Abstract classes and interfaces (C #/Java)

C #:

I. abstract class:
Abstract classes are special classes, but they cannot be instantiated. In addition, they have other characteristics of the class. It is important that abstract classes can include abstract methods, which are not supported by common classes. Abstract METHODS can only be declared in abstract classes and do not contain any implementations. The Derived classes must overwrite them. In addition, an abstract class can be derived from an abstract class. It can overwrite the abstract methods of the base class or overwrite them. If not, its derived class must overwrite them.

 

Ii. interface:
The interface is of reference type. It is similar to a class and has three similarities with the abstract class:
1. It cannot be instantiated;
2. contains an unimplemented method statement;
3. The derived class must implement unimplemented methods. The abstract class is an abstract method, and the interface is all members (not only the methods include other members );

In addition, interfaces have the following features:
In addition to methods, interfaces can also contain attributes, indexers, and events, and these members are defined as common. It cannot contain any other Members, such as constants, fields, constructors, destructor, and static members. A class can directly inherit multiple interfaces, but can only inherit one class (including abstract classes ).

Iii. Differences between abstract classes and interfaces:
1. class is the abstraction of objects. abstract classes can be understood as classes as objects. abstract classes are called abstract classes. the interface is just a behavior specification or provision. Microsoft's custom interface always carries the able field behind it to prove that it represents a class "I can do it... ". Abstract classes are more defined in a series of closely related classes, while interfaces are mostly classes with loose relationships but all implement certain functions.
2. The interface basically does not have any specific characteristics of inheritance. It only promises the methods that can be called;
3. A class can implement several interfaces at a time, but only one parent class can be extended.  
4. interfaces can be used to support callback, but inheritance does not.  
5. the abstract class cannot be sealed.  
6. The specific methods implemented by abstract classes are virtual by default, but the interface methods in the class implementing interfaces are non-virtual by default. Of course, you can also declare them as virtual.
7. Similar to a non-abstract class, an abstract class must provide its own implementation for all the members of the interface listed in the base class list of this class. However, the abstract class is allowed to map interface methods to abstract methods.
8. abstract classes implement a principle in OOP that separates mutable from immutable. Abstract classes and interfaces are defined as immutable classes, while variable class classes are implemented.  
9. A good interface definition should be specific and functional, rather than multi-functional, otherwise it will cause interface pollution. If a class only implements a function of this interface and has to implement other methods in the interface, it is called interface pollution.  
10. Avoid using inheritance to implement the build function, but use black box multiplexing, that is, object combination. As the hierarchy of inheritance increases, the most direct consequence is that when you call a class in this group, you must load all of them into the stack! The consequences can be imagined (based on the stack principle ). At the same time, some interested friends can note that Microsoft often uses the object combination method when building a class. For example, in Asp.net, the page class has server request and other attributes, but in fact they are all objects of a certain class. This object of the page class is used to call the methods and attributes of other classes. This is a very basic design principle.  
11. If an abstract class implements an interface, you can map the methods in the interface to the abstract class as an abstract method without having to implement it. Instead, you can implement the methods in the subclass of the abstract class.

Iv. Use of abstract classes and interfaces:
1. If you want to create multiple versions of a component, create an abstract class. Abstract classes provide simple methods to control component versions.
2. If the created function is used across a wide range of different objects, the interface is used. If you want to design small and concise functional blocks, use interfaces.
3. If you want to design a large functional unit, use the abstract class. If you want to provide general implemented functions among all the implementations of the component, use the abstract class.
4. abstract classes are mainly used for closely related objects. interfaces are suitable for providing general functions for irrelevant classes.

The following are some of the Image metaphors I have seen on the Internet. They are really good:
1. Planes fly and birds fly. They all inherit the same interface "fly". However, f22 belongs to the aircraft abstract class and pigeon belongs to the bird abstract class.
2. just like all doors (abstract class), I can't give you a door (I can't instantiate it), but I can give you a specific door or wooden door (polymorphism ); it can only be a door. You cannot say it is a window (single inheritance); a door can have a lock (Interface) or a doorbell (multiple implementations ). A door (abstract class) defines what you are and an interface (LOCK) specifies what you can do (one interface is best to do only one thing, you cannot require the lock to make sound (interface pollution )).

