java abstract classes and interfaces

1. Abstract class

We know that objects are generated through classes, but not all classes can describe specific objects.

If a class does not contain enough information to describe a specific object, it becomes an abstract class. An abstract class is an abstraction of a class of objects that have the same characteristics but different performance details. For example, birds all call, but different birds call in different ways. The concept of "call" can be abstracted.

  Define the following bird:

public abstract class Bird {
    private String color;
 
    public String getColor() {
        return color;
    }
    public void setColor(String color) {
        this.color = color;
    }
    public Bird(){
    }
    public abstract void sing();//鸣叫
}
//喜鹊
class  Magpie extends Bird{
public void sing() {
System.out.println("I can sing in a whisper. ");
}
}

Abstract class characteristics:

(1) The abstract class is modified with the abstract keyword

(2) The abstract method in the abstract class is modified with the abstract keyword, and there is no method body (specific implementation ).

(3) Abstract classes can contain non-abstract methods

(4) Abstract classes do not need to contain abstract methods (there is no point in designing them as abstract classes...), but classes that contain abstract methods must be abstract classes

(5) Abstract classes are essentially classes and can only be inherited

(6) Abstract classes cannot be instantiated or new. We said before that it does not describe specific objects, and it certainly cannot be instantiated. (7) Abstract classes can have instance variables and construction methods. The contract stipulates what you can do. The software design process relies on abstraction rather than concrete implementation.

Like the USB interface on our computers, whether you have a hard drive, a USB flash drive, or a mobile phone, as long as you implement the interface specified by USB, you can connect to the computer.

In the above abstract class example, I want to add a "fly" function. Considering that not all birds can fly, such as penguins, ostriches, ducks, etc., what should I do?

It is definitely inappropriate to modify abstract classes: 1. It violates the opening and closing principle, 2: It will cause all subclasses to inherit the "flying" function, and we will see ducks flying all over the sky ^_^ .

We can define the following interface:

public interface IFly {
void fly();
 }

// Magpie can implement the interface in IBird:

class  Magpie extends Bird implements IFly {
public void sing() {
System.out.println("I can sing in a whisper. ");
}
        public void fly(){
            System.out.println("我会飞了！");
        }
}

Magpie has implemented the fly interface in IFly and can finally fly.

So what if we need to add a swimming function to the birds? Think for yourself. . .

Characteristics of interface:

(1) Modifiers: public, abstract, default (don’t write)

(2) Keywords: interface

(3) The methods in the interface are abstract methods and cannot be implemented .

(4) The methods in the interface are public abstract by default, and the implementation class must use public modification.

(5) All methods in the interface must be implemented in the implementation class (except abstract classes).

(6) Variables in the interface are public static final by default.

(7) A class can implement multiple interfaces.

3. Application scenarios

1) Abstract classes reflect the "is a" relationship. If the implementations of certain classes have something in common, you can abstract an abstract class and let the abstract class implement common code. , and the personalized methods are implemented by each subclass.

2) The interface reflects the "like a" relationship and represents the abstract behavior of objects of different types. For example, airplanes and birds can fly, and the interface for flying can be detached, but they are not the same kind.

3) In software design, when you need to separate the interface from the implementation and encapsulate changes, interface-oriented programming is particularly important.

For example, in IOC thinking, the client does not care what class you are, the specific object is injected by the container.

Another example is the interaction between two systems. A good design is that both parties provide interfaces and do not care about the internal implementation. This reduces coupling while encapsulating changes.

Many design principles, design ideas and design patterns reflect the importance of interface-oriented programming: opening and closing principle, interface isolation, dependency inversion, adapter pattern, etc.