There is one more thing we need to consider with the Diamond Problem — compatibility. It may happen that your perfectly valid code works today but stops working tomorrow. Your code works correctly. Then someone comes and adds default foo method to the second interface. When you recompile your code — it breaks.
Like we said in previous section, the problem is about deciding which thing wins when we have two of them. There are two foo methods, one accepting long, other accepting double. Can you easily tell which one is going to be used? The answer is: the former, accepting long parameter.
We have two methods with different signatures. We want to call the method and we pass invalid value — value of a different type. We have two things and we cannot decide which one to use. In the Diamond Problem with default interface implementations Java shows a compilation error but with method overloading it just chooses one method over another.
Also, it has the same implications when it comes to breaking the compatibility — imagine that someone comes and adds another foo int i method.
It breaks the compatibility. While accepting different numbers is a plausible situation, there is actually much more serious place where you may hit this issue. Source compatibility issue with Google Guava library post shows when Guava library added new override when accepting params array versus explicit parameters. Actually, we can inherit implementation since Java 8 — is it a multiple inheritance or not?
And it has the same implications. Skip to content This is the fourth part of the Types and Programming Languages series. For your convenience you can find other parts in the table of contents in Part 1 — Do not return in finally The Diamond Problem, sometimes called Deadly Diamond of Death, is a problem in which we inherit the same thing through multiple base entities.
Table of Contents. The complete Clock class:. Let's check our exception handling by inputting floats and strings as input.
We also check, what happens, if we exceed the limits of the expected values:. We will now create a class "Calendar", which has lots of similarities to the previously defined Clock class. Instead of "tick" we have an "advance" method, which advances the date by one day, whenever it is called. Adding a day to a date is quite tricky. We have to check, if the date is the last day in a month and the number of days in the months vary.
As if this isn't bad enough, we have February and the leap year problem. As a little useful gimmick, we added a possibility to output a date either in British or in American Canadian style. At last, we will introduce our multiple inheritance example. We are now capable of implementing the originally intended class CalendarClock, which will inherit from both Clock and Calendar. The method "tick" of Clock will have to be overridden. However, the new tick method of CalendarClock has to call the tick method of Clock: Clock.
The "diamond problem" sometimes referred as the "deadly diamond of death" is the generally used term for an ambiguity that arises when two classes B and C inherit from a superclass A, and another class D inherits from both B and C. If there is a method "m" in A that B or C or even both of them has overridden, and furthermore, if it does not override this method, then the question is which version of the method does D inherit?
It could be the one from A, B or C. Let's look at Python. If you call the method m on an instance x of D, i. If we transpose the order of the classes in the class header of D in "class D C,B :", we will get the output "m of C called".
Principially, two possibilities are imaginable: "m of C" or "m of A" could be used. We call this script with Python2. Only for those who are interested in Python version2: To have the same inheritance behaviour in Python2 as in Python3, every class has to inherit from the class "object". Our class A doesn't inherit from object, so we get a so-called old-style class, if we call the script with python2. Multiple inheritance with old-style classes is governed by two rules: depth-first and then left-to-right.
If you change the header line of A into "class A object :", we will have the same behaviour in both Python versions. We have seen in our previous implementation of the diamond problem, how Python "solves" the problem, i. Let's apply the method m on an instance of D. And the Snake class ended up calling the breathe method of the Animal class. In the second example, the Snake class inherits two breathe methods.
The breathe method of the Animal and Reptile class. But, it is not necessary that we use all the features it provides. It ensures that the child class gets only a single instance of the common base class. In other words, the Snake class will have only one instance of the LivingThing class. The Animal and Reptile classes share this instance. This solves the compile time error we receive earlier. Classes derived from abstract classes must override the pure virtual functions defined in the base class.
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