Introducing Classes
🧠 One Golden Rule of Java
Before learning anything else, remember this:
In Java, everything happens inside a class.
You cannot write Java code outside a class.
So to understand Java, we must first understand what a class is.
🏠 What Is a Class? (Real-Life Example)
Think of it like this 👇
Class → House plan (blueprint)
Object → Real house built using that plan
📌 A plan is not a house
📌 A class is not an object
A class only describes something.
An object is the real thing created from it.
✏ A Very Simple Class Example
class Box {
double width;
double height;
double depth;
}
Don’t panic 😄
Let’s read this in plain English:
“Box is a type.
Every Box has width, height and depth.”
✔ This code does not create a box
✔ It only tells Java what a box looks like
📦 What Is an Object?
An object is a real instance created from a class.
Box mybox = new Box();
Read it slowly 👇
“Create a new Box and name it mybox.”
Now mybox is a real box stored in memory.
🧩 Why Do We Need Objects?
Because only objects:
Store values
Occupy memory
Do real work
📌 Class = idea
📌 Object = reality
🔗 Accessing Data Using Dot (.)
mybox.width = 10;
mybox.height = 20;
mybox.depth = 15;
Read like English:
“Width of mybox is 10.”
The dot (.) means:
👉 belongs to
🧮 A Complete Very Simple Program
class Box {
double width;
double height;
double depth;
}
class BoxDemo {
public static void main(String[] args) {
Box mybox = new Box();
mybox.width = 10;
mybox.height = 20;
mybox.depth = 15;
double volume = mybox.width * mybox.height * mybox.depth;
System.out.println("Volume is " + volume);
}
}
Output:
Volume is 3000.0
📌 This is core Java.
Take your time here — don’t rush.
🧠 Important Beginner Rule
Each object has its own data.
Box b1 = new Box();
Box b2 = new Box();
✔ b1 and b2 are different boxes
✔ Changing b1 does NOT affect b2
🪜 Creating an Object Happens in Two Steps
Step 1: Create reference
Box b;
👉 No object yet.
Step 2: Create object
b = new Box();
Usually written together:
Box b = new Box();
🧨 What Does new Mean?
new Box();
Means:
“Hey Java, give me memory for a Box.”
Java:
Allocates memory
Returns the address
Stores it in the reference variable
⚠ A Very Common Beginner Mistake
Box b1 = new Box();
Box b2 = b1;
❌ No new box is created
✔ Both variables point to the same box
Think of it as:
👉 Two names for one object
🛠 What Is a Method?
A method is a block of code that does some work.
void sayHello() {
System.out.println("Hello");
}
Methods help us:
Reuse code
Keep programs clean
Add behavior to objects
🧮 Method Inside a Class
class Box {
double width, height, depth;
double volume() {
return width * height * depth;
}
}
Now the box knows how to calculate its own volume.
📞 Calling a Method
Box b = new Box();
b.width = 10;
b.height = 20;
b.depth = 15;
System.out.println(b.volume());
The dot means:
“Call volume() of b.”
🔁 Understanding return
double volume() {
return width * height * depth;
}
return means:
“Send the result back.”
📥 Method with Parameters
int square(int x) {
return x * x;
}
Calling the method:
square(5); // 25
square(10); // 100
📌 x → parameter
📌 5, 10 → arguments
❌ Bad Way to Set Object Data
b.width = 10;
b.height = 20;
b.depth = 15;
Problems:
Easy to forget a value
Poor design
✅ Better Way: Use a Method
void setDim(double w, double h, double d) {
width = w;
height = h;
depth = d;
}
Use it like this:
b.setDim(10, 20, 15);
✔ Cleaner
✔ Safer
✔ Professional approach
🏗 Constructor (VERY IMPORTANT)
A constructor:
Runs automatically
When an object is created
Used to set initial values
class Box {
double width, height, depth;
Box() {
width = 10;
height = 10;
depth = 10;
}
}
This runs automatically:
Box b = new Box();
🔢 Constructor with Values
Box(double w, double h, double d) {
width = w;
height = h;
depth = d;
}
Use:
Box b = new Box(10, 20, 15);
✔ Clean
✔ Safe
✔ Interview-ready
🔁 this Keyword (Very Simple)
this means:
👉 current object
Box(double width, double height, double depth) {
this.width = width;
this.height = height;
this.depth = depth;
}
Left side → object variable
Right side → parameter
🗑 Garbage Collection (No Fear)
Java automatically removes unused objects.
✔ You don’t delete objects
✔ Java manages memory for you
❤️ Mentor Advice for Beginners
If you are new to Java:
1️⃣ Read slowly
2️⃣ Type the code yourself
3️⃣ Understand — don’t memorize
4️⃣ Practice one concept at a time
🎯 Final Thoughts
Classes and objects are the foundation of Java.
Once this chapter is clear, everything else becomes easier.
You’re doing great — keep going 💪