Olá pessoal, meu nome é Konstantin. Estou desenvolvendo em Java no Tinkoff.ru e adoro o SOLID. Neste artigo, formularemos o princípio de substituição de Lisk, mostraremos sua conexão com o princípio Aberto-Fechado, aprenderemos a formar adequadamente a hierarquia de herança e responderemos à pergunta filosófica sobre se um quadrado é um retângulo.

, , , .

-. , , , . . ( LSP) — , - . — , LSP, -.

:

, , -, .

:

, : o1 S o2 T, P, T, P , o2 o1, S — T.

, LSP, , , , . -, , .

LSP

, :

void drawShape(Shape shape) {
   if (shape instanceof Square) {
       drawSquare((Square) shape);
   } else {
       drawCircle((Circle) shape);
   }
}

. Shape, , drawCircle, , — .

drawShape Shape. , , Shape , . Shape, -.

, LSP . , , . :

public class Rectangle {
    private int width;
    private int height;

    public int getWidth() {
        return width;
    }

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

    public int getHeight() {
        return height;
    }

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

    public int perimeter() {
        return 2 * height + 2 * width;
    }
}

. , . , , , , , — Square. ?

, «» (ISA). , , ISA , , .

— . , ISA. , Square Rectangle. , , , , .

, Square height width, , side, . , ( , , ), . , Square setWidth setHeight. , :

    @Override
    public void setWidth(int width) {
        super.setWidth(width);
        super.setHeight(width);
    }

    @Override
    public void setHeight(int height) {
        super.setHeight(height);
        super.setWidth(height);
    } 

Square . . Square. . , , , , .

, Rectangle :

public class RectangleTest {

    @Test
    public void perimeter() {
        Rectangle rectangle = new Rectangle();
        rectangle.setHeight(5);
        rectangle.setWidth(7);

        int result = rectangle.perimeter();

        assertEquals(24, result);
    }

} 

, Square . Square ISA Rectangle, , Rectangle Square. , :

public class RectangleTest {

    @Test
    public void perimeter() {
        Rectangle rectangle = initRectangle();
        rectangle.setHeight(5);
        rectangle.setWidth(7);

        int result = rectangle.perimeter();

        assertEquals(24, result);
    }

    protected Rectangle initRectangle() {
        return new Rectangle();
    }

}

public class SquareTest extends RectangleTest {

    @Override
    protected Rectangle initRectangle() {
        return new Square();
    }

}

, SquareTest.perimeter , perimeter 24, , 28. : , , ? — . , Rectangle, Square. Square Rectangle , Rectangle ( ) , Rectangle Square. Square — Rectangle, LSP .

— ?

— , , , Square — Rectangle. , Square Rectangle. , .

, ISA . , , . , .

. — , , . , , .

?

: . . , .

, , , , , . Shape Square Rectangle.

, . , . Quadrangle , , , , . .

- , , .

, , . , , , , , -.

1996 — The Liskov Substitution Principle. . , , C++. , , LSP .