amana2
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Liskov Substitution Principle
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// Example of violation of LSP class Rectangle { public int setWidth(int a) { width = a; } public int setHeight(int a) { height = a; } public int area { return (width * height); } } class Square extends Rectangle { public int setWidth(int a) { width = height = a; } public int setHeight(int a) { width = height = a; } } class RectangleTest { /* * This test fails for objects of class Square because they do not * respect the semantics of the setWidth and setHeight operations * in the superclass. */ @Test public void testArea(Rectangle r) { r.setWidth(2); r.setHeight(5); assert(r.area() == 2*5); } } /* * "Fixing" the test method through violation of OCP: */ @Test public void testArea(Rectangle r) { if (r instaceOf Rectangle) { r.setWidth(2); r.setHeight(5); assert(r.area() == 2*5); } } /* * As we have seen, the problem is not easy to solve, but most * important, it is originated by a wrong understanding of the * generalization relation. No matter how similar two objects look * in the real world, the generalization is applied to their computer * representation (e.g. class). Consequently, it only makes sense to * use generalization when the subclass will correspond to the semantics * of the superclass. Otherwise, it is better to use two separate classes. * * We could even force their relation in some cases by using a more * complex class structure, but we must bear in mind that having those * classes related may not be worth the complexity. */ public abstract class Rectangle { public int area { return (width * height); } } class PureRectangle extends Rectangle { public int setWidth(int a) { width = a; } public int setHeight(int a) { height = a; } } class Square extends Rectangle { public int setSide(int a) { width = height = a; } } /* * Now, client classes of the Rectangle class can only expect the semantics * of the area operation of the subclasses of Rectangle to be consistent with * the Rectangle specification. * * On the other hand, clients of each subclass will not have any problem with * semantics of the other subclass. */ |