Liskov Substitution and Interface Segregation

Honest subtypes and focused contracts

The Liskov Substitution Principle (LSP), from Barbara Liskov: objects of a subtype must be usable wherever the supertype is expected without breaking correctness. Subtypes must honour the parent's contract: they may not strengthen preconditions (demand more from callers), weaken postconditions (promise less) or break invariants. The classic violation is Square extends Rectangle: code that sets width to 5 and height to 4 expects area 20, but a square that keeps sides equal returns 16. Another is a subclass that throws UnsupportedOperationException for methods its parent supports. The Interface Segregation Principle (ISP): clients should not be forced to depend on methods they do not use. A fat Worker interface with work(), eat() and attendMeeting() forces a Robot to implement eat(). Split it into small, role-based interfaces (Workable, Feedable) so each class implements only what makes sense and clients depend only on what they call.

An LSP violation and an ISP fix

Square breaks callers that rely on Rectangle's contract; small interfaces avoid fake methods.

class Rectangle {
    protected int width, height;
    void setWidth(int w)  { width = w; }
    void setHeight(int h) { height = h; }
    int area() { return width * height; }
}

class Square extends Rectangle {               // LSP violation
    @Override void setWidth(int w)  { width = height = w; }
    @Override void setHeight(int h) { width = height = h; }
}

void resize(Rectangle r) {
    r.setWidth(5);
    r.setHeight(4);
    assert r.area() == 20;                      // fails for a Square (area 16)
}
// better: make Square and Rectangle separate immutable Shapes

// ISP: small interfaces instead of one fat one
interface Printer { void print(Document d); }
interface Scanner { Document scan(); }
class BasicPrinter implements Printer { public void print(Document d) { } }
class OfficeMachine implements Printer, Scanner {
    public void print(Document d) { }
    public Document scan() { return new Document(); }
}

A replacement driver

If your regular driver is replaced, the substitute must still stop at red lights and take you where you asked. A "driver" who refuses to drive on highways is not a valid substitute, even with the same uniform.

त्वरित जाँच: Which is a sign of a Liskov Substitution Principle violation?

• A subclass adds a new helper method
• A subclass throws UnsupportedOperationException for a method the parent supports
• A subclass overrides toString
• A class implements two interfaces