在我的一次采访中，我被要求解释接口类和抽象类之间的区别。
以下是我的回答:
Methods of a Java interface are implicitly abstract
and cannot have implementations. A Java abstract class can have
instance methods that implements a default behaviour.
Variables declared in a Java interface are by default final. An
abstract class may contain non-final variables.
Members of a Java interface are public by default. A Java abstract
class can have the usual flavours of class members like private,
protected, etc.
A Java interface should be implemented using keyword “implements”; A
Java abstract class should be extended using keyword “extends”.
An interface can extend another Java interface only, an abstract class
can extend another Java class and implement multiple Java interfaces.
A Java class can implement multiple interfaces but it can extend only
one abstract class.
然而，面试官并不满意，他告诉我这种描述代表了“书本知识”。
他让我给出一个更实际的回答，用实际的例子解释我什么时候会选择抽象类而不是接口。
我哪里错了?
这个世界上没有什么是完美的。他们可能期待的是更实际的方法。
但在你解释之后，你可以用稍微不同的方法添加这些行。
Interfaces are rules (rules because you must give an implementation to them that you can't ignore or avoid, so that they are imposed like rules) which works as a common understanding document among various teams in software development.
Interfaces give the idea what is to be done but not how it will be done. So implementation completely depends on developer by following the given rules (means given signature of methods).
Abstract classes may contain abstract declarations, concrete implementations, or both.
Abstract declarations are like rules to be followed and concrete implementations are like guidelines (you can use it as it is or you can ignore it by overriding and giving your own implementation to it).
Moreover which methods with same signature may change the behaviour in different context are provided as interface declarations as rules to implement accordingly in different contexts.
编辑:Java 8简化了在接口中定义默认方法和静态方法。
public interface SomeInterfaceOne {
void usualAbstractMethod(String inputString);
default void defaultMethod(String inputString){
System.out.println("Inside SomeInterfaceOne defaultMethod::"+inputString);
}
}
现在，当一个类将实现SomeInterface时，并不强制为interface的默认方法提供实现。
如果我们有另一个具有以下方法的接口:
public interface SomeInterfaceTwo {
void usualAbstractMethod(String inputString);
default void defaultMethod(String inputString){
System.out.println("Inside SomeInterfaceTwo defaultMethod::"+inputString);
}
}
Java不允许扩展多个类，因为这会导致“钻石问题”，即编译器无法决定使用哪个超类方法。如果使用默认方法，接口也会出现菱形问题。因为如果一个类同时实现这两个
SomeInterfaceOne and SomeInterfaceTwo
并且没有实现常见的默认方法，编译器不能决定选择哪一个。
为了避免这个问题，在java 8中必须实现不同接口的通用缺省方法。如果任何类实现了上述两个接口，它必须提供defaultMethod()方法的实现，否则编译器将抛出编译时错误。
是的，从技术上讲，你的回答是正确的，但你的错误之处在于，你没有向他们表明你理解选择其中一个的利弊。此外，他们可能担心将来升级时代码库的兼容性问题。这种类型的回答可能有帮助(除了你说的):
"Choosing an Abstract Class over an Interface Class depends on what we
project the future of the code will be.
Abstract classes allow better forward-compatibility because you can
continue adding behavior to an Abstract Class well into the future
without breaking your existing code --> this is not possible with an
Interface Class.
On the other hand, Interface Classes are more flexible than Abstract
Classes. This is because they can implement multiple interfaces. The
thing is Java does not have multiple inheritances so using abstract
classes won't let you use any other class hierarchy structure...
So, in the end a good general rule of thumb is: Prefer using Interface
Classes when there are no existing/default implementations in your
codebase. And, use Abstract Classes to preserve compatibility if you
know you will be updating your class in the future."
祝你下次面试好运!
