Java Packages and interface

Java Packages and interface

1) What are packages ? What is use of packages ?

Ans :The package statement defines a name space in which classes are stored.If you omit the package, the classes are put into the default package.

Signature... package pkg;

Use: * It specifies to which package the classes defined in a file belongs to. * Package is both naming and a visibility control mechanism.

2) What is difference between importing "java.applet.Applet" and "java.applet.*;" ?

Ans :"java.applet.Applet" will import only the class Applet from the package java.applet

Where as "java.applet.*" will import all the classes from java.applet package.3) What do you understand by package access specifier?

Ans : public: Anything declared as public can be accessed from anywhere

private: Anything declared in the private can’t be seen outside of its class.

default: It is visible to subclasses as well as to other classes in the same package.4) What is interface? What is use of interface?

Ans : It is similar to class which may contain method’s signature only but not bodies.

Methods declared in interface are abstract methods. We can implement many interfaces on a class which support the multiple inheritance.

5) Is it is necessary to implement all methods in an interface?

Ans : Yes. All the methods have to be implemented.

6) Which is the default access modifier for an interface method?

Ans : public.

7) Can we define a variable in an interface ?and what type it should be ?

Ans : Yes we can define a variable in an interface. They are implicitly final and static.

8) What is difference between interface and an abstract class?

Ans : All the methods declared inside an Interface are abstract. Where as abstract class must have at least one abstract method and others may be concrete or abstract.

In Interface we need not use the keyword abstract for the methods.

9) By default, all program import the java.lang package.

True/False

Ans : True

10) Java compiler stores the .class files in the path specified in CLASSPATH

environmental variable.

True/False

Ans : False

11) User-defined package can also be imported just like the standard packages.

True/False

Ans : True

12) When a program does not want to handle exception, the ______class is used.

Ans : Throws

13) The main subclass of the Exception class is _______ class.

Ans : RuntimeException

14) Only subclasses of ______class may be caught or thrown.

Ans : Throwable

15) Any user-defined exception class is a subclass of the _____ class.

Ans : Exception

16) The catch clause of the user-defined exception class should ______ its

Base class catch clause.Ans : Exception

17) A _______ is used to separate the hierarchy of the class while declaring an

Import statement.

Ans : Package

18) All standard classes of Java are included within a package called _____.

Ans : java.lang

19) All the classes in a package can be simultaneously imported using ____.

Ans : *

20) Can you define a variable inside an Interface. If no, why? If yes, how?

Ans.: YES. final and static

21) How many concrete classes can you have inside an interface?

Ans.: None

22) Can you extend an interface?

Ans.: Yes

23) Is it necessary to implement all the methods of an interface while implementing the interface?

Ans.: No

24) If you do not implement all the methods of an interface while implementing , what specifier should you use for the class ?

Ans.: abstract

25) How do you achieve multiple inheritance in Java?

Ans: Using interfaces.

26) How to declare an interface example?

Ans : access class classname implements interface.

27) Can you achieve multiple interface through interface?

a)True

b) false

Ans : a.

28) Can variables be declared in an interface ? If so, what are the modifiers?

Ans : Yes. final and static are the modifiers can be declared in an interface.

29) What are the possible access modifiers when implementing interface methods?

Ans : public.

30) Can anonymous classes be implemented an interface?

Ans : Yes.

31) Interfaces can’t be extended.

a)True

b)False

Ans : b.

32) Name interfaces without a method?

Ans : Serializable, Cloneble & Remote.

33) Is it possible to use few methods of an interface in a class ? If so, how?

Ans : Yes. Declare the class as abstract.