Interview Questions and Answers for 'This' | Search Interview Question

Interview Questions and Answers for 'This' - 21 question(s) found - Order By Newest

Very frequently asked. Among first few questions in almost all interviews. Among Top 5 frequently asked questions. Frequently asked in Indian service companies (HCL,TCS,Infosys,Capgemini etc based on multiple feedback ) and Epam Systems

Q1. Difference between == and .equals() ?

Core Java

Ans. "equals" is the method of object class which is supposed to be overridden to check object equality, whereas "==" operator evaluate to see if the object handlers on the left and right are pointing to the same object in memory.

x.equals(y) means the references x and y are holding objects that are equal. x==y means that the references x and y have same object.

Sample code:

String x = new String("str");
String y = new String("str");

System.out.println(x == y); // prints false
System.out.println(x.equals(y)); // prints true

Sample Code for equals

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Frequently asked at Manhattan Associates ( Based on 2 feedback )

Q2. What is a Lambda Expression ? What's its use ?

Core Java

Ans. Its an anonymous method without any declaration.

Lambda Expression are useful to write shorthand Code and hence saves the effort of writing lengthy Code.

It promotes Developer productivity, Better Readable and Reliable code.

Sample Code for lambda

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Q3. What are the common uses of "this" keyword in java ?

Core Java

Ans. "this" keyword is a reference to the current object and can be used for following -

1. Passing itself to another method.

2. Referring to the instance variable when local variable has the same name.

3. Calling another constructor in constructor chaining.

Sample Code for this keyword

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Very Frequently asked. Have been asked in HCL Technologies very frequently ( based on 3 feedback ). Among first few questions in many interviews.

Q4. Differences between abstract class and interface ?

Core Java

Ans. Abstract classes can have both abstract methods ( method declarations ) as well as concrete methods ( inherited to the derived classes ) whereas Interfaces can only have abstract methods ( method declarations ).

A class can extend single abstract class whereas it can implement multiple interfaces.

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Q5. Can we use both "this()" and "super()" in a constructor ?

Core Java

Ans. No, because both this and super should be the first statement.

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Frequently asked in high end product companies. Frequently asked in Deloitte.

Q6. How is Hashmap internally implemented in Java ?

Core Java

Ans. https://medium.com/javarevisited/internal-working-of-hashmap-in-java-97aeac3c7beb#:~:text=HashMap%20internally%20uses%20HashTable%20implementation,the%20entries%20into%20the%20map.

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Q7. Difference Between this() and super() ?

Core Java

Ans. 1.this is a reference to the current object in which this keyword is used whereas super is a reference used to access members specific to the parent Class.

2.this is primarily used for accessing member variables if local variables have same name, for constructor chaining and for passing itself to some method whereas super is primarily used to initialize base class members within derived class constructor.

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Q8. What is "this" keyword used for ?

Ans. Used to represent an instance of the class in which it appears.

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Q9. Why can't we use this in static context ?

Core Java

Ans. Static methods can be called using instance references wherein this would have made sense but static method can also be called using Class name wherein this would mean nothing and hence forbidden.

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Q10. What is the difference between authentication and authorization ?

Authentication

Ans. Authentication is the process of verifying the identity and credentials of the user to authenticate him into the system.

whereas

Authorization is the process by which access to a segment , method or resource is determined.

Authorization is usually a step next to authentication.

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Q11. What is a Webdriver ?

Testing

Ans. Selenium WebDriver is a tool for automating web application testing.It helps in replicating the manual tester behavior like keyboard entry, mouse events etc and then matching the output against the expected.

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Q12. Can we use "this" within static method ? Why ?

Core Java

Ans. No. Even though "this" would mean a reference to current object id the method gets called using object reference but "this" would mean an ambiguity if the same static method gets called using Class name.

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Q13. Difference between Assert and Verify ?

Testing

Ans. Assert works only if assertions ( -ea ) are enabled which is not required for Verify.Assert throws an exception and hence doesn't continue with the test if assert evaluates to false whereas it's not so with Verify.

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Very frequently asked. Usually followed by questions related to private constructor and synchronized access. Frequently asked in JPMorgan and TCS (Based on 2 feedback)

Q14. Explain Singleton Design Pattern ?

Design

Ans. http://www.buggybread.com/2014/03/java-design-pattern-singleton-interview.html

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Q15. In majority of the situations it won't make much sense whether you append this with the instance method call.Can you tell a situation wherein this keyword would make sense in a instance method ?

Core Java

Ans. Within instance method of the parent class that has other multiple methods that have been overridden by the derived classes.

In such case a simple method call from the common method will always be made to the method definition in the parent class. But If we use this.methodCall , this will be polymorphic and will be made to the respective derived object overriding method.

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Q16. What is Jenkins ?

Jenkins

Ans. It is a continuous integration tool written in Java.

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Q17. Which of the following is not the use of this keyword ?

a. Passing itself to another method
b. To call the static method
c. Referring to the instance variable when local variable has the same name
d. Calling another constructor in constructor chaining

Ans. To call the static method

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Q18. Does compiler treats it differently if we don't prefix this to the member element and its implicit as its the only variable available with that name ?

Ans. It makes no difference whether we add this to the variable or not. The only use of this is in the cases where there are multiple variables with the same name and we want to distinguish between the member variable and local variable. In case this is not added ,this is automatically added by the compiler in the byte code.

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Q19. Have you ever tried mocking static methods ?

Testing

Ans. Yes, that can be done using Power Mock. Mockito doesnt provide a way to mock static methods.

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Q20. Can we have a sub enum within an enum ?

Core Java

Ans. Yes

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Q21. What measures would you take to avoid Java threading issues in a multi-threaded environment ?

^{This question was recently asked at 'Fifth Third Bank'.This question is still unanswered. Can you please provide an answer.}

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Q22. What will be the output of following code ?

public class BuggyBread {

   private int x;

   private BuggyBread(int x){
      x = x;
   };

   public static void main(String[] args){
      BuggyBread buggybread = new BuggyBread(5);
      System.out.println(buggybread.x);
   }
}

Core Java

a. compilation error

b. undefined

c. 0

d. 5

Ans.c. 0

Q23. What is the problem with this code ?

public class Car extends Vehicle{
   int x;

   Car(int y){
      x = 5;
   }

   Car(){
      this(5);
      super();
   }
}    Reference

Core Java

a. We cannot overload constructors

b. Constructors should have type

c. we should have called super() before this(5)

d. We cannot have both super() and this() in a constructor

Ans.d. We cannot have both super() and this() in a constructor

Q24. What is the problem with this code ?

public class Car extends Vehicle{
   int x;

   Car(int y){
      x = 5;
   }

   Car(){
      super();
      this.x = 5;
   }
}

Core Java

a. We cannot overload constructors

b. We cannot call super in overloaded constructor

c. we cannot have this and super in constructor

d. There is no problem

Ans.d. There is no problem

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