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Core Java - Interview Questions and Answers for 'Inner classes' - 24 question(s) found - Order By Newest

 Q1. Why do we need Inner classes ? Cant we just work with outer classes wherever we implement Inner classes ?Core Java
Admin
info@buggybread.com
Ans. Yes, we can substitute outer classes wherever we need to have inner classes but Inner classes have advantage in certain cases and hence preferred -

Ease - Why to implement a class outside if its objects are only intended to be part of an outer object. Its easy to define the class within another class if the use is only local.

Protection - Making a call an outer exposes a threat of it being used by any of the class. Why should it be made an outer class if its object should only occur as part of other objects.

For example - You may like to have an class address whose object should have a reference to city and by design thats the only use of city you have in your application. Making Address and City as outer class exposes City to any of the Class. Making it an inner class of Address will make sure that its accessed using object of Address.

  Sample Code for inner class

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 Q2. What is the difference between the following two code lines ?

1. new OuterClass().new InnerClass();

2. new OuterClass.InnerClass();
Core Java2016-06-30 14:45:37

Ans. In first case we are trying to initialize Inner class object using the instance of Outer Class whereas in second case we are trying to initialize the Inner class object directly using the Outer class name.

In second case , Inner class is "static inner class" as we cannot access "non static inner class" using Classname alone.

In first case, the inner class could be either "static inner class" or "non static inner class".

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  Q3. What are the different types of inner classes ?Core Java
Anonymous
Ans. Simple Inner Class,
Local Inner Class,
Anonymous Inner Class,
Static Nested Inner Class.

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 Q4. What is the difference between inner class and sub class ?Core Java2017-06-18 16:30:30

Ans. Inner Class is a class that is nested within another class whereas sub class is a class that extends or inherit another class.

Inner class can only be accessed using reference of outer class ( Class name , if inner class is static or object reference, if inner class is non static ) whereas Sub class is accessed directly.

In terms of memory, inner class is stored just like another class having it's own body whereas sub class carries the body of parent class as well as it's own fields in memory.

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 Q5. Which access specifier can be used with Class ?Core Java
Anonymous
Ans. For top level class we can only use "public" and "default". We can use private with inner class.

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 Q6. Difference between nested and inner classes ?
Anonymous
Ans. Inner class is a type of nested class.

Inner classes are non static nested classes i.e the one that are associated with the instance of outer class.

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 Q7. Explain use of nested or inner classes ?Core Java
Anonymous
Ans. Sometime we just need classes or class objects just to be used as part of a particular class or objects. Making them non nested won't make any difference as far as functionality is concerner but making them Nested provide a level of convenience and protection fro, being used anywhere else. Moreover it helps reducing the Code.

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 Q8. What is the benefit of inner / nested classes ?Core Java
Admin
info@buggybread.com
Ans. You can put related classes together as a single logical group.

Nested classes can access all class members of the enclosing class, which might be useful in certain cases.

Nested classes are sometimes useful for specific purposes. For example, anonymous inner classes are useful for writing simpler event-handling code with AWT/Swing.

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 Q9. Explain Static nested Classes ?
Admin
info@buggybread.com
Ans. The accessibility (public, protected, etc.) of the static nested class is defined by the outer class.

A static nested class is not an inner class, it's a top-level nested class.

The name of the static nested class is expressed with OuterClassName.NestedClassName syntax.

When you define an inner nested class (or interface) inside an interface, the nested class is declared implicitly public and static.

Static nested classes can be declared abstract or final.

Static nested classes can extend another class or it can be used as a base class.

Static nested classes can have static members.

Static nested classes can access the members of the outer class (only static members, obviously).

The outer class can also access the members (even private members) of the nested class through an object of nested class. If you don't declare an instance of the nested class, the outer class cannot access nested class elements directly.

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 Q10. Explain Inner Classes ?Core Java
Admin
info@buggybread.com
Ans. The accessibility (public, protected, etc.) of the inner class is defined by the outer class.

Just like top-level classes, an inner class can extend a class or can implement interfaces.

Similarly, an inner class can be extended by other classes, and an inner interface can be implemented or extended by other classes or interfaces.

An inner class can be declared final or abstract.Inner classes can have inner classes, but you will have a hard time reading or understanding such complex nesting of classes.

