Java抽象类和方法

用了Java很多年，很少涉及到抽象类的使用。现在快毕业了，找工作的时候，首当其冲的面试题就是“Java 抽象类和接口的区别”。

好吧，为了以后自己方便，也为了看到这篇文章的读者方便，引用一下官网的解释。

An abstract class is a class that is declared abstract —it may or may not include abstract methods.

抽象类可以含有非抽象方法，也可以不含抽象方法。

Abstract classes cannot be instantiated, but they can be subclassed.

抽象类可以被继承，但不能new。

An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), like this:

抽象方法没有方法体，以分号结尾，如下：

abstract void moveTo(double deltaX, double deltaY);

If a class includes abstract methods, the class itself must be declared abstract , as in:

public abstract class GraphicObject {
   // declare fields
   // declare non-abstract methods
   abstract void draw();
}

When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, the subclass must also be declared abstract .含有抽象方法的类必须是抽象类。他的子类必须实现他的所有抽象方法，否则子类必须声明为抽象的。

以上是抽象类和抽象方法的基本概念。接下来看一下他和接口的关系：说不定会让你吃一惊。

 All of the methods in an interface (see the Interfaces section) are implicitly abstract, so the abstract modifier is not used with interface methods (it could be—it's just not necessary).接口中的方法都是抽象方法，虽然没有声明。当日声明了也是可以的。

Unlike interfaces, abstract classes can contain fields that are not static and final , and they can contain implemented methods. Such abstract classes are similar to interfaces, except that they provide a partial implementation, leaving it to subclasses to complete the implementation. If an abstract class contains only abstract method declarations, it should be declared as an interface instead.

如果一个抽象类只含有抽象方法，那就把它声明为接口吧。这就是共同点。否则的话，抽象类中的非抽象方法可以有部分功能代码，子类可以完成这些方法。抽象类中的变量也无需是静态和常量。而接口中的变量都是final的。

Multiple interfaces can be implemented by classes anywhere in the class hierarchy, whether or not they are related to one another in any way. Think of Comparable or Cloneable , for example.

By comparison, abstract classes are most commonly subclassed to share pieces of implementation. A single abstract class is subclassed by similar classes that have a lot in common (the implemented parts of the abstract class), but also have some differences (the abstract methods).

抽象类虽然像接口，但他毕竟是一个类，就有类的特征。而接口是给实现他的泪增加属性的。可能是借口和具体类直接的分立太明显了，所以SUN提出了抽象类吧。

曾经听一哥们说：

抽象类被继承后，你可以往抽象类中添加非抽象方法，这样他的子类都具有了这些方法。而接口被实现后，往里面增加方法后，子类必须重写，否则编译出错了。这就是他们看起来的大区别吧。