 

Java:

Differences between abstract classes and interfaces

Abstract class and interface are two mechanisms supported for the definition of abstract classes in Java. It is precisely because of the existence of these two mechanisms that give Java powerful object-oriented capabilities. Abstract class and interface have great similarity in support for the definition of abstract classes, and can even be replaced with each other, therefore, when defining abstract classes, many developers may choose abstract classes and interfaces at will.

In fact, there is a big difference between the two. Their choices even reflect the understanding of the nature of the problem domain, and whether the understanding of the design intent is correct and reasonable. This article will analyze the differences between them and try to provide developers with a basis for selection between them.

I. understanding abstract classes

Abstract class and interface are used for abstract classes in Java language (abstract classes in this article are not translated from abstract class, it represents an abstract body, abstract class is a method used to define abstract classes in the Java language. Please note that it is defined). So what is an abstract class and what benefits can it bring to us by using abstract classes?

In the concept of object-oriented, we know that all objects are depicted through classes, but this is not the case. Not all classes are used to depict objects. If a class does not contain enough information to depict a specific object, such classes are abstract classes. Abstract classes are often used to represent the abstract concepts we have come up with in the analysis and design of problem domains. They are abstractions of a series of seemingly different but essentially identical specific concepts.

For example, if we develop a graphic editing software, we will find that some specific concepts such as circles and triangles exist in the problematic domain. They are different, however, they all belong to the concept of shape. The concept of shape does not exist in the field of problem. It is an abstract concept. Abstract concepts cannot be instantiated because they do not have specific concepts in the problem field.

In the Object-Oriented field, abstract classes are mainly used to hide types. We can construct a fixed abstract description of a group of actions, but this group of actions can have any specific implementation method. This abstract description is an abstract class, and any possible implementations of this group are represented as all possible Derived classes. The module can operate on an abstract body. Because the module depends on a fixed abstract body, it may not be allowed to be modified. At the same time, the behavior function of this module can be extended by deriving from this abstract body. Readers familiar with OCP must know that abstract classes are the key to implementing an open-closed principle Principle of object-oriented design.

Ii. View abstract class and interface in terms of syntax definition

At the syntax level, the Java language provides different definitions for abstract class and interface. The following describes how to define an abstract class named demo. You can use abstract class to define a demo abstract class as follows:

Abstract class demo {

Abstract void Method1 ();

Abstract void method2 ();

...

}

The following method is used to define the demo abstract class using the interface:

Interface demo {

Void Method1 ();

Void method2 ();

...

}

In the abstract class method, the demo can have its own data members or non-Abstarct member methods. In the implementation of the interface method, demo can only have static data members that cannot be modified (that is, they must be static final, but generally do not define data members in the interface). All member methods are abstract. In a sense, interface is a special form of abstract class.

From the programming point of view, abstract class and interface can be used to implement the idea of "Design by contract. However, there are some differences in usage.

Abstract class represents an inheritance relationship in Java. A class can only use an inheritance relationship once. However, a class can implement multiple interfaces. Maybe this is a compromise between Java designers and Java's support for multi-inheritance.

Secondly, in the definition of abstract class, we can assign the default behavior of the method. However, in the interface definition, a method cannot have default behavior. to bypass this restriction, you must use a delegate. However, this increases complexity and sometimes causes great trouble.

Another serious problem still exists when the default behavior cannot be defined in the abstract class, which may cause maintenance trouble. Because if you want to modify the interface of the class (usually expressed by abstract class or interface) to adapt to the new situation (for example, adding a new method or adding a new parameter to the used method) it will be very troublesome and may take a lot of time (especially when there are many derived classes ). However, if the interface is implemented through abstract class, you may only need to modify the default behavior defined in abstract class.

Similarly, if the default behavior cannot be defined in the abstract class, the implementation of the same method will appear in every derived class of the abstract class, in violation of the "one rule, one place" principle, resulting inCodeRepetition is also not conducive to future maintenance. Therefore, be careful when selecting abstract class and interface.

Iii. abstract class and Interface

The difference between abstract class and interface is discussed from the perspective of syntax definition and programming. The difference between these layers is relatively low-level and non-essential. This article will analyze the differences between abstract class and interface on another level. The author believes that only by analyzing at this level can we understand the essence of the two concepts.

