Rec 02

Problems

01. Liskov Substitution Principle and Inheritance

When dealing with questions involve inheritance, always think about whether the Liskov Substitution Principle is violated or not. The latter can be done by seeing whether the specifications set by the parent class are violated or not.

In this question, firstly, we don't set any specification for our Rectangle. But, when we try to inherit the Square class from the Rectangle class, we explicitly specify the specification, which is

the fours sides for a Square should be of the same length.

Since the parent class Rectangle doesn't have any specification, so there is no chance for Square to violate LSP. Thus till now, Square conform to LSP

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Then, we make a new specification for our Rectangle class, which is

if the most recent call to setWidth is setWidth(w) and the most recent call to setHeight is setHeight(h), then getArea() must return w*h

Here, we didn't override the Rectangle::setHeight and Rectangle::setWidth in the Square class.

Now, since we add a new specification to the parent class, we need to verify whether LSP still holds or not. To verify that, we can easily find that it will violate the first specification of our Square class, which is the four sides of the Square should be of the same length.

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What if we override the Rectangle::setHeight and Rectangle::setWidth in the Square class to set both the height and width to be the same length? Now, we may violate the second specification of our Rectangle class because now the getArea will not return w*h. Instead, it may return either w*w or h*h.

Thus, now we can conclude the Square cannot be inheritted from Rectangle.

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Conversely, a Rectangle cannot be inheritted from a Square since a Square needs four sides to be equal, a Rectangle don't need this specification and thus can easily violate the LSP.

02. Interface and Inheritance

A class cannot inherit from multiple classes at the same time.

Let's see the following example, where we have A::foo and B::foo. Class C inherits from A and B.

If we do not override foo. What will C::foo call? A::foo or B::foo? We don't know. So does the compiler. Thus, we cannot inherit a class from multiple classes.

A class can implement multiple interfaces at the same time

Suppose now A and B become two interfaces. Recall that all the methods in interface are public abstract, thus have no implementation. And all concrete classes that implement an interface must override the methods in the interface to provide a concrete implementation.

So, now what will happen if we call C::foo? By the knowledge of Method Invocation, we can clearly notice that C::foo will call the foo that is overriden in the class C itself. Thus, there is no problem for a class to implement several interfaces.

Tips

1. When using LSP, think about whether the properties/specifications between these two classes are violated or not.

2. A class cannot inherit from multiple classes at the same time but a class can implement multiple interfaces at the same time.