🚀 Day 7 of My Automation Journey – Default Values & Constructors in Java

#beginners #codenewbie #java #oop

Today I learned two very important concepts in Java:

✅ Default Values
✅ Constructors
✅ Object Initialization

This is where Java becomes more practical and object-oriented.

🔹 What Are Default Values in Java?

When we declare instance variables (class-level variables) and do not assign values, Java automatically assigns default values.

📌 Default Values of Data Types

Data Type   Default Value
int                0
double               0.0
float                0.0f
boolean              false
char                 '\u0000' (empty character)
String               null

👉 Important: Default values are assigned only to instance variables, not local variables.

🔹 What Is a Constructor?

A constructor:
Has the same name as the class
Is automatically called when object is created
Has no return type
Is used to initialize object-specific values

Syntax

public ClassName(parameters) {
    // initialization
}

🔹 Real Example – SuperMarket Program

Let’s understand the code step by step.

package java_Module_2_Constructor;

public class SuperMarket {

    String prod_name, Quantity;
    int price, discount;

    public SuperMarket(String prod_name, String Quantity, int price, int discount) {
        // TODO Auto-generated constructor stub
        this.prod_name = prod_name;
        this.Quantity = Quantity;
        this.price = price;
        this.discount = discount;
        // System.out.println(prod_name + Quantity + price + discount); 

    }

    public static void main(String[] args) {

        SuperMarket prod_1 = new SuperMarket("Pepsi ", "1/2 litre ", 35 , 5);
        SuperMarket prod_2 = new SuperMarket("Pepsi ", "1 litre ", 55 , 5 );
        SuperMarket prod_3 = new SuperMarket("Pepsi ", "1 1/2 litre ", 75 , 10);
        SuperMarket prod_4 = new SuperMarket("Pepsi ", "2 litre ", 95 , 15);    

        prod_1.sell();
        prod_2.sell();
        prod_3.sell();
        prod_4.sell();


    }

    private void sell() {
        // TODO Auto-generated method stub
        System.out.println(prod_name);
        System.out.println(Quantity);
        System.out.println(price);
        System.out.println(discount);
        System.out.println("-------------------");
    }

}

Result:

Pepsi 
1/2 litre 
35
5
-------------------
Pepsi 
1 litre 
55
5
-------------------
Pepsi 
1 1/2 litre 
75
10
-------------------
Pepsi 
2 litre 
95
15
-------------------

🔹 What Is this Keyword?

this refers to the current object.

Example:

this.price = price;

Left side → instance variable
Right side → constructor parameter

It removes confusion when variable names are same.

🔹 How Object Creation Works

When you write:

SuperMarket prod_1 = new SuperMarket("Pepsi", "1 litre", 55, 5);

Step-by-step:

Memory is allocated
Default values are assigned
Constructor is called
Values are initialized
Object is ready to use

🔹 Why Constructor Is Important?

Without constructor:

Variables remain default (0, null, false)

No proper initialization

With constructor:

Each object can have different values

Real-world modeling becomes possible

🧠 Real-Life Understanding

Think of constructor like a form you fill when creating a product:

Product Name
Quantity
Price
Discount

Every product object will have its own data.

🎯 Interview Points

✔ Constructor has no return type
✔ Automatically called when object is created
✔ Used to initialize instance variables
✔ If not written, Java provides default constructor
✔ this keyword refers to current object

🚀 Summary of Day 7

Today I learned: