Classes

COMP122 Lectures 2021-03-02

There are two methods of creating objects:

• Copying and adjusting existing ones.
• Instantiating a template.

Classes

A class is a template of blueprint for objects:

• It specifies which attributes and methods should exist.
• An object can be an instance of the class.

For example you may have many instances of rectangles that follow the following class:

classDiagram
class Rectangle{
	-int size
	-int colour
	+area() int
}

This is an UML class diagram.

Class Diagrams

The boxes are laid out in the following order:

1. Name
2. Attributes
3. Methods

Java Objects & Classes

To create objects in Java you need to:

• Define a class.
• Instantiate a new object of that class.

Defining a Class

public class Rectangle{
	// Attributes
	private int side;
	private int colour;
	// Methods
	public int area() {
		return side * side;	
	}
}

Instantiating an Object

Every class defines a type and so variables can be declared like this:

Rectangle r;

This defines a variable of the type Rectangle called r.

Object can be instantiated using the keyword new like this:

r = new Rectangle();