Java interfaces and the understanding of abstract classes

Java interface and abstract class usage summary, usage scenariosCategory: Java 2014-07-09 15:22 279 people read Comments (0) favorite reports

Original reproduced from: http://yinny.iteye.com/blog/1152430

Interface

1 because Java does not support multiple inheritance, there is an interface, a class can inherit only one parent class, but can implement multiple interfaces, the interface itself can inherit multiple interfaces.

The member variables in the 2 interface are public static final type by default. The initialization that must be displayed.

The method in the 3 interface is the public abstract type by default. An implicit declaration.

The 4 interface has no constructor method and cannot be instantiated.

The 5 interface cannot implement another interface, but it can inherit multiple interfaces.

Class 6 If you implement an interface, you must implement all the abstract methods inside the interface, otherwise the class will be defined as an abstract class.

Abstract class

1 If you declare a class as abstract, this class cannot generate objects and can only be used by inheritance.

2 The abstract method must exist in an abstract class.

3 There can be general variables and general methods in the abstract class.

4 Subclass inheriting an abstract class must implement an abstract method, unless the subclass is an abstract class.
private void Print () {}; This statement represents a null implementation of the method.
abstract void print (); This statement represents the abstraction of the method, without implementation.

The difference between an interface and an abstract class

The 1 interface can contain only abstract methods, and abstract classes can contain common methods.
The 2 interface can only define static constant properties, and abstract classes can define either normal properties or static constant properties.
The 3 interface does not contain a constructor method, and an abstract class can contain a constructor method.

An abstract class cannot be instantiated, but it does not mean that it can have constructors, that abstract classes can have constructors, and that they inherit class extensions

The 1 interface is the core, which defines what to do, contains many methods, but does not define how these methods should be done.
2 If many classes implement an interface, each will implement those methods in code
3 If the implementation of some classes has something in common, then an abstract class can be abstracted, allowing the abstract class to implement the common code of the interface, while the individual methods are implemented by the subclasses.

So, abstract class is to simplify the implementation of the interface, he not only provides the implementation of public methods, so that you can quickly develop, but also allow your class to fully implement all the methods, there is no tight coupling problem.


The application is simple.
1 Priority Definition Interface
2 If there is more than one interface to the existing common part, use the abstract class and then integrate it.

the difference between an interface and an abstract class--I'm sure you'll never confuse it.

I think, for you to use the object-oriented programming language programmer, "interface" this noun must not be unfamiliar, but I do not know if you have such doubts: what is the purpose of the interface? What is the difference between it and an abstract class? Can you use abstract classes instead of interfaces? And, as programmers, you must often hear the phrase "interface-oriented programming," What does it mean? What is the connotation of the mind? What is the relationship with object-oriented programming? This article will answer these questions.

1. What is the relationship between interface-oriented programming and object-oriented programming?

First, interface-oriented programming and object-oriented programming are not lateral, it is not an independent programming idea that is more advanced than object-oriented programming, but is attached to the object-oriented ideology, which belongs to its part. Or, it is one of the essence of thought in object-oriented programming system.

2. The nature of the interface

An interface, on the surface, consists of a collection of several method definitions without the principal code, with a unique name that can be implemented by a class or other interface (or, optionally, inherited). It may look like this in form:

Interface InterfaceName
{
void Method1 ();
void Method2 (int para1);
void Method3 (String para2,string para3);
}


So what is the nature of the interface? Or what the meaning of the interface exists. I think it can be considered from the following two points of view:

1) An interface is a set of rules that specify a set of rules that a class or interface that implements this interface must have. Embodies the nature "if you are ... You must be able to ... "the idea.

For example, in nature, people can eat, that is, "If you are human, you must be able to eat". So the simulation to the computer program, there should be a IPerson (the custom interface name from the "I") interface, and there is a method called Eat (), and then we stipulate that each class that represents "human", must implement the IPerson interface, which simulates the nature "If you are human, you must be able to eat" This rule.

