Liskov Substitution

Subtypes must be substitutable for their base types.

Explanation: A subclass should be able to replace its superclass without altering the correctness of the program.

Java Example

Bad Design (Violating LSP)

class Rectangle {
    int width, height;

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

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

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

class Square extends Rectangle {
    void setWidth(int width) {
        this.width = width;
        this.height = width; // Violates LSP
    }

    void setHeight(int height) {
        this.width = height;
        this.height = height; // Violates LSP
    }
}

Here, Square is a subclass of Rectangle, but it breaks LSP because changing the width also changes the height, making it behave differently from a rectangle.

Good Design (Following LSP)

interface Shape {
    int getArea();
}

class Rectangle implements Shape {
    int width, height;

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

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

class Square implements Shape {
    int side;

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

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

Now, both Rectangle and Square can be used interchangeably without breaking behavior.

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Python Example

Bad Design (Violating LSP)

class Rectangle {
    private int width;
    private int height;
    
    public Rectangle(int width, int height) {
        this.width = width;
        this.height = height;
    }
    
    public void setWidth(int width) {
        this.width = width;
    }
    
    public void setHeight(int height) {
        this.height = height;
    }
    
    public int getArea() {
        return this.width * this.height;
    }
}

class Square extends Rectangle {
    public Square(int side) {
        super(side, side);
    }
    
    @Override
    public void setWidth(int width) {
        this.width = width;
        this.height = width;  // Violates LSP
    }
    
    @Override
    public void setHeight(int height) {
        this.width = height;
        this.height = height;  // Violates LSP
    }
}

👎 Problem: A Square should not behave like a Rectangle. Setting width also changes the height, breaking expected behavior.

Good Design (Following LSP)

// Abstract Shape interface
interface Shape {
    int getArea();
}

class Rectangle implements Shape {
    private int width;
    private int height;
    
    public Rectangle(int width, int height) {
        this.width = width;
        this.height = height;
    }
    
    @Override
    public int getArea() {
        return this.width * this.height;
    }
}

class Square implements Shape {
    private int side;
    
    public Square(int side) {
        this.side = side;
    }
    
    @Override
    public int getArea() {
        return this.side * this.side;
    }
}

👍 Fix: Now, Square and Rectangle can be used interchangeably without breaking behavior.