Overview of Using Classes

• One of the major aspects of a programming language is reusability - why re-invent the wheel if you don't have to?

• In computer science, software reusability is one of the foundations of good software.

• A program that re-uses code that has already been tested and works correctly will have less bugs and the programmer can spend more time on the new and unique aspects of the problem they are solving.

• Object-oriented programming languages like Java support software reusability directly in their syntax with the ability to create and use classes.

• A class is a complex data type that allows a program to to declare and manipulate objects that represent complex values.

• In Java, everything is a class, including your program. You may recall that your program name comes right after the keyword class.

• The Point class and the String class are both examples we have already looked at - they are built-in classes. Java has a number of built-in, pre-defined classes that you can use in your programs.

• Remember the basic syntax for a declaration is

   class	identifier;

• When using a built-in class, make make sure you have the proper import statements.

  Also remember that you use new to allocate the memory for an object, that using new with primitives is a syntax error, and it is optional for objects of class String. For all other classes, using new is a required step, so you will need to remember to code this extra step for all other classes.

  Note: From now on when we discuss how class objects work, we will just use the term objects, with the understanding that we are not talking about primitives or String objects.

• Every class in Java is made up of two components:

  1. attributes - the values an object of the class can store or the states that it can represent.

  2. behaviors - the ways in which an object of the class behaves or can be manipulated. An object's behavior is defined by the methods that belong to the class; you manipulate objects with method calls.

• Declaring an object of a particular class is call instantiating the class. Each object of that class your program declares is an instance of the class.

• One way to think of the difference between a class and an object is that a class represents an idea and an object is a concrete instance of that idea. For instance, you could have a class called Car that represents a car.

  When you actually want a car, you have to declare it. You can declare multiple cars, if your program needs to. You also need to create each car (instantiate it). Below is a small example.

  Example Declaration with the Car Class

  Car	miata = new Car();
  Car	vwBug = new Car();
  

• In the example, Car is the class and both miata and vwBug are instances of the class Car. The Car class is not a pre-defined class, declarations work the same way for all classes.

• Recap: Creating objects entails three basic steps:

  1. Declare the variable.

  2. Use new to set aside memory in the computer to store its value, which is also called instantiating the object in OO. A more generic term for creating the memory for a variable is called memory allocation.

  3. Give the object its value.