From here, I think you can see a little bit of object-oriented thinking. One of the core of object-oriented thinking is to simulate the real world and abstract the real world into classes, and the whole program relies on each kind of instance to communicate with each other and to cooperate with each other to complete the system function, which is very accord with the real world running condition and the essence of object oriented thought.

2) The interface is an abstract representation of the same thing on a certain granularity view. Note here I emphasize that in a certain granularity view, because the concept of "homogeneous thing" is relative, it differs because of the granularity view.

For example, in my eyes, I am a person, and a pig there is an essential difference, I can accept my classmates and I are similar this statement, but can not accept me and a pig is the same. However, if in the eyes of a zoologist, I and pig should be the same, because we are animals, he can think that "man" and "pig" have achieved ianimal this interface, and he in the study of animal behavior, I and pig will not be treated separately, but from the "animal" This larger grain size study, But he would think I was fundamentally different from a tree.

Now for a geneticist, the situation is different, because the organism can inherit, so in his eyes, I not only with the pig no difference, and a mosquito, a bacterium, a tree, a mushroom and even a SARS virus is no different, Because he would think that we all achieved idescendable this interface (note: Descend VI. Heredity), that is, we are all heritable things, he will not study us separately, but will all creatures as the same kind of research, in his eyes there is no human and virus, only the genetic material and non-hereditary material. But at least, I have a difference with a stone.

But the unfortunate thing happened, some day, there appeared a great man, his name is Lenin, he was familiar with Max, Engels of dialectical materialism thought after the masterpiece, quite experience, so he made a famous definition: the so-called material, is to be reflected in the consciousness of the objective reality. At this point, I and a stone, a trace of air, an idiom and transmission of mobile phone signal electromagnetic field has no difference, because in Lenin's eyes, we are can be reflected in the consciousness of the objective reality. If Lenin was a programmer, he would say: the so-called matter is all the instances that are generated by all classes that simultaneously implement the "Ireflectabe" and "Iesse" two interfaces. (Note: Reflect v. reflect esse N. Objective reality)

You might think that my example above is a nonsense, but that's what the interface is all about. Object-oriented thinking and one of the core is called polymorphism, what is polymorphism? To put it bluntly is to treat the same things indiscriminately at a certain granularity view level. And the reason to do this is because there is an interface exists. Like the geneticist, he knew that all living creatures had idescendable interfaces, and that, as long as they were creatures, there must be descend (), so that he could study together, rather than study each creature and end up exhausted.

Maybe I can't give you a visual impression of the nature and function of the interface. Then in the following example and in the analysis of several design patterns, you will be more intuitive to experience the connotation of the interface.

3. Overview of interface-oriented programming

Through the above, I think we have an understanding of the interface and the concept of interface, then what is interface-oriented programming? My personal definition is: in the system analysis and architecture, distinguish between hierarchy and dependency, each level is not directly to its upper layer to provide services (that is, not directly instantiated in the upper layer), but by defining a set of interfaces, only to the upper layer exposes its interface function, the upper layer for the lower layer is only the interface dependency, not the specific class.

The benefits of doing so are obvious, and first of all, a great benefit to system flexibility. When the lower layer needs to change, as long as the interface and interface functions are not changed, the upper layer does not have to make any changes. You can even replace the entire lower layer without altering the upper code, as if we were replacing a WD 60G hard drive with a Seagate 160G hard drive, instead of having to make any changes in the other parts of the computer, we could unplug the original hard drive and plug it into the new hard drive, because the rest of the computer is not dependent on the hard drive. Instead of relying on an IDE interface, you can replace the hard drive by implementing the interface. From this point of view, the interface in the program is very similar to the interface in reality, so I always think that the word interface (interface) is really in the likeness!

Another advantage of using the interface is that different parts or levels of developers can work in parallel, like making a hard disk without the CPU, and do not wait for the display, as long as the interface is consistent, reasonable design, can be developed in parallel, thereby improving efficiency.

