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

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Q131. Can we have a final argument in a method ? What is the use of that if we do so ?

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Q132. why is "".equals(str); safer than str.equals("")?

Core Java

Ans. str.equals("") this statement will throw NullPointerException if str is null where as "".equals(str) works fine even if str is null

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Q133. Write a method that would solve two problems at the same time ?

Core Java

Ans. We can return value for one problem solving or computation and set the value for other in some member element.

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Q134. Can a constructor be declared static ? Why ?

Core Java

Ans. No.

When we declare a method static, it means that "this belongs to class as whole and not particular instance". The whole purpose of constructor is to initialize a object and hence there is no sense having static constructor.

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Q135. Can constructors be final ?

Core Java

Ans. No. They are never inherited and therefore are not subject to hiding or overriding.

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Q136. Have you ever worked on continuations in Java ?

Core Java

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Q137. What is the use of this keyword in javascript ?

Javascript

Ans. This keyword refers to the current instance of the class.

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Q138. What are inline functions ? Do we have inline functions in Java ?

Core Java

Ans. Inline functions , just like C++ Macros is an optimized technique used by compiler to reduce the execution time. If the function is working on pre identified values ( which aren't resolved at runtime ), the function can execute the method and evaluate the outcome at compile time only instead of making a function call at runtime.

In Java, the optimizations are usually done at the runtime or JVM level. At runtime, the JVM perform some analysis to determine which methods to inline. Java compiler would never inline any method and there is no way in java for the developer to explicitly define inlining of methods as it's take intrinsically care of during runtime only.

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Q139. What is the default size of list ? How it is growable ?

Core Java

Ans. Default size is 10.
Regard how size will increase.

Inside add() method there is one method called ensureCapacity() which will check list has enough size available.

This ensureCapacity() method if found size of list need to increase then it will call grow() method which will create new list of extra size and copy existing list data to new one and return.

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Q140. Can a method return more than one value at a time in Java ?

Core Java

Ans. object / native type returned can only be one but an object can comprise of a array , collection or a group of different value types.

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Q141. Difference between Base Class, Derived Class , Abstract Class and Concrete Class ?

Core Java

Ans. Base Class is a relative term used to signify the Parent class in Parent - Child or Base - derived relationship.

Derived Class is a relative term used to signify the Child class in Parent - Child or Base - derived relationship.

Abstract Class is a class that is not allowed to be instantiated and hence can serve only a base class for it's usage.

Concrete Class is the class which is supposed to be instantiated and hence provide definition for all implementing interface methods and extending abstract methods.

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Q142. Can an abstract class be a derived class ?

Core Java

Ans. Yes, an abstract class can extend another class.

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Q143. What is the relationship between Base Class and Abstract Class ?

Core Java

Ans. Abstract Class is a class that's only allowed to be a Base class for it's usage. It can never be instantiated on it's own.

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Q144. What is a base class ? Is Abstract class and Base class the same thing ?

Core Java

Ans. Base Class is a relative term. For example - Class Vehicle is a Base class of Class Car. Even though Base class is derived in this case , there is no restriction on instantiation of Vehicle class.

Abstract Class is a class that is meant only to be a Base Class and not allowed to be instantiated.

Abstract Class is a Base Class that isn't allowed to be instantiated and is only meant for carrying definition to derived class and help participate in runtime polymorphism.

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Q145. What are the biggest challenges for Java technology in the coming years?

Core Java

Ans. JVM - With Cloud and Serverless, this could be a hurdle for it to be preferred technology.

Confusion between OOP and Functional Programming.

Slower speed of innovation due to wider adoption.

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Q146. Explain difference between web-based application and Core Java Application ?

Java

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Q147. Difference between List and ArrayList ?

Core Java

Ans. List is an interface whereas ArrayList is an implementation of List.

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Q148. If you override hashcode to always return true, how a hash based collection will behave?

Core Java

Ans. At that bucket, it will form a linked list depending on what equals method evaluates for that object.

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Q149. Difference between String constant and String non-constant pools?

Core Java

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Q150. Difference between "extend" and "implement" in java?

Core Java

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Q151. Can we serialize objects with only private variables in Java ?

Core Java

Ans. It depends on how we are serializing. The Serialization API doesn't worry about private variables and convert it into binary representation.

If we are using a library to map it to JSON / XML using XML Mappers, it may create trouble.

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Q152. What is input Stream ?

Core Java

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Q153. What will happen if there are no collections in java ?

Core Java

Ans. Collections in Java is nothing but a library implementation for data structures and algorithm. If it's not available , we might have to include some other library or provide our own implementation.

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Q154. What will happen if there are no enums in Java ?

Core Java

Ans. Enums in Java is a facility to have objects upfront and use them as constants. Even if such a facility is not available , a workaround could be achieved.

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Q155. What will happen if there are no interfaces in Java ?

Core Java

Ans. Abstract classes can take care of that to a certain extent. Though they are little heavier than Interfaces but An abstract class with all abstract methods and no instance variables will be able to help with everything that currently an interface does.

The only problem is that a class can only extend one class whereas it can implements multiple interfaces and that is the reason Interfaces were introduced in Java, i.e to get over the problem of multiple inheritance.

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Q156. What are the different ways to create objects in Java ?

Core Java

Ans. 1. Using new operator
3. Using Class.forName or Reflection
4. Using clone
5. Using Deserialization

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Q157. Can we create object in static block ?

Core Java

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Q158. Explain servlet lifecycle

Java EE

Ans. Init -> service -> destroy.

Out of these, init and destroy are called once per servlet whereas service is called every time when a request is made to the servlet.

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Q159. in the following class:
class A {
void methoda(Object o) {
Sysout("Object");
}

void methoda(String s) {
Sysout("String");
}

public static void main(String []args) {
A a = new A();
a.methoda(null);
}
}

what will be printed?

Core Java

Ans. String

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Q160. How many read and write operations are provided at the same time by ConcurrentHashMaps ?

Core Java

Ans. ConcurrentHashMaps provide unlimited read operation fro multi threaded environment. For write operation only one thread can change value for particular segment of map.

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