Clarification regarding Java Interface and Inheritance

[Geek_Guest]

Java Specification says that we cannot create an object for an Interface but in the init (ServletConfig config) Syntax we are passing an object of ServleConfig which is an interface. Similarly in the service method we pass ServletRequest, ServletResponse objects.

Please clarify Java says that Multiple inheritance is not possible but every class extends Object class and we can extend a class explictly. It means that a class is extending 2 classes please clarify

Question asked by visitor suman

[eswar_ambati]

Hi Suman,

We cant create an object to Interface but we can assign an object reference.

For example:
A -> Interface
B -> Class implements A

We can set a reference to Interface object likethis:
A newObject = new B();

In init method we are passing ServeltConfig object reference not creating new object. The object will be created by webserver which contains Implementation classes of ServeltConfig Interface. Same case for service() also.

Java doesn't support multiple inheritance for Classes. Every class extends java.lang.Object class if it doesn't extend any other class. Try the following example you can know the difference.

public class A { }

public class B extends A { }

compile those two classes.
run the following commands:
javap A
javap B

check the output and find the difference

Regards,
Eswar

[parvesh]

1. Although we can't create object of interfaces but can create reference of it which we are sending.


2. what u r considering is not multiple inheritence but multi level inheritence
in multiple inher.. two classes/ entites are simultaneously extending by third one.

[parvesh]

1. Although we can't create object of interfaces but can create reference of it which we are sending.


2. what u r considering is not multiple inheritence but multi level inheritence
in multiple inher.. two classes/ entites are simultaneously extending by third one.

[vinotha]

Hi,

for your first doubt, since u r passing the arguments (ServletRequest request),
the object is not created.it'll be created only when ServletRequest request=new ServletRequest();

for your second doubt,
if a class extends a class, then it won't extend the Object class directly,
it will extend through that super class only