static void main(String[] args) {
	String s = "Hello World";
	
	//examples of invoking methods on an instance
	int len = s.length();
	String l = s.toLowerCase();
	char[] myArray = s.toCharArray();
}