Explain the differences between abstract classes and interfaces in Java

InJavaAbstract class andInterfaceYesAbstract classTwo mechanisms defined. The existence of these two mechanisms gives 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, many developers may choose abstract class and interface when defining abstract classes. 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.

　　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 the most core OCP (open-closed principle) Principle of object-oriented design.

　　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-abstract 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 (Because Java does not support multiple inheritance-reinjection ). 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. This is 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, when 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 for a lot of 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 same method will appear in every derived class of the abstract class, in violation of the "one rule, one place" principle, code duplication is also not conducive to future maintenance. Therefore, be careful when selecting abstract class and interface.

　　Abstract class and interface from the design concept level

The preceding section mainly discusses the differences between abstract class and interface from the perspective of syntax definition and programming. The differences between these layers are relatively low-level and non-essential. This section analyzes the differences between abstract class and interface on the other layer. The author believes that only by analyzing at this level can we understand the essence of the two concepts.

As mentioned above, abstract 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 have been simplified or ignored )? Below we will list possible solutions and analyze these different solutions from 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 principle ), in the definition of the door, the inherent behavior method of the door concept is mixed with the behavior method of another concept "alarm. One problem is that the modules that rely solely on the concept of door will change because of the change 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, there are two problems: 1. We may not understand the problem field clearly. Is the concept of alarmdoor actually a door or an alarm? 2. If we have no problem in understanding the problem field, for example, through analysis of the problem field, we find that alarmdoor is essentially consistent with door in concept, 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, 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" link. 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, so 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 indicates the "is-a" relation, and interface indicates the "like-a" relation. You can use this 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.

　　Summary

1. abstract class represents an inheritance relationship in Java. A class can only use an inheritance relationship once. However, a class can implement multiple interfaces.

2. abstract class can have its own data members or non-Abstarct member methods. In interface, only static data members that cannot be modified (that is, they must be static final, but data members are not defined in interfaces) are allowed. All member methods are abstract.

3. abstract class and interface have different design concepts. Abstract class represents the "is-a" relation, and interface represents the "like-a" relation.

4. classes that implement abstract classes and interfaces must implement all the methods in them. Abstract classes can have non-abstract methods. There is no implementation method in the interface.

5. the variables defined in the interface are of the public static final type by default and must be given the initial values. Therefore, the Implementation class cannot be redefined or changed.

6. the variables in the abstract class are friendly by default. Their values can be redefined in the subclass or assigned again.

7. The methods in the interface are public and abstract by default.

　　Conclusion

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.