抽象类是声明为抽象的类——它可以包含也可以不包含抽象方法。抽象类不能被实例化，但可以被子类化。

换句话说，用abstract关键字声明的类在java中称为抽象类。它可以有抽象方法(没有主体的方法)和非抽象方法(有主体的方法)。

重要提示: 抽象类不能用于实例化对象，它们可以用于创建对象引用，因为Java的运行时多态性方法是通过使用超类引用实现的。因此，必须能够创建对抽象类的引用，以便可以使用它指向子类对象。您将在下面的示例中看到该特性

abstract class Bike{  
  abstract void run();  
}  

class Honda4 extends Bike{  
    void run(){
        System.out.println("running safely..");
    }  

    public static void main(String args[]){  
       Bike obj = new Honda4();  
       obj.run();  
    }  
} 

简单地说，您可以将抽象类看作是具有更多功能的接口。

你不能实例化一个接口，这也适用于一个抽象类。

On your interface you can just define the method headers and ALL of the implementers are forced to implement all of them. On an abstract class you can also define your method headers but here - to the difference of the interface - you can also define the body (usually a default implementation) of the method. Moreover when other classes extend (note, not implement and therefore you can also have just one abstract class per child class) your abstract class, they are not forced to implement all of your methods of your abstract class, unless you specified an abstract method (in such case it works like for interfaces, you cannot define the method body).

public abstract class MyAbstractClass{
  public abstract void DoSomething();
}

否则，对于抽象类的普通方法，“继承者”可以像往常一样只使用默认行为或重写它。

例子:

public abstract class MyAbstractClass{

  public int CalculateCost(int amount){
     //do some default calculations
     //this can be overriden by subclasses if needed
  }

  //this MUST be implemented by subclasses
  public abstract void DoSomething();
}

Java类在以下条件下变成抽象类:

1. 至少有一个方法被标记为abstract:

public abstract void myMethod()

在这种情况下，编译器会强制您将整个类标记为抽象类。

2. 该类被标记为abstract:

abstract class MyClass

如前所述:如果你有一个抽象方法，编译器会强迫你将整个类标记为抽象。但是即使你没有任何抽象方法，你仍然可以把类标记为抽象的。

常见的使用:

抽象类的一个常见用途是提供类的大纲，类似于接口。但与接口不同的是，它已经可以提供功能，即类的某些部分已经实现，而另一些部分只是用方法声明来概述。(“摘要”)

抽象类不能被实例化，但是可以基于抽象类创建具体类，然后可以实例化抽象类。要做到这一点，你必须继承抽象类并重写抽象方法，即实现它们。

An abstract class is one that isn't fully implemented but provides something of a blueprint for subclasses. It may be partially implemented in that it contains fully-defined concrete methods, but it can also hold abstract methods. These are methods with a signature but no method body. Any subclass must define a body for each abstract method, otherwise it too must be declared abstract. Because abstract classes cannot be instantiated, they must be extended by at least one subclass in order to be utilized. Think of the abstract class as the generic class, and the subclasses are there to fill in the missing information.

解决方案——基类(抽象)

public abstract class Place {

String Name;
String Postcode;
String County;
String Area;

Place () {

        }

public static Place make(String Incoming) {
        if (Incoming.length() < 61) return (null);

        String Name = (Incoming.substring(4,26)).trim();
        String County = (Incoming.substring(27,48)).trim();
        String Postcode = (Incoming.substring(48,61)).trim();
        String Area = (Incoming.substring(61)).trim();

        Place created;
        if (Name.equalsIgnoreCase(Area)) {
                created = new Area(Area,County,Postcode);
        } else {
                created = new District(Name,County,Postcode,Area);
        }
        return (created);
        }

public String getName() {
        return (Name);
        }

public String getPostcode() {
        return (Postcode);
        }

public String getCounty() {
        return (County);
        }

public abstract String getArea();

}

使用abstract关键字声明的类称为抽象类。 抽象是一个隐藏数据实现细节，只向用户显示功能的过程。抽象让您关注对象做了什么，而不是它是如何做的。

抽象类的主要内容

An abstract class may or may not contain abstract methods.There can be non abstract methods. An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), like this: ex : abstract void moveTo(double deltaX, double deltaY); If a class has at least one abstract method then that class must be abstract Abstract classes may not be instantiated (You are not allowed to create object of Abstract class) To use an abstract class, you have to inherit it from another class. Provide implementations to all the abstract methods in it. If you inherit an abstract class, you have to provide implementations to all the abstract methods in it.

