Core Java 面试题与答案

问题 91. What modifiers are allowed for methods in an Interface?

Only public and abstract modifiers are allowed for methods in interfaces.

问题 92. What must a class do to implement an interface?

It must provide all of the methods in the interface and identify the interface in its implements clause.

问题 93. What are the differences between an interface and an abstract class?

• An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.
• Abstract class are used only when there is a "IS-A" type of relationship between the classes. Interfaces can be implemented by classes that are not related to one another and there is "HAS-A" relationship. 

• You cannot extend more than one abstract class. You can implement more than one interface. 

• Abstract class can implement some methods also. Interfaces can not implement methods. 

• With abstract classes, you are grabbing away each class’s individuality. With Interfaces, you are merely extending each class’s functionality.
  

• As per Java 8, interface can have method body as well.
  

问题 94. What is the difference between interface and abstract class?

Abstract class defined with methods. Interface will declare only the methods. Abstract classes are very much useful when there is a some functionality across various classes. Interfaces are well suited for the classes which varies in functionality but with the same method signatures.

问题 95. Access specifiers: "public", "protected", "private", nothing?

public : Public class is visible in other packages, field is visible everywhere (class must be public too)
private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.
protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.
default : What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.