Added OOPs concepts demos

Author: SuryanshuAgrawal

src/main/java/com/thealgorithms/oops/AbstractionDemo.java

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+package com.thealgorithms.oopconcepts;
+
+abstract class Vehicle {
+    abstract void start();
+    abstract void stop();
+}
+
+class Car extends Vehicle {
+    @Override
+    void start() {
+        System.out.println("Car engine started");
+    }
+
+    @Override
+    void stop() {
+        System.out.println("Car stopped");
+    }
+}
+
+public class AbstractionDemo {
+    public static void main(String[] args) {
+        Vehicle v = new Car();
+        v.start();
+        v.stop();
+    }
+}

src/main/java/com/thealgorithms/oops/EncapsulationDemo.java

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+package com.thealgorithms.oopconcepts;
+
+class Account {
+    private String holder;
+    private double balance;
+
+    public String getHolder() {
+        return holder;
+    }
+
+    public void setHolder(String holder) {
+        this.holder = holder;
+    }
+
+    public double getBalance() {
+        return balance;
+    }
+
+    public void deposit(double amount) {
+        if (amount > 0) balance += amount;
+    }
+
+    public void withdraw(double amount) {
+        if (amount > 0 && amount <= balance) balance -= amount;
+    }
+}
+
+public class EncapsulationTest {
+    public static void main(String[] args) {
+        Account acc = new Account();
+        acc.setHolder("Suryanshu");
+        acc.deposit(50000);
+        acc.withdraw(12000);
+        System.out.println(acc.getHolder() + " has balance ₹" + acc.getBalance());
+    }
+}

src/main/java/com/thealgorithms/oops/InheritanceDemo.java

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+package com.thealgorithms.oopconcepts;
+class Vehicle {
+    int tyres;
+    String colour;
+    String companyName;
+
+    void setTyres(int tyres) {
+        this.tyres = tyres;
+        System.out.println("Number of tyres: " + tyres);
+    }
+
+    void setColour(String colour) {
+        this.colour = colour;
+        System.out.println("Colour of vehicle: " + colour);
+    }
+
+    void setCompanyName(String companyName) {
+        this.companyName = companyName;
+        System.out.println("Company name: " + companyName);
+    }
+}
+
+class Truck extends Vehicle {
+    int wheels = 8, headlights = 2;
+    String model;
+
+    Truck(String model) {
+        this.model = model;
+        System.out.println("This is a truck: " + model);
+    }
+}
+
+class Motorcycle extends Vehicle {
+    int wheels = 2, headlights = 1;
+    String model;
+
+    Motorcycle(String model) {
+        this.model = model;
+        System.out.println("This is a motorcycle: " + model);
+    }
+}
+
+public class InheritanceDemo {
+    public static void main(String[] args) {
+        Motorcycle mc = new Motorcycle("Dream");
+        mc.setColour("Black");
+        mc.setTyres(2);
+        mc.setCompanyName("Honda");
+
+        Truck tk = new Truck("Tata Ultra");
+        tk.setTyres(8);
+        tk.setColour("White");
+        tk.setCompanyName("Tata");
+    }
+}

src/main/java/com/thealgorithms/oops/PolymorphismDemo.java

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+package com.thealgorithms.oopconcepts;
+
+class Payment {
+    void pay() {
+        System.out.println("Processing generic payment");
+    }
+}
+
+class CreditCardPayment extends Payment {
+    @Override
+    void pay() {
+        System.out.println("Payment done via Credit Card");
+    }
+}
+
+class UpiPayment extends Payment {
+    @Override
+    void pay() {
+        System.out.println("Payment done via UPI");
+    }
+}
+
+public class PolymorphismExample {
+    public static void main(String[] args) {
+        Payment p1 = new CreditCardPayment();
+        Payment p2 = new UpiPayment();
+        Payment p3 = new Payment();
+
+        p1.pay();
+        p2.pay();
+        p3.pay();
+    }
+}