All classes in Kotlin have a common superclass Any , that is the default superclass for a class with no supertypes declared:. Note: Any is not java. Object ; in particular, it does not have any members other than equals , hashCode and toString. Please consult the Java interoperability section for more details. If the derived class has a primary constructor, the base class can and must be initialized right there, using the parameters of the primary constructor.
If the derived class has no primary constructor, then each secondary constructor has to initialize the base type using the super keyword, or to delegate to another constructor which does that. Note that in this case different secondary constructors can call different constructors of the base type:.
As we mentioned before, we stick to making things explicit in Kotlin.
And unlike Java, Kotlin requires explicit modifiers for overridable members we call them open and for overrides:. The override modifier is required for Derived. If it were missing, the compiler would complain.
If there is no open modifier on a function, like Base. The open modifier has no effect when added on members of a final class i. A member marked override is itself open, i. If you want to prohibit re-overriding, use final :. Overriding properties works in a similar way to overriding methods; properties declared on a superclass that are then redeclared on a derived class must be prefaced with override , and they must have a compatible type. Each declared property can be overridden by a property with an initializer or by a property with a getter method.
You can also override a val property with a var property, but not vice versa. This is allowed because a val property essentially declares a getter method, and overriding it as a var additionally declares a setter method in the derived class. Note that you can use the override keyword as part of the property declaration in a primary constructor. For instance, in C , the base method or property can only be overridden in a subclass if it is marked with the virtual, abstract, or override modifier. Implementation inheritance is the mechanism whereby a subclass re-uses code in a base class.
By default the subclass retains all of the operations of the base class, but the subclass may override some or all operations, replacing the base-class implementation with its own. The base class comprises operations to compute the sum of the squares between two integers. The subclass re-uses all of the functionality of the base class with the exception of the operation that transforms a number into its square, replacing it with an operation that transforms a number into its square and cube respectively. In most quarters, class inheritance for the sole purpose of code reuse has fallen out of favor.
An alternative technique, explicit delegation , requires more programming effort, but avoids the substitutability issue. Whereas public inheritance represents an " is-a " relationship and delegation represents a " has-a " relationship, private and protected inheritance can be thought of as an "is implemented in terms of" relationship. Another frequent use of inheritance is to guarantee that classes maintain a certain common interface; that is, they implement the same methods.
The parent class can be a combination of implemented operations and operations that are to be implemented in the child classes.
Often, there is no interface change between the supertype and subtype- the child implements the behavior described instead of its parent class. Inheritance is similar to but distinct from subtyping. The relationship can be expressed explicitly via inheritance in languages that support inheritance as a subtyping mechanism. In programming languages that do not support inheritance as a subtyping mechanism , the relationship between a base class and a derived class is only a relationship between implementations a mechanism for code reuse , as compared to a relationship between types.
Inheritance, even in programming languages that support inheritance as a subtyping mechanism, does not necessarily entail behavioral subtyping. It is entirely possible to derive a class whose object will behave incorrectly when used in a context where the parent class is expected; see the Liskov substitution principle.
In some OOP languages, the notions of code reuse and subtyping coincide because the only way to declare a subtype is to define a new class that inherits the implementation of another. For example, consider a class Person that contains a person's name, date of birth, address and phone number.
We can define a subclass of Person called Student that contains the person's grade point average and classes taken, and another subclass of Person called Employee that contains the person's job-title, employer, and salary. In defining this inheritance hierarchy we have already defined certain restrictions, not all of which are desirable:. The composite reuse principle is an alternative to inheritance. This technique supports polymorphism and code reuse by separating behaviors from the primary class hierarchy and including specific behavior classes as required in any business domain class.
This approach avoids the static nature of a class hierarchy by allowing behavior modifications at run time and allows one class to implement behaviors buffet-style, instead of being restricted to the behaviors of its ancestor classes. Implementation inheritance is controversial among programmers and theoreticians of object-oriented programming since at least the s. Among them are the authors of Design Patterns , who advocate interface inheritance instead, and favor composition over inheritance.
For example, the decorator pattern as mentioned above has been proposed to overcome the static nature of inheritance between classes. As a more fundamental solution to the same problem, role-oriented programming introduces a distinct relationship, played-by , combining properties of inheritance and composition into a new concept. According to Allen Holub , the main problem with implementation inheritance is that it introduces unnecessary coupling in the form of the "fragile base class problem" :  modifications to the base class implementation can cause inadvertent behavioral changes in subclasses.
Using interfaces avoids this problem because no implementation is shared, only the API. Reportedly, Java inventor James Gosling has spoken against implementation inheritance, stating that he would not include it if he were to redesign Java. Complex inheritance, or inheritance used within an insufficiently mature design, may lead to the yo-yo problem. If a development team combined multiple layers of inheritance with the single responsibility principle it created many super thin layers of code, many which would only have 1 or 2 lines of code in each layer.
Before teams learned the hard way that 2 or 3 layers was optimum number of layers balancing the benefit of code reuse against the complexity increase with each layer, it was not uncommon to work on inheritance frameworks with 10 and up to 30 layers. For instance 30 layers made debugging a significant challenge simply to know which layer needed debugged. PowerBuilder built one of the best code library that primarily used inheritance, it was built with 3 to 4 layers.
The number of layers in an inheritance library is critical and must be kept at or below 4 layers otherwise the library becomes too complex and time consuming to use. Another issue with inheritance is that subclasses must be defined in code, which means that program users cannot add new subclasses. Other design patterns such as Entity—component—system allow program users to define variations of an entity at runtime. From Wikipedia, the free encyclopedia. This article has multiple issues.
Please help improve it or discuss these issues on the talk page.
You can force a class to be strictly an inheritable class by using the "abstract" keyword. When you define a class with abstract, any attempt to instantiate a separate instance of it will result in a fatal error. This is useful for situations like a base class where it would be inherited by multiple child classes yet you want to restrict the ability to instantiate it by itself. Even when autoloading SPL is used, class inheritance does not seem to work. Simply the PHP engine is unable to find parent inherited class.
PHP 5. Similar to functions, unless they are in conditionals, it is possible to define classes anywhere within your script and they still generate instances. It is also possible to define child classes before the parent class, however, this seems to act the same way as a conditional, meaning that you need to define these classes before they can be used.
For multiple single inheretance to work the order of class definition is very important if you're going up more than two levels in inheretence.
Here's fun, an attempt to make some degree of multiple inheritance work in PHP using mixins. It's not particularly pretty, doesn't support method visibility modifiers and, if put to any meaningful purpose, could well make your call stack balloon to Ruby-on-Rails-esque proportions, but it does work. This page is not clear about the nature and specifics of Object Inheritance especially that you can only make the visibility of an inherited method or property weaker and not stronger within the subclass that is inheriting these from its parent class.