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Q161. What are Util Classes ? What are its advantages ?

Ans. Classes having only static methods in java is called util classes. As they don't have specific object state, they are basically intend to have shared code and state through static elements and methods. They save resources by sharing code.

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Q162. What was the need of creating an abstract class in Java when we cannot instantiate an object of it ?

Core Java

Ans. The objective of an abstract class is to provide an interface as well as code reuse. Though we cannot instantiate an instance of abstract class but we can reuse the code by extending such a class. Moreover abstract class provides interfacing to outside components just like interfaces.

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Q163. Why can't we create an instance of abstract class ? Why is it restricted by compiler ?

Core Java

Ans. Because Abstract class is incomplete i.e there are abstract methods that needs to be defined by extending classes.

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Q164. What is the importance of Abstract Class ?

Core Java

Ans. Abstract classes provide a mechanism of interfacing ( using abstract method ) as well as code reuse through inheritance ( extending abstract class )

Comparing to concrete class they have an advantage of providing interface which a concrete class doesn't provide.

Comparing to interfaces they have an advantage of providing code reuse through inheritance which interfaces dont provide.

Sample Code for abstract class

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Q165. Make the following class immutable

public class Employee {

   private long employeeId;

   private String employeeName;

   public long getEmployeeId() {
      return employeeId;
   }

public void setEmployeeId(long employeeId) {
      this.employeeId = employeeId;
   }

   public String getEmployeeName() {
      return employeeName;
   }

public void setEmployeeName(String employeeName) {
      this.employeeName = employeeName;
   }
}

Core Java

Ans. public class Employee {

   private long employeeId;

   private String employeeName;

   public Employee(long employeeId,String employeeName){
      this.employeeId = employeeId;
      this.employeeName = employeeName;
   }

   public long getEmployeeId() {
      return employeeId;
   }

   public long getEmployeeName() {
      return employeeName;
   }
}

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Q166. What are var args ?

or

What is the use of var args ?

Core Java

Ans. Var args is the feature of Java that allows a method to have variable number of arguments. Var args are used when we are not sure about the number of arguments a method may need. Internally java treats them as array.

Declarations of methods with var args

void method(int... x){};
void method(int x, int... y){};

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Q167. Why following method declarations are not valid ?

void method(int... x, int y){};
void method(int... x,int... y){};

Core Java

Ans. Because var args are not only allowed with the last argument in the method declaration.

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Q168. Do you see any problem with this code

public class BuggyBread {

   public static void main(String[] args) {
      method("Hello","World");
   }

   private static void method(String... args){
      for(String arg:args){
         System.out.println(arg);
      }
   }
}

Core Java

Ans. No, It will compile and execute perfectly fine. As we are using method with var args , we can call it with 0 to n arguments.

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Q169. Do you see any problem with this code

public class BuggyBread {

   public static void main(String[] args) {
      method();
   }

   private static void method(String... args){
      for(String arg:args){
         System.out.println(arg);
      }
   }
}

Core Java

Ans. This will compile fine as we can provide 0 arguments for a var arg but will print nothing upon execution.

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Q170. Do you see any problem with this code

public class BuggyBread {

   public static void main(String[] args) {
      method("Hello");
   }

   private static void method(String... args){
      for(String arg:args){
         System.out.println(arg);
      }
   }

   private static void method(String arg){
      System.out.println(arg);
   }
}

Core Java

Ans. It will compile fine and the method call will bind to method with specific argument and not with var arg

i.e private static void method(String arg)

It will print "Hello" upon execution

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Q171. Do you see any problem with this code

public class BuggyBread {

   public static void main(String[] args) {
      method("Hello");
   }

   private static void method(String... args){
      for(String arg:args){
         System.out.println(arg);
      }
   }

   private static void method(String[] arg){
      System.out.println(arg);
   }
}

Core Java

Ans. Yes, it will give compilation error and java will complain about duplicate method. Java treat var args internally as arrays and hence would result in same byte code for both method's syntax.

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Q172. Will this code work

public class BuggyBread {

   public static void main(String[] args) {
      method("Hello","World");
   }

   private static void method(String[] arg){
      System.out.println(arg);
   }
}

What all possible changes we can make to method signature to make it work ?

