What is an abstract class in Java?

To understand interfaces we must first know abstract classes. So what is an abstract class?

Problem description: We have many objects in life that cannot be described specifically. For example: we can say that a quadrilateral has four sides. Or to be more specific, two sides of a rectangle are symmetrical and equal, and four sides of a square are symmetrical and equal. But for ordinary graphics, it is difficult to describe in detail. Converting to Java language means: for a very specific class, we can easily define its various properties and methods, but for some classes, it is difficult for us to understand how to implement its methods. . At this time we can use abstract classes.

The syntax format of an abstract class is as follows:

1 public abstract class 类名{2   abstract 返回值类型 方法名(参数列表);//定义抽象方法      3 }

The keyword to define an abstract method is: abstract.

A class defined using abstract is called an abstract class, and a method defined using abstract is called an abstract method.

Abstract methods have no method bodies and themselves have no meaning, so abstract classes cannot be instantiated.

When it is inherited by its subclass, the subclass must implement all the abstract methods in it.

As long as there is an abstract method in a class, this class must be defined as an abstract class.

After understanding the abstract class, we continue to think about the following questions. Let’s look at the following piece of code:

public abstract class shape 
{//定义一个图形抽象类public abstract void toString();//toString抽象方法public abstract void paint();//画图的抽象方法}public class triangle extends shape
{public void toString()
    {
        system.out.println("toString");
    }public void paint()
    {
        system.out.println("paint");
    }
}

It seems like there is no problem. What if we don’t want some classes to have a paint method? Put the paint method in another class? But other classes also need to use this method. Java's syntax stipulates that classes cannot inherit from multiple classes. What should I do? At this time we can consider using interfaces.

Interface is an extension of abstract class, which solves the shortcoming of multiple inheritance in Java. We can encapsulate the paint method into an interface, and then let the subclass of the graphics class that needs to be used implement this interface. In Java, a class can implement multiple interfaces. We can encapsulate different methods in interfaces and leave them to the class for use.

The keyword of interface is interface. Its definition is as follows:

public interface 接口名  extends 类名  implements 接口,....{  //定义常量
  public static final 数据类型 常量名=值;  //定义抽象方法
  public abstract 返回值类型 方法名 (参数列表);        
}

It should be noted that: 1. The interface cannot be instantiated 2. The access modifier of the interface can only be public 3. The interface will The public abstract static final keyword is provided by default

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