As mentioned above, Abstarct class represents an inheritance relationship in Java. To make the inheritance relationship reasonable, there must be a "is a" relationship between the parent class and the derived class, that is, the concept of the parent class and the derived class should be essentially the same. For an interface, it is not required that the implementer of the interface and the interface definition are essentially consistent in concept, but only implement the contract defined by the interface. In order to make the discussion easier to understand, we will explain it through a simple example below.

Consider this example. Suppose there is an abstract concept about the door in our problem field. The door has two actions: open and close, in this case, abstract class or interface can be used to define a type that represents the abstract concept. The definitions are as follows:

Use abstract class to define door:

Abstract class door {

Abstract void open ();

Abstract void close ();

}

Use the interface method to define the door:

Interface door {

Void open ();

Void close ();

}

For other specific door types, extends can use the door defined in abstract class or implements to use the door defined in interface mode. It seems that there is no big difference between abstract class and interface.

If you want the door to have the alarm function. How can we design the class structure for this example (in this example, we mainly want to demonstrate the differences between abstract class and interface in the design concept, other irrelevant issues are simplified or ignored.) The following describes possible solutions and analyzes these solutions at the design concept layer.

Solution 1:

Add an alarm method to the door definition as follows:

Abstract class door {

Abstract void open ();

Abstract void close ();

Abstract void alarm ();

}

Or

Interface door {

Void open ();

Void close ();

Void alarm ();

}

The alarmdoor with alarm function is defined as follows:

Class alarmdoor extends door {

Void open (){... }

Void close (){... }

Void alarm (){... }

}

Or

Class alarmdoor implements door {

Void open (){... }

Void close (){... }

Void alarm (){... }

}

This method violates a core principle in object-oriented design, ISP (interface segregation priciple ), in the definition of door, the inherent behavior methods of the door concept are mixed with the behavior methods of another concept "alarm. One problem is that the modules that rely solely on the door concept will change due to changes in the concept of "alarm" (for example, modifying the parameters of the alarm method.

Solution 2:

Since open, close, and alarm belong to two different concepts, they should be defined in abstract classes that represent these two concepts according to the ISP principle. The two concepts are defined by abstract class. Both concepts are defined by interface. One is defined by abstract class, and the other is defined by interface.

Obviously, because the Java language does not support multiple inheritance, both concepts are defined using abstract class. The latter two methods are feasible, but their selection reflects the understanding of the concept nature in the problem field, and whether the reflection of the design intent is correct and reasonable. Let's analyze and explain them one by one.

If both concepts are defined using the interface method, two problems are identified:

1. We may not understand the problem. Is alarmdoor actually a door or an alarm?

2. If we have no problem in understanding the problem field, for example, we have found that alarmdoor is essentially consistent with door through analysis of the problem field, therefore, our design intent cannot be correctly revealed during implementation, because the definitions of these two concepts (both using the interface method definition) do not reflect the above meaning.

If our understanding of the problem field is: alarmdoor is essentially a door in concept, it also has the alarm function. How can we design and implement it to clearly reflect what we mean? As mentioned above, abstract class represents an inheritance relation in Java, and the inheritance relation is essentially a "is a" relation. So we should use the Abstarct class method to define the concept of door. In addition, alarmdoor has the alarm function, indicating that it can complete the behaviors defined in the alarm concept. Therefore, the alarm concept can be defined through interface. As follows:

Abstract class door {

Abstract void open ();

Abstract void close ();

}

Interface alarm {

Void alarm ();

}

Class alarmdoor extends door implements alarm {

Void open (){... }

Void close (){... }

Void alarm (){... }

}

This implementation method can clearly reflect our understanding of the problem field and correctly reveal our design intent. Abstract class represents the "is a" relation, and interface represents the "like a" relation. You can use it as a basis for your selection, of course, this is based on the understanding of the problem field. For example, if we think that alarmdoor is essentially an alarm and has the door function, then the above definition method will be reversed.

Abstract class and interface are two methods of defining abstract classes in Java. They have great similarity. However, their choices often reflect the understanding of the concept nature in the problem field, and whether the reflection of the design intent is correct and reasonable, because they represent different relationships between concepts (although they all implement the required functions ). This is actually a common use of language. I hope readers can understand it in detail.