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 Q11. Explain Method Local Inner Classes ?Core Java
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Ans. You can create a non-static local class inside a body of code. Interfaces cannot have local classes, and you cannot create local interfaces.

Local classes are accessible only from the body of the code in which the class is defined. The local classes are completely inaccessible outside the body of the code in which the class is defined.

You can extend a class or implement interfaces while defining a local class.

A local class can access all the variables available in the body of the code in which it is defined. You can pass only final variables to a local inner class.

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 Q12. Explain about anonymous inner classes ?Core Java
Admin
info@buggybread.com
Ans. Anonymous classes are defined in the new expression itself, so you cannot create multiple objects of an anonymous class.

You cannot explicitly extend a class or explicitly implement interfaces when defining an anonymous class.

An anonymous inner class is always created as part of a statement; don't forget to close the statement after the class definition with a curly brace. This is a rare case in Java, a curly brace followed by a semicolon.

Anonymous inner classes have no name, and their type must be either a subclass of the named type or an implementer of the named interface

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 Q13. Which of the following is not type of inner classes ?

a. Simple Inner Class
b. Static Nested Inner Class
c. Method Nested Static Inner Class
d. Anonymous Inner Class
Anonymous
Ans. Method Nested Static Inner Class

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 Q14. Can we access private members of the parent class ? i.e Are private members of the class visible in the derived class ?Core Java2016-08-04 09:24:34

Ans. No, If both Parent and Derived are outer classes.

public class Vehicle {
private static String manufacturingDate = "2016";
}

public class Car extends Vehicle{
public static void main(String args[]){
System.out.println(manufacturingDate); // error - The field Vehicle.manufacturingDate is not visible
}
}

Yes, If derived is the inner class of Parent.

public class Vehicle {
private static String manufacturingDate = "2016";
static public class Car extends Vehicle{
public static void main(String args[]){
System.out.println(manufacturingDate); // no problem
}
}
}

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 Q15. Can we have an inner class within Interface ?Core Java2016-10-24 11:18:24

Ans. Yes, we can define an inner class within interface.Inside the interface, the inner class is implicitly public static.

So the following is legit in Java

public interface BuggyBreadInterface {
   void doSomething();

   public class BuggyBreadClass implements BuggyBreadInterface{
      void doSomething(){
         System.out.println("Do something");
      }
   }
}

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 Q16. how to access the methods of an inner class?Core Java2017-01-14 11:11:38

Ans. It depends on the type of inner class

To access non static inner class method

new OuterClass().new InnerClass().getMethod();

To access static method of static inner class

new OuterClass.InnerClass().getMethod();

  Sample Code for inner class

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 Q17. Upon compiling the following class, What .class files will get created

public class OuterClass{
class InnerClass{
}
}
Core Java2017-01-30 10:34:16

Ans. OuterClass.class AND OuterClass$InnerClass.class

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 Q18. Can we declare a class private ?Core Java2017-06-07 16:40:34

Ans. We can declare an inner class as private and hence would be restricting it's access from outside. We cannot make outer class as private.

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 Q19. How can we hide a class in Java ?Core Java2017-06-07 16:42:57

Ans. By encapsulating it within another class and declaring it private. In such a case, it will only be accessible through parent class or parent class object.

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 Q20. Can an inner class be subclass of it's parent class ?Core Java2017-06-18 16:36:17

Ans. Yes that can be done

public class OuterClass {
public class InnerClass extends OuterClass {
}
}

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 Q21. Is this code legal in java i.e nested class within interface implementing the parent interface ?

public interface MyInterface {
public class MyClass implements MyInterface {
}
}
Core Java2017-06-18 16:37:53

Ans. Yes that's legal in java

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 Q22. Can an interface be defined inside a class just like we can have inner classes inside a class ?Core Java2017-06-23 08:13:43

Ans. Yes , we can have an interface that is part of a class.

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 Q23. Why can we have a static inner class but not a static Top level class in Java ?Core Java2017-08-23 20:14:59

 This question is still unanswered. Can you please provide an answer.


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 Q24. Which of following can be nested into another ?Core Java
a. class within another class
b. class within interface
c. interface within class
d. All of above

Ans.d. All of above



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