The document discusses key concepts related to classes in Java including primitive types, defining a class with fields and methods, allocating objects of a class, using constructors, and static variables and methods. It provides examples of defining a Person class with fields for name and age, creating Person objects using new, and accessing object fields directly and via methods. It also covers static variables that are shared among all class instances and how they can be accessed through the class or an object.

1. CLASSES IN JAVA
Primitive Types
Example of a Class
Allocating Objects of a Class
Protecting Class data
Constructors
Static data and Static Methods

2. Primitive types in Java
Type Contains Default Size Min Max
boolean true/false false 1 bit n/a n/a
char Unicode character u0000 16 bits u0000 uFFFF
byte signed integer 0 8 bits -128 127
short signed integer 0 16 bits -32768 32767
int signed integer 0 32 bits large large
long signed integer 0 64 bits huge huge
float IEEE 754 floating-pt 0.0 32 bits large large
double IEEE 754 floating-pt 0.0 64 bits huge huge

3. Class (simple example)
• Suppose you need to represent “a person”
• You need a person to consist of:
• A first name
• A last name
• An Age
Interesting questions:
1) Is this the official representation of a person?
2) Does the compiler already know this representation?

4. User Defined Type (non-primitive)
public class Person
{
String firstName;
String lastName;
int age;
}
note: a public class that only contains public data structures and no methods (a.k.a.
functions) is a essentially a struct in the programming language C/C++

5. Allocating a Class (non-primitive) object
int i = 3; // allocates storage for an int and assigns it to 3
// i can be used to refer to the storage
Person p; // sets p to be a reference to a Person…
// no storage is allocated
Person p = new Person();
// sets p to be a reference to a Person…
// storage is allocated

6. Changing a data member of an object
Person p = new Person();
p.firstName = “George”;
p.lastName = “Washington”;
p.age = 40;
Note: allowing data to be accessed so freely, can lead to
users corrupting the data. Ex) integers may be assigned
where String belong, etc.

7. Protecting your Class data
public class Person
{
private String firstName;
private String lastName;
private int age;
}
Note:
Person p = new Person();
p.age = 40; // will cause an error because age is private

8. Public Method setting Private data
public class Person
{
private String firstName;
private String lastName;
private int age;
public void setAge(int a) { age = a; }
}
Note:
Person p = new Person();
p.setAge(40); // will set the age to 40

9. Constructor Method
public class Person
{
private String firstName;
private String lastName;
private int age;
public Person(String f, String l, int a)
{
firstName = f;
lastName = l;
age = a;
}
public setAge(int a) { age = a; }
}
Person p = new Person(“George”, “Washington” , 40);
//Will create a Person object and initialize all 3 fields

10. Static (Class) Variables
public class Person
{
private String firstName;
private String lastName;
private int age;
public static int personCount;
public Person(String f, String l, int a)
{
firstName = f;
lastName = l;
age = a;
personCount++;
}
public setAge(int a) { age = a; }
}

11. Static (Class) Variables
Person p1 = new Person(“George”, “Washington” , 40);
Person p2 = new Person(“Abraham”, “Lincoln” , 41);
Person p3 = new Person(“Theodore”, “Roosevelt” , 42);
System.out.print(p2.personCount); // would output 3
System.out.print(Person.personCount); // would output 3

12. Variables related to Classes in Java
• Local variables: Variables defined inside methods, constructors or
blocks are called local variables. The variable will be declared and
initialized within the method and the variable will be destroyed when
the method has completed.
• Instance variables: Instance variables are variables within a class
but outside any method. These variables are instantiated when the
class is loaded. Instance variables can be accessed from inside any
method, constructor or blocks of that particular class.
• Class variables: Class variables are variables declared with in a
class, outside any method, with the static keyword.

13. Private Statics / Public Access Methods
public class Person
{
private String firstName;
private String lastName;
private int age;
private static int personCount;
public Person(String f, String l, int a)
{
firstName = f;
lastName = l;
age = a;
personCount++;
}
public static void printCount() { System.out.print(personCount); }
public setAge(int a) { age = a; }
}

14. Classes composed of Classes
Finally Note:
Class type definitions are made up of combinations of primitive
types and other already existing classes.
public class Group
{
Person person1;
Person person2;
Person person3;
static int groupsCreated;
}

15. Hello World in Java (revisited)
public class HelloWorld
{
public static void main(String[] args)
{
System.out.println("Hello World!!!");
}
}
Notes:
HelloWorld is a class that you created
Main is a public static method of the Class HelloWorld
is the entry point of your class. (leftover from C/C++)
System is a class
out is a variable of the class System
out is static because it is referenced by it’s class name not an object
println is a void method