Liskov Substitution Principle

Liskov Substitution Principle enables you to replace objects of a parent class(Bird) with objects of a subclass(Eagle,Penguin) without breaking the application.

Liskov Substitution Principle extends the open/closed Principle.

public class Bird {
    private int wings;
    private int health;

    public Bird(int wings, int health) {
        this.wings = wings;
        this.health = health;
    }

    private booelan fly() {
        if (health > 0 && wings==2) {
             System.out.println("The bird is flying.");
            return true;
        } else {
            System.out.println("The bird cannot fly.");
           return false;
        }
    }

     // Getters and setters for health and wings can be added here if needed
}

public class BirdAnalysisClient {
    // This method accepts a Bird object or any of its subtypes 
    // (e.g., Eagle, Penguin
    public void crossLake(Bird bird) {
        if (bird.canFly()) {
            System.out.println("The bird can cross the lake.");
        } else {
            System.out.println("The bird cannot cross the lake.");
        }
    }
}
// Test works fine no issue . Looks good .
public class BirdAnalysisTester {
    public static void main(String[] args) {
        BirdAnalysisClient client = new BirdAnalysisClient();

        Bird genericBird = new Bird(1,2);
        client.crossLake(genericBird);
    }
}

public class Eagle extends Bird {

public Eagle(boolean isHealthy, int wings) { 
    super(isHealthy, wings);
 }

}

// Logic works fine no issue.
public class BirdAnalysisTester {
    public static void main(String[] args) {
        BirdAnalysisClient client = new BirdAnalysisClient();

        Bird genericBird = new Eagle(1,2);
        client.crossLake(genericBird);
    }
}

// Liskov Substitution priniciple not followed .
// This child class will break application on use instead of parent class.
public class Penguin extends Bird {

    public Penguin(int wings, int health) {
        super(wings, health);        
    }

    @Override 
    public boolean canFly() { 
    throw new UnsupportedOperationException("Penguins cannot fly"); 
    } 
}

// Logic works fails, code throws an error . 

public class BirdAnalysisTester {
    public static void main(String[] args) {
        BirdAnalysisClient client = new BirdAnalysisClient();

        Bird genericBird = new Penguin(1,2);
        client.crossLake(genericBird);
    }
}