Week 4: Writing classes/03A Writing a class: file contents

A class is defined by the class identifier. 
As stated earlier, all code in Java must be written inside classes. 
However, not all the code that is written is directed at the objects that are created from the class. 


In Java, the class name must be exactly the same as the file name. 
Specifically, a file must have a public class defined by its name. 

For example, the file Car.java must have a class definition of
public class Car
 
...and from the file OwnFineClass.java definition
public class OwnFineClass

 

You can write MORE THAN 1 CLASS in the SAME FILE, but ONLY 1 can be PUBLIC. 
The others are initialized without the public attribute.

A class has different features. Features can be public or private. 
When an object is created from a class, the client of the object has direct access to the public features. 
The client in this context is the program code through which the entity is accessed 
(it does not mean, for example, the program user or any other physical person).







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Attributes
Let's start by looking at the attributes of the class (note the two t's). 
An attribute is a variable of an object. 
They are therefore used to define the information content of the objects that make up the class. 
Each object in a class has its own values for all the attributes defined in the class.



As an example, consider a class that models a student. 
The class has three attributes: name, student number and credits:

public class Student {
    // Attributes are usually defined at the start of the class
    String name;
    String studentId;
    int studyPoints;
}


 

ATTRIBUTES are thus defined inside the class, but OUTSIDE ANY METHODS.
Typically, attribute variables are assigned values in the CONSTRUCTOR based on PARAMETERS YOU PASS IN. 




So let's write a constructor for the Student class:
public class Student {
    // Attributes are usually defined at the start of the class
    String name; 
    String studentId; 
    int studyPoints;
	
    // Constructor
    public Student(String n, String id, int sp) {
        name = n;
        studentId = id;
        studyPoints = sp;
    }
}
 



Now new student objects can be created using the new operator:
public class Testclass {
    public static void main(String[] args) {
        Student sally = new Student("Sally Student", "12345", 10);
        Student sam = new Student("Sam Student", "99999", 35);
    }
}