Class definitions

Classes are used to create objects. They contain

fields to store data
constructors allowing us to create create objects
methods that implement operations that object can perform

A Java class contains certain patterns. A common pattern includes

A header
and a Body in between a pair of braces {}

Aims

Elements that make up a class definition e.g. fields, constructors and methods
Assigning values to variables
operators that allow you to manipulate data
printing out information
code that follows different paths according to a condition
how to write headers
how to include comments
projects contain a readme file

Class header

Always starts with upper-case letter it can be told apart from method/variable names
above the class header is a comment

*Important tip - you can leave the class body empty and it will compile

Fields

Store data persistently within an object
also called instance variables
must start with a type of value e.g. int
it is best to initialise fields using a constructor

Constructors

a constructors header does not declare a return type
A constructors name must always match that of its class
Ensure the object is set up properly when it is first created
constructors initialise data
write a comment

Methods

Methods implement the behaviour of an object
Must include a method return type
if it returns something, must finish with a return statement
write a comment

Accessors and Mutators

An Accessors is also called a getter. A Mutator is called a setter.

Access Modifiers

Are usually private.

When writing Java

Use this method:

public class ClassName
{
    Fields
    Constructors
    Methods
}

Parameters

Formal parameters

A formal parameter is a parameter declaration. We write Formal parameters when we are defining a method or constructor. A formal parameter goes in the method or constructor header. If we do not need any parameters we still need the brackets.

Actual Parameters

An actual parameter is often called an argument.

A method / constructors signature lists the argument types required.

Scope

Scope is the area of source code recognised by Java compiler. The scope of a field is the whole of its enclosing class. A methods scope is enclosed in a class scope.

A scope (also called a block) is a unit of code indicated by a pair of curly brackets. Scopes are often nested.

Pay attention to the indentation of you code.

Printing from methods

Literals

A literal is some text that represents a literal value e.g. 42 or “Hello”.

Conditionals

Conditionals change the flow of our code using if, if-else, while, do-while etc… The main use of conditionals is to test operators using statements. The conditions are listed in B&K Appendix 3

Local variables

Used within a method or constructor to temporarily hold data. Local variables never have private or public in front of them. These are created when a method is called, and destroyed when the method is finished.

Fields, Parameters and local variables

The types of variable in any field has a type
Fields are defined outside of constructors and methods
Fields are used to store data persistently through an objects life time
Fields have a class scope
Any field with a private cannot be accessed outside of the class
Formal Parameters and local variables only last for the life of its calling constructor / method
Formal parameters only exist in the header
Local Variables are defined inside a method or constructor.
Local variables have a scope that is limited by its calling method.

Static typing and Dynamic type checking

For any variable there are are two steps required before we can use it

Saying what the type and name are (Declaration)
initializing the variable with a value (Initialization)

When modelling our own objects, we can choose any name we want, as long as it follows the set rules. After a variable has been declared and initialized we can use the name to access its value.

Concept Summary

object creation Some objects cannot be constructed unless extra information is provided
field Fields store data for an object to use. Fields are also known as instance variables.
comment Comments are inserted into the source code of a class to provide explanations to human readers. They have no effect on the functionality of the class.
constructor Constructors allow each object to be set up properly when it is first created.
scope The scope of a variable defines the section of source code from which the variable can be accessed.
lifetime The lifetime of a variable describes how long the variable can exist before it is destroyed.
assignment statement Assignment statements store the value represented by the right hand side of the variable named on the left.
assessor method Assessor methods return information about the state of an object.
mutator method Mutator methods change the state of an object.
println The method System.out.println prints its parameter to the terminal.
conditional statement A conditional statement takes one of two possible outcomes depending on the result of a test.
boolean expression Boolean expressions have only two possible values: true and false. They are usually found as a choice between two paths of a conditional statement.
local variable A local variable is a variable declared and used inside a method. Its scope and lifetime are limited to that method.