Cheat Sheet for OOPs concept in Java

 

1. Class and Object:

   • Class: Blueprint or template for creating objects.
   • Object: Instance of a class; represents a real-world entity.
   • 
     

   java
   // Class definition
   
   class Car {
       
       // Fields (attributes)
   
       String model;
       int year;
   
       
       // Methods (behavior)
       void start() {
           
         // implementation
   
       }
   }
   
   
   // Creating an object
   
   Car myCar = new Car();
   

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5. Encapsulation:

   • Bundling of data (attributes) and methods that operate on the data into a single unit (class).
   • Access modifiers (public, private, protected) control access to class members.

   java
   class BankAccount {
       
       private double balance;
   
       
       public void deposit(double amount){
             // implementation
   
       }
       public double getBalance() {
             return balance;
   
       }
   
   }
   

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9. Inheritance:

   • A mechanism where a new class inherits properties and behaviors of an existing class.
   • extends is used to implement inheritance.

   java
   class Animal {
         void eat() {
             // implementation
   
          }
   
   }
   
   
   class Dog extends Animal {
         void bark() {
             // implementation
        }
    }

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13. Polymorphism:

    • Ability of a method to do different things based on the object it is acting upon.
    • Achieved through method overloading and overriding.

    java
    class Shape {
          void draw() {// implementation}
    
    }
    
    
    class Circle extends Shape {
          void draw() {
            // overridden implementation
        }
    
    }
    
    
    // Polymorphic behavior
    
    Shape myShape = new Circle();
    
    myShape.draw();
    

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17. Abstraction:

    • Concept of hiding the complex implementation details and showing only essential features of an object.
    • Abstract classes and interfaces are used to achieve abstraction.

    java
    abstract class Shape {
          abstract void draw();
    
    }
    
    
    class Circle extends Shape {
          void draw() {
            // implementation
        }
    
    }
    

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21. Interface:

    • A collection of abstract methods.
    • Classes implement interfaces to provide specific implementations for those methods.

    java
    interface Printable {
          void print();
    
    }
    
    
    class Document implements Printable {
          public void print() {
            // implementation
        }
    
    }
    

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25. Constructor:

    • Special method used for initializing objects.
    • It has the same name as the class and no return type.

    java
    class Person {
       String name;
       // Constructor
        
       public Person(String n) {
            name = n;
        }
    
    }
    
    
    // Creating an object with a constructor
    
    Person person1 = new Person("John");
    

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29. Getter and Setter:

    • Methods used to retrieve and update the values of private fields.

    java
    class Student {
          private String name;
    
        
          // Getter
        
          public String getName() {
            return name;
        }
    
        
          // Setter
        
          public void setName(String n) {
            name = n;
        }
    
    }