The difference between interface and abstract class

The difference between interface and abstract class

There is a big difference between interface and abstract class, and the choice of them even reflects Whether the understanding of the nature of the problem domain and the design intention is correct and reasonable.

1. Similarities

A. Both are abstract classes and cannot be instantiated.

B. Both interface implementation classes and subclasses of abstract class must implement the declared abstract methods.

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2. Differences

A. The interface needs to be implemented, and implements must be used. Abstract class needs to be inherited and uses extends.

B. A class can implement multiple interfaces, but a class can only inherit one abstract class.

C. Interface emphasizes the implementation of specific functions, while abstract class emphasizes ownership relationships.

D. Although both the interface implementation class and the subclasses of abstract class must implement the corresponding abstract methods, the implementation forms are different. Every method in the interface is an abstract method, which is only declared (declaration, without method body), and the implementation class must implement it. Subclasses of abstract class can be implemented selectively.

3. Application occasions of interface

A. Classes need specific interfaces for coordination, regardless of how they are implemented.

B. It exists as an identifier that can realize a specific function, or it can be a pure identifier without any interface methods.

C. A group of classes needs to be treated as a single class, and the caller only contacts this group of classes through the interface.

D. Specific multiple functions need to be implemented, and these functions may have no connection at all.

4. Application occasions of abstract class

In a word, when both a unified interface and instance variables or default methods are needed, then Can use it. The most common ones are:

A. Define a set of interfaces, but do not want to force each implementation class to implement all interfaces. You can use abstract class to define a set of method bodies, or even empty method bodies, and then let subclasses choose the methods they are interested in to cover.

B. In some cases, pure interfaces alone cannot satisfy the coordination between classes, and variables representing states in the class are also required to distinguish different relationships. The intermediary role of abstract can satisfy this very well.

C. Standardizes a set of mutually coordinated methods, some of which are common, independent of state, and can be shared without the need for subclasses to implement them separately; while other methods require each subclass to implement it according to its own A specific state to achieve a specific function.

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