Liskov Substitution Principle

Subtypes must be substitutable for base types

Liskov Substitution Principle (LSP)

Subtypes must be substitutable for their base types without altering the correctness of the program. If class B extends class A, you should be able to use B anywhere A is expected without surprises.

java

// ❌ LSP Violation - Rectangle/Square
// Classic violation: Square extends Rectangle
class Rectangle {
    protected int width, height;

    public void setWidth(int w) { width = w; }
    public void setHeight(int h) { height = h; }
    public int area() { return width * height; }
}

class Square extends Rectangle {
    @Override
    public void setWidth(int w) {
        width = w;
        height = w; // SURPRISE! Also changes height
    }
    @Override
    public void setHeight(int h) {
        width = h;  // SURPRISE! Also changes width
        height = h;
    }
}

// This breaks when using Square as Rectangle:
void resize(Rectangle r) {
    r.setWidth(5);
    r.setHeight(10);
    assert r.area() == 50; // FAILS for Square! Area = 100

java

// ✅ LSP Applied
// Fix: Use a common Shape interface
interface Shape {
    double area();
}

class Rectangle implements Shape {
    private final int width, height;
    Rectangle(int w, int h) { width = w; height = h; }
    public double area() { return width * height; }
}

class Square implements Shape {
    private final int side;
    Square(int s) { side = s; }
    public double area() { return side * side; }
}

// Both work correctly when treated as Shape
void printArea(Shape s) {
    System.out.println("Area: " + s.area()); // Always correct!
}