Abstract Classes in Java

Abstract classes serve as base classes that cannot be instantiated. They often contain abstract methods that must be implemented by subclasses.

// Abstract base class
public abstract class Creature {
    // Abstract method requiring implementation
    public abstract void consume();
}

// Concrete subclass
public class Feline extends Creature {
    @Override
    public void consume() {
        System.out.println("Cat eats fish");
    }
}

// Another concrete subclass
public class Canine extends Creature {
    @Override
    public void consume() {
        System.out.println("Dog eats meat");
    }
}

The Final Keyword

The final modifier prevents modification of classes, methods, or variables.

Final Classes

public final class ImmutableClass {
    // Cannot be extended
}

Final Methods

public class Parent {
    public final void lockedMethod() {
        // Cannot be overridden
    }
}

Final Variables

public class Constants {
    // Final instance variable
    private final int MAX_VALUE = 100;
    
    // Final static variable
    public static final double PI = 3.14159;
}

Static Members

Static members belong to the class rather than insatnces.

Static Fields

public class Employee {
    private String name;
    public static String company = "TechCorp";
    
    public void displayInfo() {
        System.out.println(name + " works at " + company);
    }
}

Static Methods

public class MathUtils {
    public static int square(int num) {
        return num * num;
    }
}

Interfaces in Java

Interfacse define contracts that implementing classes must follow.

Basic Interface

public interface Connectable {
    void establishConnection();
    void terminateConnection();
}

// Concrete implementation
public class Printer implements Connectable {
    @Override
    public void establishConnection() {
        System.out.println("Printer connected");
    }
    
    @Override
    public void terminateConnection() {
        System.out.println("Printer disconnected");
    }
}

Interface Features

public interface NetworkDevice {
    // Constant field
    int MAX_CONNECTIONS = 8;
    
    // Abstract method
    void transmitData();
    
    // Default method (Java 8+)
    default void logStatus() {
        System.out.println("Device operational");
    }
    
    // Static method (Java 8+)
    static void displayProtocol() {
        System.out.println("TCP/IP");
    }
}