声明抽象类 在声明期间在类之前指定abstract关键字将使其抽象。看看下面的代码:

abstract class AbstractDemo{ }

声明抽象方法 在声明过程中在方法之前指定abstract关键字将使方法抽象。看看下面的代码，

abstract void moveTo();//no body

为什么我们需要抽象类

In an object-oriented drawing application, you can draw circles, rectangles, lines, Bezier curves, and many other graphic objects. These objects all have certain states (for ex -: position, orientation, line color, fill color) and behaviors (for ex -: moveTo, rotate, resize, draw) in common. Some of these states and behaviors are the same for all graphic objects (for ex : fill color, position, and moveTo). Others require different implementation(for ex: resize or draw). All graphic objects must be able to draw or resize themselves, they just differ in how they do it.

对于抽象超类来说，这是一个完美的情况。您可以利用这些相似性，并将所有图形对象声明为继承自相同的抽象父对象(例如:GraphicObject)，如下图所示。

首先，声明一个抽象类GraphicObject，以提供所有子类完全共享的成员变量和方法，例如当前位置和moveTo方法。GraphicObject还声明了抽象方法，如draw或resize，这些方法需要由所有子类实现，但必须以不同的方式实现。GraphicObject类看起来像这样:

abstract class GraphicObject {

  void moveTo(int x, int y) {
    // Inside this method we have to change the position of the graphic 
    // object according to x,y     
    // This is the same in every GraphicObject. Then we can implement here. 
  }

  abstract void draw(); // But every GraphicObject drawing case is 
                        // unique, not common. Then we have to create that 
                        // case inside each class. Then create these    
                        // methods as abstract 
  abstract void resize();
}

在子类中使用抽象方法 GraphicObject的每个非抽象子类，如Circle和Rectangle，必须为draw和resize方法提供实现。

class Circle extends GraphicObject {
  void draw() {
    //Add to some implementation here
  }
  void resize() {
    //Add to some implementation here   
  }
}
class Rectangle extends GraphicObject {
  void draw() {
    //Add to some implementation here
  }
  void resize() {
    //Add to some implementation here
  }
}

在main方法中，你可以像这样调用所有的方法:

public static void main(String args[]){
   GraphicObject c = new Circle();
   c.draw();
   c.resize();
   c.moveTo(4,5);   
}

在Java中实现抽象的方法

在java中有两种实现抽象的方法

抽象类(0到100%) 接口(100%)

具有构造函数、数据成员、方法等的抽象类

abstract class GraphicObject {

  GraphicObject (){
    System.out.println("GraphicObject  is created");
  }
  void moveTo(int y, int x) {
       System.out.println("Change position according to "+ x+ " and " + y);
  }
  abstract void draw();
}

class Circle extends GraphicObject {
  void draw() {
    System.out.println("Draw the Circle");
  }
}

class TestAbstract {  
 public static void main(String args[]){

   GraphicObject  grObj = new Circle ();
   grObj.draw();
   grObj.moveTo(4,6);
 }
}

输出:

GraphicObject  is created
Draw the Circle
Change position according to 6 and 4

记住两条规则:

如果类中抽象方法和具体方法都很少， 将其声明为抽象类。 如果类只有抽象方法，则将其声明为接口。

引用:

TutorialsPoint - Java抽象 Java抽象类方法 Java文档-抽象方法和类 JavaPoint - Java中的抽象类