Java

TOPIC 36 – CONSTRUCTORS I

LESSON NOTE - REVIEW

ABOUT THIS UNIT

Object-oriented programming is a programming approach that involves the grouping of associated data and functionality inside of objects. This then allows the programmer to think of problems in terms of real world objects instead of a large series of variables and separated operations.

In this unit, we will look at new concepts relating to object-oriented programming (OOP). This will include far more details on constructing objects and adding functionality to objects. We will then also learn about protecting data from taking on illegal values.

RECAP

We have already created classes that contain only instance variables (also known as data fields). We have used these classes to create objects (or instances).

EXAMPLE

Here is a simple Point class that contains two instance variables (data fields) called x and y:

public class Point

{

public double x;

public double y;

}

We can now create Point objects and access the instance variables (data fields).

public class Tester

{

public static void main(String[] args)

{

//Create a Point object named p.

Point p = new Point();

//We can set the values of the data fields of object p.

p.x = 5.0;

p.y = 7.2;

//We can print the values of p by doing:

System.out.println("(" + p.x + ", " + p.y + ")");

}

}

Executing the code above would output:

(5.0, 7.2)

REVISTING TERMINOLOGY

In the example above, the class name is Point. We created a Point object named p. The object p can also be referred to as an instance of the class Point.

The term object and the term instance can be used interchangeably.

CLASS NAMING CONVENTION

Reminder: Classes should be named in the same way as variables except the first character should always be an upper case letter.

OBJECTS IN MEMORY

Consider the following code:

Point p = new Point();
p.x = 5.0;

p.y = 6.1;

The above code will create an object in memory. The object p will be a memory reference to the location in memory where the data fields are stored.

It will look like this: