Java class

1. Class Declaration:
   • In Java, a class is declared using the class keyword followed by the class name. It serves as a blueprint for creating objects.
   • Example: public class MyClass { }
2. Fields (Instance Variables):
   • Fields, also known as instance variables, are the attributes or properties of a class.
   • They represent the state of objects and hold data specific to each object created from the class.
   • Example: public int age;
3. Constructors:
   • Constructors are special methods used for initializing objects.
   • They have the same name as the class and are called automatically when an object is created.
   • Constructors can be parameterized or non-parameterized.
   • Example: public MyClass() { }
4. Methods:
   • Methods are functions defined within a class that define the behavior of objects.
   • They perform operations and manipulate the data of objects.
   • Methods can have parameters and return values.
   • Example: public void displayInfo() { }
5. Object Creation and Usage:
   • Objects are instances of a class created using the new keyword followed by the class name and optional arguments to constructors.
   • Once created, objects can access fields and methods of the class.
   • Example: MyClass obj = new MyClass();

In summary, class declaration defines the structure of a class, fields hold the state of objects, constructors initialize objects, methods define the behavior of objects, and object creation allows us to instantiate and use objects based on the class blueprint. These aspects form the foundation of object-oriented programming in Java.

Example programme -Run on your IDE

public class studentApp {

public static void main(String[] args) {

Student student1=new Student(“Pekka”,”123456″);

Student student2=new Student(“Tommi “,”123457”,10,true);

student1.bePresent();

student1.addStudyrecord(15);

student1.addStudyrecord(25);

student2.addStudyrecord(5);

student1.printInfo();

student2.printInfo();

}

}

class Student{

public String name;

public String studentNumber;

public int credits;

public boolean attendentStatus;

public Student(){

name=” “;

studentNumber=” “;

credits=0;

attendentStatus=false;

}

public Student(String name,String studentNumber ){

this.name=name;

this.studentNumber=studentNumber;

credits=0;

attendentStatus=false;

}

public Student(String name,String studentNumber,int credits,boolean attendentStatus ){

this.name=name;

this.studentNumber=studentNumber;

this.credits=credits;

this.attendentStatus=attendentStatus;

}

public void addStudyrecord(int creditAmount)

{

if (attendentStatus) {

credits=credits+creditAmount;}

else

{

System.out.println(“Student is absent cant add records”);

}

}

public void printCreditInfo()

{

System.out.println(“Has “+ credits+” Credits”);

}

public void bePresent()

{

attendentStatus=true;

}

public void beAbsent()

{

attendentStatus=false;

}

public void printInfo()

{

System.out.println(“name :”+ name);

System.out.println(“Student number :”+ studentNumber);

printCreditInfo();

if (attendentStatus) {

System.out.println(“Student is present”);

}

else

{

System.out.println(“Student is Absent”);

}

}

}

explanation

In the provided example, there are two classes: studentApp and Student. Let’s break down each part and explain how classes are used in Java:

1. Class Declaration:
   • The studentApp class is the main class that contains the main method, serving as the entry point of the Java application.
   • The Student class defines the blueprint for creating student objects.
2. Fields (Instance Variables):
   • In the Student class, there are several instance variables (name, studentNumber, credits, attendentStatus) representing the attributes of a student.
3. Constructors:
   • The Student class has three constructors: a default constructor, a parameterized constructor with name and student number, and a parameterized constructor with all attributes.
   • Constructors are used to initialize the state of objects when they are created.
4. Methods:
   • The addStudyrecord(int creditAmount) method in the Student class is used to add study credits to a student’s record, but only if the student is present (attendentStatus=true).
   • The bePresent() and beAbsent() methods are used to change the attendance status of a student.
   • The printInfo() method prints information about a student, including name, student number, credits, and attendance status.
5. Object Creation and Usage:
   • In the main method of the studentApp class, two Student objects (student1 and student2) are created using different constructors, demonstrating constructor overloading.
   • Methods like addStudyrecord, bePresent, and printInfo are called on these objects to perform actions and display information about the students.