Core Java

Ans. It won't work as java won't find the method definition for method with 2 arguments.

We can either declare the method as

private static void method(String arg,String arg2)

or

private static void method(String... arg)

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Q173. What is Boxing conversion ?

Core Java

Ans. Boxing conversion converts expressions of primitive type to corresponding expressions of reference type.

For example

boolean to Boolean
long to Long
double to Double

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Q174. Can we declare a class private ?

Core Java

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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Q175. How can we hide a class in Java ?

Core Java

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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Q176. Can an inner class be subclass of it's parent class ?

Core Java

Ans. Yes that can be done

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

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Q177. 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 Java

Ans. Yes that's legal in java

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Q178. Can a source file contain more than one class declaration ?

Core Java

Ans. Yes, but only one of them can be declared public.

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Q179. Can an interface be defined inside a class just like we can have inner classes inside a class ?

Core Java

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

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Q180. Design a class diagram for a farm.

Design

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Q181. Does Wrapper classes produces immutable objects ?

Core Java

Ans. Yes

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Q182. What are the advantage of Abstract classes over interfaces with respect to Java 7 ? and What changed in Java 8 to help facilitate that in Java 8 ?

Core Java

Ans. Abstract Classes provide default implementations of methods that are inherited by the classes that extend them, which was not the case for Interfaces. This changed in Java 8, where default implementations are provided for methods.

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Q183. Why do we use Thread Class as well as Runnable Interface for creating threads in Java ?

Core Java

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Q184. Why can we have a static inner class but not a static Top level class in Java ?

Core Java

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Q185. Can we use primitive types with Collection classes ?

Core Java

Ans. No, As collection classes involve use of Generics, they cannot accept primitive types.

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Q186. Is immutability an advantage with Wrapper classes over primitive types ?

Core Java

Ans. There is no concept of de referencing with primitive types and hence they are implicitly immutable. Having wrapper classes as mutable offers disadvantages compared to primitive types. Wrapper classes being immutable offer similar advantage as primitive types.It actually overshadows the disadvantage wrapper class could have if they are immutable.

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Q187. Which immutable classes have you worked with ? Can you name some immutable Collections ? How to make an immutable collection ?

Core Java

Ans. string and wrapper class objects

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Q188. What are the benefits of immutability or Immutable classes ?

Core Java

Ans. Immutable objects are thread-safe so you will not have any synchronization issues.Immutable objects are good for Map keys and Set elements, since these typically do not change once created.Immutability makes it easier to write, use and reason about the code (class invariant is established once and then unchanged)Immutability makes it easier to parallelize your program as there are no conflicts among objects.The internal state of your program will be consistent even if you have exceptions.References to immutable objects can be cached as they are not going to change.

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Q189. Explain OOps concepts.

Core Java

Ans. There are four main OOP concepts in Java. These are:

Abstraction. Abstraction means using simple things to represent complexity. We all know how to turn the TV on, but we don?t need to know how it works in order to enjoy it. In Java, abstraction means simple things like objects, classes, and variables represent more complex underlying code and data. This is important because it lets avoid repeating the same work multiple times.

Encapsulation. This is the practice of keeping fields within a class private, then providing access to them via public methods. It?s a protective barrier that keeps the data and code safe within the class itself. This way, we can re-use objects like code components or variables without allowing open access to the data system-wide.

Inheritance. This is a special feature of Object Oriented Programming in Java. It lets programmers create new classes that share some of the attributes of existing classes. This lets us build on previous work without reinventing the wheel.

Polymorphism. This Java OOP concept lets programmers use the same word to mean different things in different contexts. One form of polymorphism in Java is method overloading. That?s when different meanings are implied by the code itself. The other form is method overriding. That?s when the different meanings are implied by the values of the supplied variables. See more on this below.

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Q190. Which are the thread safe / synchronized / concurrent classes in java collections ?

Core Java

Ans. Stack
Properties
Vector
BlockingQueue
ConcurrentMap
ConcurrentNavigableMap

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