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Oops in Java
Classes and Objects
Learning Outcome
5
Use constructors to initialize objects
4
Create an object using new
3
Differentiate class vs object
2
Understand what an object is
1
Understand what a class is in Java
6
Use basic modifiers (public/private, static/final)
Variables: Know how to store data like int, double, String
Basic Java Syntax: Know how to write class, main method, and print output.
Before Starting ,Lets Recall
Methods (Functions): Understand how methods work, how to pass values and return results.
Arrays: Understand how methods work, how to pass values and return results
Think of a bakery where cakes are prepared every day
- Before making a cake, the baker refers to a recipe book that explains how to prepare it.
- Using the same recipe, the baker can make many cakes to serve customers.
The bakery does not keep just one recipe.
It stores many related recipes together in one place so that they are easy to find and manage.
Once a cake is ready, it has specific characteristics, such as:Flavor,Size
A cake is not just something to look at.
Different actions can be performed on it:
-
The cake can be cut
-
The cake can be eaten
After the cake is completely eaten, the plate is no longer needed, so it is thrown away.
Similarly in java,
The recipe represents a class, as it defines how something should be created.
Each cake made using the recipe represents an object, as it is a real instance.
The flavor and size represent variables, as they store the state of the cake.
Actions like cutting and eating represent methods, as they define behavior.
Clearing the plate after the cake is finished represents garbage collection, as unused items are removed automatically.
The initial decisions made while baking represent a constructor, as it initializes the cake.
The bakery that stores many recipes together represents a package, as it organizes related classes.
What is Class and Object in Java?
Object: The Real Thing
An object is a real instance created from a class. It uses the class blueprint to store actual data and perform specific actions.
Real Example: ChocolateCake and VanillaCake are objects created from the CakeRecipe class.
Class: The Blueprint
Bakery Analogy: A cake recipe is like a class—it defines ingredients and baking steps but isn't the actual cake itself.
A class is a template that defines what an object will contain (variables) and what actions it can perform (methods).
Java Syntax for Classes and Objects
Class Syntax
class ClassName {
// Fields (variables)
// Methods (functions)
}ClassName variable =
new ClassName();Object Creation
Complete Working Example
class Cake {
String flavor;
void bake() {
System.out.println("Baking " + flavor + " cake");
}
}
public class Main {
public static void main(String[] args) {
Cake c1 = new Cake();
c1.flavor = "Chocolate";
c1.bake();
}
}
Constructors in Java
A constructor is a special method used to initialize objects. It runs automatically when an object is created.
Key Rules
No return type, not even void.
Name must be the same as the class name.
Executed automatically upon using the new keyword.
class ClassName {
// Constructor
ClassName() {
// initialization code
}
}
Syntax
class Student {
String name;
int age;
// Constructor
Student(String n, int a) {
name = n;
age = a;
}
}
public class Main {
public static void main(String[] args) {
Student s1 = new Student("Rahul", 20);
}
}
Example:
Types of Constructor
Default Constructor
-
Provided automatically by the compiler
-
Sets default values
class Student {
// Compiler adds: Student() {}
}
No-Argument Constructor
-
Written by the programmer
-
Used for custom initialization
class Student {
Student() {
System.out.println("Object created");
}}
Parameterized Constructor
-
Accepts arguments for initialization
-
Avoids extra setter calls
class Student {
Student(String name) {
System.out.println("Name:"+ name);
}}
Copy Constructor
-
Creates a new object using another object’s data
-
Must be defined manually
class Student {
Student(Student s) {
System.out.println("Object copied");
}}
Destructors in Java
A destructor is a special method used to clean up resources before an object is destroyed.
No Explicit Destructor
Unlike C++, Java does not use manual destructors.
Java uses Garbage Collection (GC) to automatically remove unused objects.
The finalize() method exists for cleanup but is deprecated and rarely used.
class Cake {
protected void finalize() {
System.out.println("Object is destroyed");
}
}
Example:
Access Modifiers in Java
Access modifiers control who can see or use variables and methods in a class.
Private
- Accessible only within the classitself.
public
- Accessible from anywhere. No restrictions.
Protected
- Same package + subclasses.
Default
- No keyword. Accessible only in same package.
Non-Access Modifiers in Java
Non-access modifiers change the behavior of variables, methods, or classes.
Static
- Belongs to Class, shared by all objects.
Final
- Cannot be changed once assigned.
Summary
5
Modifiers control access and behavior
4
Java uses garbage collection instead of destructors
3
Objects store data and perform actions
2
An object is a real example of a class
1
A class is a blueprint
Quiz
How do you create an object in Java?
A. ClassName obj;
B. ClassName obj = new ClassName();
C. new ClassName;
D. obj = ClassName();
How do you create an object in Java?
A. ClassName obj;
B. ClassName obj = new ClassName();
C. new ClassName;
D. obj = ClassName();
Quiz-Answer
Class and Object
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