Microsoft PowerPoint - Introduction to Reserved Word - This_1

Introduction tothe this

reserved word

Java - Supplemented Learning

By: Keenan Ratushniak

This power point is intended for first time students

learning Java and builds on Java foundations. It is

intended to give a quick review to the java reserved

word this. It is a very early introduction to the use

and placements of it.

Class Level Variables When writing a class, your instance data, is your

class level variables

Any methods within your class can use and assign

values to these variables

They are Global to the class

Method Variables within Class Methods can accept parameters and declare variables

within its body

These variables are local to the method

In other words they dont exist outside the method,

and other methods cant access them directly

This is just like a counter variable within a for loop.for (int counter = 0; counter < max; counter++)

When to use thisAt this introduction level to java there are 2 conditions that must

be met to signal the possible use of the reserve word this

Conditions:

1. Your method must accept a parameter or declare a variable

and

2. You must CHOOSE to name your instance data variable and

method variable the same

How this works The this reserve word is used to tell the compiler

that within a method there are 2 variables of the same

name. One being your instance data variable and one

being your local method variable

You apply this to the variable that is your Instance

Data or class level variable

In this Code Snippet the this reference is not needed. The constructor uses a variable of a different name The getDiameter() Method directly uses the Instance Data

public class SphereEnhanced{

//Instance Data

private double diameter;

//Constructor

public SphereEnhanced(double diameterInit){diameter = diameterInit;

}

//Method to get diameter

public double getDiameter(){

return diameter;}

This basic construction will be used throughout this tutorial to show you when thethis reference is used and not used.

Common Programming Practice In the next example you will see a code similar to

the previous one

The use of this is not needed with common

programming

The example illustrates roughly what is

happening under the covers

Common Programming Practicepublic class Contact{

//Instance Data

String name;

//Constructor

public Contact(String newName){

name = newName;

}

}


//Instance Data

String name;

//Constructor


name = newName;

}

}

Local Variable


//Instance Data

String name;

//Constructor


name = newName;

}

}

Local Variable

Assignment From Local to Instance Data


//Instance Data

String name;

//Constructor


name = newName;

}

}

Local Variable


Instance Datanow has Valuefrom Constructor.


//Instance Data

String name;

//Constructor


name = newName;

}

}

Local Variable


Instance Datanow has Valuefrom Constructor

Compiler is Happyand Understands

Attempt Without Using This In this example modified from above the

parameter variable is the same name as the

instance data

The compiler cannot tell which parameter or

variable you are trying to apply the assignment to

Attempt Without Using Thispublic class Contact{

//Instance Data

String name;

//Constructor

public Contact(String name){

name = name;

}

}


//Instance Data

String name;

//Constructor


name = name;

}

}

Local Variable?


//Instance Data

String name;

//Constructor


name = name;

}

}

Local Variable?Local Variable?


//Instance Data

String name;

//Constructor


name = name;

}

}


OR instance data??


//Instance Data

String name;

//Constructor


name = name;

}

}


OR instance data??

OR instance data??


//Instance Data

String name;

//Constructor


name = name;

}

}


OR instance data??

OR instance data??

Compiler is Sad andConfused

How to apply this The following code is exactly as the previous with

one slight modification

The this reserve word is added to the instance data

variable that we want to use within a method

How to apply this to a variable: this.name

How to apply thispublic class Contact{

//Instance Data

String name;

//Constructor


this.name = name;

}

}


//Instance Data

String name;

//Constructor


this.name = name;

}

}

Local Variable


//Instance Data

String name;

//Constructor


this.name = name;

}

}

Local Variable

Assignment From Local to this reference of Instance Data


//Instance Data

String name;

//Constructor


this.name = name;

}

}

Local Variable


this is referringto this classinstance datavariable (theclass level)


//Instance Data

String name;

//Constructor


this.name = name;

}

}

Local Variable


this is referringto this classinstance datavariable (theclass level)

Compiler is Happyand Understands

Applying to Methods The same procedure is used for methods

If declaring a variable in the method with the

same name as the instance data you must use the

this reference

Applying to MethodsThis is the common practice way of coding like with the constructor


//Instance Dataprivate double diameter;

//Constructorpublic SphereEnhanced(double diameterInit){

diameter = diameterInit;}

//Method to set diameterpublic double setDiameter(double newDiameter){

diameter = newDiameter;}

Applying to MethodsUsing the this reference for the method



//Constructorpublic SphereEnhanced(double diameterInit){

diameter = diameterInit;}

//Method to set diameterpublic double setDiameter(double diameter){

this.diameter = diameter;}

Applying to MethodsUsing the this reference for the method and constructor



//Constructorpublic SphereEnhanced(double diameter){


//Method to set diameterpublic double setDiameter(double diameter){


Summary Using the this reference isnt common programming but is up

to the programmers discretion on when they want to use it

It is used when you choose to name a methods parameters or

declare variables inside its body the same as your instance data

Using this implicitly makes that variable reference your

instance data variable.

Documents

Microsoft PowerPoint - Introduction to Reserved Word - This_1