Java - Classes

JPatterson

What is a class?

public class _Alpha{

public static void main(String [] args){

}}You have been using classes all year – you just didn’t know

What is a class?

public class _Alpha{

public static void main(String [] args){

}}When a class file has a main you can execute the file

What is a class?

public class _AlphaClass{

}We are going to make class files without the main method.

What is a class?

public class Student{

}You still need to save the file as the class name with the .java extension – in this case it would be Student.java

Attributes / Fields

public class Student{

}What do we need to know about a student? These will be the ATTRIBUTES of the class.

Attributes / Fields

public class Student{

private String name;private int idNumber;private double gpa;

}All attributes are declared to be private. Attributes are the global variables of the class.

Attributes / Fields

public class Student{

private String name;private int idNumber;private double gpa;

}NOTICE – you only DECLARE the attributes you do NOT initialize them here…we will do that later.

Attributes / Fields

public class Student{

private String name;private int idNumber;private double gpa;

}So far we have DECLARED the global or instance variables of the class

Constructors

public class Student{

private String name;private int idNumber;private double gpa;

public Student( ){

}

}

To initialize the variables we have to write a method called a CONSTRUCTOR

Constructors

public class Student{

private String name;private int idNumber;private double gpa;

public Student( ){

}

}

CONSTRUCTORS are public and have the same name as the class – ALWAYS, NO EXCEPTIONS

Constructors

public class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

}

This constructor is called the DEFAULT constructor. We want to initialize the attributes to generic starting values.

Constructors

public class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

}

Classes can have more than 1 constructor. Next we are going to make a SECONDARY constructor

Constructorspublic class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

}

In this case the secondary constructor will allow your client to pass over values to the class and then store them in the attributes.

Constructorspublic class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

}

FLEXIBLITY – having more than 1 constructor makes your class more flexible. There are now 2 ways to initialize your attributes.

Constructorspublic class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

}

INITIALIZE ALL ATTRIBUTES IN EVERY CONSTRUCTOR

CLASSpublic class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

}

So – let’s look at a client program that might use this class.

Declaring and Instantiating Classes

public class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

}

public class Example{

public static void main (String [] args)

{ Scanner scan = new Scanner (System.in);

Student s1 = new Student ();

}}

Declaring and Instantiating Classes

public class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

}

public class Example{

public static void main (String [] args)

{ Scanner scan = new Scanner (System.in);

Student s1 = new Student ();

}}

s1

name “ “

idNumber 0

Gpa 0

s1 is a Student OBJECTthat was declared with the default constructorso all the attributes are set to the default values.

Declaring and Instantiating Classes

public class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

}

public class Example{

public static void main (String [] args)

{ Scanner scan = new Scanner (System.in);

Student s1 = new Student (); Student s2 = new Student(“Sookie”, 10025, 4.0);

}}s1

name “ “

idNumber 0

Gpa 0

name “ Sookie “

idNumber

10025

Gpa 4.0

s2

Declaring and Instantiating Classes

public class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

}

public class Example{

public static void main (String [] args)

{ Scanner scan = new Scanner (System.in);

Student s1 = new Student (); Student s2 = new Student(“Sookie”, 10025, 4.0);

}}s1

name “ “

idNumber 0

Gpa 0

name “ Sookie “

idNumber

10025

Gpa 4.0

s2

“Sookie” is passed over to n

Declaring and Instantiating Classes

public class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

}

public class Example{

public static void main (String [] args)

{ Scanner scan = new Scanner (System.in);

Student s1 = new Student (); Student s2 = new Student(“Sookie”, 10025, 4.0);

}}s1

name “ “

idNumber 0

Gpa 0

name “ Sookie “

idNumber

10025

Gpa 4.0

s2

The attribute name is assigned n

Accessorspublic class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

}

Now, let’s look at accessors.

Accessors are used to give information about the class.

What is your name?What is your idNumber?What is your gpa?

If I ask you, “what is your name?” what data type are you going to tell me?

Accessorspublic class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

}

Now, let’s look at accessors.

Accessors are used to give information about the class.

What is your name?What is your idNumber?What is your gpa?

If I ask you, “what is your name?” what data type are you going to tell me?

STRING – that is the return type of the accessor.

The return type is the data type of the value returned or given by the method

Accessorspublic class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

public String getName(){

return name;}

}

Now, let’s look at accessors.

Accessors are used to give information about the class.

What is your name?What is your idNumber?What is your gpa?

If I ask you, “what is your name?” what data type are you going to tell me?

STRING – that is the return type of the accessor.

The return type is the data type of the value returned or given by the method

Mutatorspublic class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

public void setName(String n){

name = n;}

}

Now, let’s look at mutators.

Mutators are used to change information about the class.

What if on the first day of class I check my roll sheet and you are listed as Orvil but you go by your middle name? I would want to change your name to Sam.

I could call a mutator method that would set your name to a new and improved value.

Mutatorspublic class Student{

private String name;private int idNumber;private double gpa;

public Student( ){ name = “ “;

idNumber = 0;gpa = 0;

}

public Student( String n, int id, double g ){ name = n;

idNumber = id;gpa = g;

}

public void setName(String n){

name = n;}

}

Now, let’s look at mutators.

Mutators are used to change information about the class.

What if on the first day of class I check my roll sheet and you are listed as Orvil but you go by your middle name? I would want to change your name to Sam.

I could call a mutator method that would set your name to a new and improved value.