This article first came here. Finally, I would like to say one more word: the essence of object-oriented is the simulation of reality, this is also the soul of this article. Therefore, more from the reality of thinking about object-oriented things, to improve the system analysis and design ability has great spleen benefits.

In the next article, I'll use an example to illustrate the basic methods of interface programming.

And the third, I will analyze some of the classic design patterns of interface-oriented programming ideas, and analysis. The interface-oriented idea in the net layered architecture.

Supplement to this article:

Carefully read everyone's reply, very happy to discuss technical issues with you. Thanks to the affirmation of friends, but also to express their opinions and questioned friends, which prompted me to think more deeply about something, hoping to take this progress. Here I would like to add something to discuss some of the more concentrated questions in the replies.

1. About "interface" in "interface-oriented programming" and "interfaces" in specific object-oriented languages two words

See a friend that "interface-oriented programming" in the word "interfaces" should be more than the interface in the simple programming language range. I think it makes sense. I did not write that very well here. I think "interface" in an object-oriented language refers to a specific code structure, such as an interface defined in C # with the interface keyword. The "interface" in "interface-oriented programming" can be described as a kind of structural part that hides concrete underlying classes and implements polymorphism from the perspective of software architecture and from a more abstract level. In this sense, if an abstract class is defined and the purpose is to achieve polymorphism, then I think it is reasonable to refer to this abstract class as an "interface". But is it unreasonable to use abstract class to realize polymorphism? Discussed in the second article below.

To summarize, I think that the concept of two "interfaces" is both different and interrelated. The interface in "interface-oriented programming" is an architectural component of the thought-level for polymorphism, software flexibility, and maintainability, and the "interface" in a specific language is the means by which the parts of the idea are implemented into code.

2. About abstract classes and interfaces

See in the reply this is a more intense discussion of a problem. I'm sorry I didn't consider this issue in the article. My personal understanding of this problem is as follows:

If single from the specific Code view, the two concepts are easy to blur, and even feel that the interface is redundant, because single from the specific function, in addition to multiple inheritance (C#,java), abstract class seems to completely replace the interface. However, does the presence of an interface exist to achieve multiple inheritance? Of course not. I think the difference between abstract classes and interfaces is the use of motivation. Abstract classes are used for reuse of code, and the motivation for using interfaces is to achieve polymorphism. So, if you're hesitant about using interfaces or abstract classes somewhere, think about what your motives are.

See a friend to IPerson this interface query, my personal understanding is, IPerson this interface should not be defined, the key to see the specific application is how a situation. If our project has women and man, all inherit person, and women and man most methods are the same, only one method DOSOMETHINGINWC () different (the example is vulgar, you forgive me), Then of course it is reasonable to define a Abstractperson abstract class, because it can include all other methods, the subclass only defines DOSOMETHINGINWC (), and greatly reduces the amount of duplicated code.

However, if the women and man two classes in our program basically do not have common code, and there is a personhandle class that needs to instantiate them and do not want to know that they are male or female, and simply treat them as human beings and realize polymorphism, then it is necessary to define interfaces.

All in all, the difference between an interface and an abstract class lies primarily in the motivation used, not in itself. And whether a thing should be defined as an abstract class or an interface depends on the context of the specific environment.

Furthermore, I think another difference between an interface and an abstract class is that the abstract class and its subclasses should be a general and special relationship, and the interface is just a set of rules that its subclasses should implement. (Of course, sometimes there may be general and special relationship, but we use the purpose of the interface is not here) For example, transportation is defined as abstract class, cars, airplanes, ships defined as sub-categories, is acceptable, because cars, airplanes, ships are a special means of transport. Another example is the IComparable interface, which simply says that the class that implements this interface must be able to be compared, which is a rule. If the car class implements the IComparable, just saying that there is a way in our car to compare two car instances, which may be more expensive than which car, or bigger than which, it does not matter, but we can not say that "the car is a special can be compared", which is not in the grammar.

Java interfaces and the understanding of abstract classes