Introduction to the “this” reserved word Java - Supplemented Learning By: Keenan Ratushniak.

Introduction to the “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 don’t 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 “this”At 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 the “this” 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


public 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 Data now has Value from Constructor.



//Instance Data

String name;

//Constructor


name = newName;

}

}

Local Variable


Instance Data now has Value from Constructor

Compiler 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



//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 Confused

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 “this”public 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 referring to this class instance data variable (the class level)


//Instance Data

String name;

//Constructor


this.name = name;

}

}

Local Variable


this is referring to this class instance data variable (the class level)

Compiler 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 Data


//Constructor


}

//Method to set diameter

public double setDiameter(double newDiameter){

diameter = newDiameter;}

Applying to MethodsUsing the this reference for the method


//Instance Data


//Constructor


}


public double setDiameter(double diameter){

this.diameter = diameter;}

Applying to MethodsUsing the this reference for the method and constructor


//Instance Data


//Constructor

public SphereEnhanced(double diameter){this.diameter = diameter;}


public double setDiameter(double diameter){this.diameter = diameter;}

Summary• Using the this reference isn’t 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.

Introduction to the “this” reserved word Java - Supplemented Learning By: Keenan Ratushniak.

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