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

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Q791. What is the use of final keyword ?

Ans. final keyword is a modifier that means differently when applied to variables, methods and classes.

1. Final variable cannot be changed once initialized
2. Final method cannot be overridden
3. Final class cannot be sub classed

Sample Code for final variable

Sample Code for final method

Sample Code for final class

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Q792. How would you explain the statement "Java Programs are collection of objects" ?

Core Java

Ans. Java Programs are collection of objects that communicates with each other to get a task accomplished. To add to those objects, there are common spaces ( static i.e common for objects belonging to a class ) that are used too.

We can visualize objects as departments of an organization in real world. Just like Task gets initiated in one department and then files are moved across different departments to get work done. In a similar fashion, a task is initiated in one object ( having main method ) and then information ( through POJOs / DTOs ) is moved across objects to accomplish a task.

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Q793. Why iterators of an array list are fail fast ?

Core Java

Ans. Because it's access isn't synchronized and hence access / modification by multiple threads may lead to inconsistent state.

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Q794. What problem we could have with ArrayList which aren't possible with Vectors ?

Core Java

Ans. ArrayLists aren't synchronized and hence doesn't allow synchronized access. As multiple threads can access an arraylist in parallel, it may result in an inconsistent state.

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Q795. What is the quickest way to find count of duplicate elements in an arraylist, without using iteration or loops ?

Core Java

Ans. We can copy the elements to a Set and then find the difference of count between ArrayList and Set. As Set don't allow duplicates , they will be removed in the set.

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Q796. Write a method, that compare two numbers and return the bigger one ?

Core Java

Ans. int compare(int a, int b) {
a>b? return a: return b;
}

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Q797. Write a Program for insertion sort.

Core Java

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Very frequently asked. Usually among very first few questions.

Q798. Define encapsulation in Java ?

Core Java

Ans. Encapsulation is a feature of OOP's that binds the data and it's associated methods together as a single unit and facilitate protection and data hiding by providing minimal interface to outside. For example - member variables are declared private and are accessed through public methods. Moreover we have private methods that can only be used internally and hence providing minimal interface to outside class through use of public methods.

Sample Code for encapsulation

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Q799. 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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Q800. 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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Q801. What are the different types of loops in Java ?

Core Java

Ans. for, while and do while

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Q802. What is an infinite loop ?

Core Java

Ans. Infinite loop is a programming condition wherein the control goes into an infinite loop because the loop termination condition can never be met. For example -

for(int x=1;x>0;x++){
}

in this loop, with each increment the condition x > 0 will remain true till infinity.

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Q803. Is String a Data type in Java ?

Core Java

Ans. String is not a primitive type in java. Anything defined within double quotes like "abc" is an object of String class.

Sample Code for String

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Q804. Are char,String and Char data types in java ?

Core Java

Ans. char is a primitive data type. String is a class. Char is a wrapper class for primitive char.

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Q805. Can we call a constructor for an object twice ?

Core Java

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Q806. Can we compile and execute a Java class without main method ?

Core Java

Ans. No without main method can not be executed it will throw an error illegal start of type. Main method is the entry point of application.

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Q807. Is JDK required on each machine where a Java application needs to be run ?

Core Java

Ans. No, to run application, machine needs just a JRE (Java Runtime Environment)

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Q808. What is the purpose of re throwing the exception when we can provide the handling at the first catch itself ?

Core Java

Ans. Sometimes the calls move across layers of classes and functions and hence each layer needs to perform some function like cleaning if something goes wrong deep inside.

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Q809. What is the problem we could face if we be very liberal with importing classes even if we are not using them ? For example

import java.util.*;
import java.io.*;

Core Java

Ans. It makes application little heavy as we are importing classes that aren't required.

It creates class name conflicts as similar name classes might be available across different packages. In case of such conflicts, we will have to specify the package name with the class name at the time of it's usage.

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Q810. Which java package is loaded by default and hence doesn't require an explicit import statement ?

Core Java

Ans. java.lang

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Q811. How do we define constant variables in Java ?

Core Java

Ans. By using static and final modifiers.

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Q812. 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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Q813. 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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Q814. Can an interface implements another interface ?

Core Java

Ans. No. No interface provides any implementation and hence the declaration isn't viable.

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Q815. Why doesn't java support operator overloading ?

Core Java

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Q816. What is the use of upcasting in Java ?

Core Java

Ans. Interfacing

Car extends Vehicle

Car car = new Car();

// now you need to call an API changeTires() which can change tires for all Vehicle Types and accepts argument of type Vehicle and hence you will have to upcast it

changeTires((Vehicle)car);

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Q817. Why do we pass an array of strings to main method ?

Core Java

Ans. Array of strings in the main method are the list of arguments or parameters which are sent to the application / program.

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Q818. In Java, what is the closest thing to pointers?

Core Java

Ans. References.

Unlike pointers in C , Java references are strongly typed and we cannot do pointer arithmetics with them.

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Q819. In Java, would you like to see an explicit destructor?

Core Java

Ans. Yes , a gc method to explicitly call garbage collection immediately on the object. It will provide better flexibility over application that require optimal usage of memory.

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Q820. How to calculate lcm of two numbers ?

Core Java

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