Liskov Substitution Principle (LSP) in SOLID with Java Example

The Liskov Substitution Principle (LSP) is one of the five SOLID principles of object-oriented programming, formulated by Barbara Liskov. It states:

"Objects of a superclass should be replaceable with objects of its subclass without affecting the correctness of the program."

In simpler terms, if class B is a subclass of class A, then objects of class A should be replaceable with objects of class B without breaking the application.

Why is LSP Important?

LSP ensures that a derived class extends the behavior of a base class without altering its fundamental characteristics. Violating LSP can lead to unexpected behaviors, breaking polymorphism and making code more complex to maintain.

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Example of LSP Violation

Incorrect Example (Violating LSP)

class Rectangle {
    protected int width;
    protected int height;

    public void setWidth(int width) {
        this.width = width;
    }

    public void setHeight(int height) {
        this.height = height;
    }

    public int getArea() {
        return width * height;
    }
}

class Square extends Rectangle {
    @Override
    public void setWidth(int width) {
        this.width = width;
        this.height = width; // Enforcing square behavior
    }

    @Override
    public void setHeight(int height) {
        this.width = height;
        this.height = height; // Enforcing square behavior
    }
}

public class LSPViolationExample {
    public static void main(String[] args) {
        Rectangle rect = new Square();  // Substituting subclass
        rect.setWidth(4);
        rect.setHeight(5);

        System.out.println("Expected Area: " + (4 * 5)); // Expecting 20
        System.out.println("Actual Area: " + rect.getArea()); // Output: 25 (Incorrect!)
    }
}

Why is LSP Violated Here?

• The Square class breaks the behavior of Rectangle by forcing the width and height to be the same.
• The program expects the area to be width * height = 4 * 5 = 20, but since Square modifies both dimensions, the actual area is 5 * 5 = 25, causing unexpected behavior.

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Correct Example (Following LSP)

To fix this, we should avoid modifying inherited behaviors in a way that breaks expectations. A better approach is to use separate abstractions for Square and Rectangle.

abstract class Shape {
    public abstract int getArea();
}

class Rectangle extends Shape {
    protected int width;
    protected int height;

    public Rectangle(int width, int height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public int getArea() {
        return width * height;
    }
}

class Square extends Shape {
    private int side;

    public Square(int side) {
        this.side = side;
    }

    @Override
    public int getArea() {
        return side * side;
    }
}

public class LSPExample {
    public static void main(String[] args) {
        Shape rect = new Rectangle(4, 5);
        Shape square = new Square(4);

        System.out.println("Rectangle Area: " + rect.getArea()); // 20
        System.out.println("Square Area: " + square.getArea()); // 16
    }
}

Why is This Correct?

• The Shape abstract class defines a common contract (getArea()), but Rectangle and Square implement their own behaviors separately.
• Rectangle and Square do not override each other’s behavior, ensuring LSP compliance.
• Objects of Rectangle and Square can be used interchangeably without breaking expected behavior.

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Key Takeaways

- Follow LSP by ensuring that subclasses do not break the expectations set by their base classes.
- Avoid overriding methods in a way that alters the base class's behavior incorrectly.
- Use separate abstractions when different behaviors are needed, instead of forcing a subclass to fit.
- Design classes such that a subclass can be substituted for its parent without causing unexpected behavior.

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