You can inherit from a single parent downwards as far as you want...
A extends Object; B extends A; C extends B; etc
Thus, by implication, C extends B, A, and Object.
What you can't do is this:
E extends F,G
ie. extend from two parents at the same level. This is what Java refers to a multiple inheritance.

The language was setup so that you can't have multiple inheritance. If you want multiple inheritance, you have to use interfaces. By using keyword "implements" in the class heading.

extends - for classes
implements - for interfaces

Yeah interfaces are somewhat similar - if your class "implements" the interface, it forces you to include those methods that are defined in the interface itself.

Thanks for the replies.I want to know how these implementation happens.ie extending object class invisibly and extending to Other class explicitly.

In effect, if your class does not explicity specify an "extends" superclass, then the superclass is taken to be Object. It's that simple.

ok.So it is multiple inheritence right.extending to two classes.How jdk peoples implemented that ?

It's single inheritance. Each class inherits from 1 class only, but that class, in turn,inherits from one class, etc, all the way up to Object.
When your code refers to a method or variable in a class Java looks for that method/variable in the class definition. If it finds it, OK. If not, it looks in the definition of the superclass. If it finds it there, OK. If not, it looks in that classes' superclass etc, all the way to Object. I it doesn't find it in Object you get a compile error.

Anyway its extending the object class. Or How these Object class is made super class to our class. How our class know that Object class is super class with out explicitly extending?

That's part pf the language definition. If you don't specify an "extends" then your class extends Object. It's built into the compiler.