What are Objects and Classes?

A class defines a new data type (a classification). It is the formal implementation, or blueprint, of the attributes and behaviors of the objects of that class.
An object is a specific instance of a class with defined attributes. Objects are declared as variables of a class type.
An attribute or instance variable is data the object knows about itself. For example a turtle object knows the direction it is facing or its color.
A behavior or method is something that an object can do. For example a turtle object can go forward 100 pixels.

Creating and Initializing Objects: Constructors

Each class has constructors like World() and Turtle(habitat) which are used to initialize the attributes in a newly created object.
A new object is created with the new keyword followed by the class name (new Class()). When this code executes, it creates a new object of the specified class and calls a constructor, which has the same name as the class.
- new World() creates and initializes a new object of the World class, and new Turtle(habitat) creates and initializes a new Turtle object in the World habitat.

// To create a new object and call a constructor write:
// ClassName variableName = new ClassName(parameters);
World habitat = new World();    // create a new World object
Turtle t = new Turtle(habitat); // create a new Turtle object

Constructors initialize the attributes in newly created objects. They have the same name as the class.
A constructor signature is the constructor name followed by the parameter list which is a list of the types of the parameters and the variable names used to refer to them in the constructor.
Overloading is when there is more than one constructor. They must differ in the number, type, or order of parameters.
New is a keyword that is used to create a new object of a class. The syntax is new ClassName(). It creates a new object of the specified class and calls a constructor.
A no-argument constructor is a constructor that doesn’t take any passed in values (arguments).
Parameters allow values to be passed to the constructor to initialize the newly created object’s attributes.
The parameter list, in the header of a constructor, is a list of the type of the value being passed and a variable name. These variables are called the formal parameters.
Actual parameters are the values being passed to a constructor. The formal parameters are set to a copy of the value of the actual parameters.
Formal parameters are the specification of the parameters in the constructor header. In Java this is a list of the type and name for each parameter (World(int width, int height).
Call by value means that when you pass a value to a constructor or method it passes a copy of the value.-