Deep understanding of abstract class and interface

	Abstract class	Interface
Instantiation of	No	No
Class	An inheritance relationship in which a class can use only one inheritance relationship. Multiple inheritance can be implemented by inheriting several interfaces	A class can implement multiple interface
Data members	can have their own	Static cannot be modified that must be static final, which is generally not defined here
Method	Can be private, non-abstract methods, must be implemented	Not private, default is Public,abstract type
Variable	Can have private, default is friendly type, its value can be redefined in subclass, can also be re-assigned value	Not private, default is public static final type, and must give its initial value, the implementation class can not be redefined, cannot change its values.
Design concept	Represents a "is-a" relationship	Represents a "like-a" relationship
Realize	Need to inherit, to use extends	To use implements

On the abstract class and interface from the perspective of grammar definition

At the syntactic level, the Java language gives different definitions for abstract class and interface, and the following is an example of defining an abstract class called Demo to illustrate this difference.

The way to define the demo abstract class using the abstract class is as follows:

At the syntactic level, the Java language gives different definitions for abstract class and interface, and the following is an example of defining an abstract class called Demo to illustrate this difference.

The way to define the demo abstract class using the abstract class is as follows:

Abstract class Demo {abstract void method1 (); abstract void method2 (); ｝

The way to define the demo abstract class using interface is as follows:

Interface Demo {void method1 (); void Method2 (); ...}

In the abstract class mode, the demo can have its own data members, but also can have non-ABSTARCT member methods, and in the implementation of interface mode, the demo can only have static data members can not be modified (that is, must be static final , but the data members are not generally defined in interface, all the member methods are abstract. In a sense, interface is a special form of abstract class.

The abstract class and interface from the programming perspective

From a programmatic point of view, both the abstract class and the interface can be used to implement the idea of "design by contract". However, there are some differences in the specific use.

First, the abstract class represents an inheritance relationship in the Java language, and a class can only use one inheritance relationship at a time. However, a class can implement multiple interface. Perhaps this is a compromise of the Java language designer in considering Java's support for multiple inheritance.

Second, in the definition of abstract class, we can give the method the default behavior. However, in the definition of interface, the method does not have the default behavior, and in order to circumvent this restriction, the delegate must be used, but this adds some complexity and can sometimes cause a lot of trouble.

There is another serious problem with the inability to define default behavior in an abstract class, which can cause maintenance headaches. Because if you later want to modify the interface of the class (typically represented by an abstract class or interface) to accommodate the new situation (for example, adding new methods or adding new parameters to a used method), it can be very cumbersome and may take a lot of time (for many of the derived classes , in particular). However, if the interface is implemented by an abstract class, it is possible to modify the default behavior defined in the abstract class only.

Similarly, if the default behavior cannot be defined in an abstract class, it causes the same method implementation to appear in each of the derived classes of the abstract class, violating the "one Rule,one place" principle, resulting in code duplication, which is also detrimental to future maintenance. Therefore, you should be very careful when choosing between the abstract class and the interface.

Abstract class and interface from the perspective of design concept

It mainly discusses the difference between abstract class and interface from the angle of grammar definition and programming, and the difference between these layers is comparatively low-level and non-essential. This section examines the difference between the two, from another level: the design concepts reflected by the abstract class and interface. The author thinks that the analysis from this level can understand the essence of the two concepts.

As mentioned earlier, ABSTARCT class embodies an inheritance relationship in the Java language, in order to make the inheritance relationship reasonable, there must be an "is a" relationship between the parent class and the derived class, that is, the parent class and the derived class should be the same in nature (the reference (3) is about "is a" In-depth discussion of the relationship, interested readers can refer to). For interface, it does not require that the interface and interface definitions be consistent in the concept, but only the contract that implements the interface definition. To make the discussion easy to understand, a simple example is described below.

Consider an example of the assumption that there is an abstraction about door in our problem area, that the door has the ability to execute two actions open and close, at which point we can define a type that represents the abstract concept by either an abstract class or a interface. The definitions are as follows:

Define door using the abstract class method:

Abstract class Door {abstract void open (); abstract void Close ();}

Define door using the interface method:

Interface Door {void open (); void close ();}

Other specific door types can be extends using the door defined by the abstract class or implements interface defined using door mode. It seems that there is no big difference between using abstract class and interface.

If you now require door also have the function of alarm. How do we design the class structure for this example (in this case, mainly to show the difference between the abstract class and the interface reflected in the design concept, the other aspects of the problem are simplified or ignored)? Here is a list of possible solutions and the analysis of these different scenarios from the design concept level.

Solution One:

Simply add a alarm method to the definition of door, as follows:

Abstract class Door {abstract void open (); abstract void close (); abstract void alarm ();}

Or

Interface Door {void open (); void close (); void alarm ();}

Then 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 approach violates a core principle in object-oriented design ISP (Interface segregation priciple), which mixes the behavior method inherent in door concept with another concept of "alarm" in the definition of door. One problem is that modules that rely solely on the concept of door are changed by the concept of "alarm" (e.g., modifying the parameters of the alarm method) and vice versa.

Solution Two:

Since open, close and alarm belong to two different concepts, they should be defined separately in an abstract class that represents both concepts, according to the ISP principle. These two concepts are defined using the abstract class approach, both of which are defined using the interface method, one defined using the abstract class approach, and the other defined using the interface method.

Obviously, because the Java language does not support multiple inheritance, it is not feasible for both concepts to be defined using the abstract class method. The latter two methods are feasible, but the choice of them reflects the understanding of the conceptual nature of the problem domain and the correctness and reasonableness of the design intent. We hit analysis, explain.

If both concepts are defined using the interface approach, then two questions are reflected: 1. We may not understand the problem areas clearly, alarmdoor in the concept is essentially door or alarm? 2. If our understanding of the problem area is not problematic, for example: we find that Alarmdoor is consistent in concept in nature and door by analysis of the problem domain, then we do not have the right to reveal our design intent when we implement it. Because the definitions of the two concepts, which are defined using the interface method, do not reflect the above meanings.

If our understanding of the problem area is: Alarmdoor is inherently door in concept, and it has a function of alerting. How do we design and implement to clearly reflect what we mean? As already mentioned, abstract class represents an inheritance relationship in the Java language, whereas an inheritance relationship is essentially an "is a" relationship. So for the door concept, we should use the ABSTARCT class approach to define it. In addition, Alarmdoor also has the alarm function, indicating that it can complete the alarm concept defined behavior, so the alarm concept can be defined by the interface way. As shown below:

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 kind of realization basically can clearly reflect our understanding of the problem area, and reveal our design intention correctly. In fact, abstract class represents the "is a" relationship, interface represents a "like a" relationship, you can choose as a basis, of course, this is based on the understanding of the problem areas, For example: If we think that alarmdoor in the concept is essentially an alarm, but also has the function of door, then the definition of the above method will be reversed.

Deep understanding of abstract class and interface