Week 4: Writing classes/01a Class and object

class defines the 
- structure of the object's data content and 
- the operations that can be performed on it


use 'class' to model 
- physical things (such as a student, a car, or a holiday share), 
- conceptual things (such as a bank account, a student record, a network connection), and 
- often things that have no real-world equivalent - such as a key-value pair to be used as a parameter

1 class
- create 1/more objects



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Creating objects from class

To construct an object, you call a class's dedicated method, the CONSTRUCTOR. 
The constructor RETURNS a NEW OBJECT (usually with parameters) - or, in fact, a REFERENCE to a new object. 
This reference is usually stored in a variable, through which the entity can later be used/changed and observed.


For example, to create a new list object, call the ArrayList class constructor:

ArrayList<String> nordics = new ArrayList<>();
 


The name of the constructor method is always the same as the name of the class. 
Thus, for example, the 'name of the constructor method' of the 'ArrayList class' is 'ArrayList', and 
the name of the constructor of the Random class is Random. 

When creating a new object, the 'operator new' is used, which, as its name implies, 
indicates that the intention is to create a new object from the class.



In the example, we have initialized the objects of the classes Random and StringBuilder:

import java.util.Random;

public class TestClass {
    public static void main(String[] args) {

        // class1
        // New object from the class random
        Random randomizer = new Random();
		
        // class2
        // New object from the class stringbuilder
        StringBuilder address = new StringBuilder("Javaroad 12");
    }
}