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

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Q271. Write a method it will return reverse of the string(with out using reverse method of String or StringBuffer)

Ans. String s1 = "akhilesh";
char[] ch = s1.toCharArray();
for (int i = ch.length-1 ; i >=0 ; i--) {
System.out.print(ch[i]);
}

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Q272. Which collection stores element in insertion order ?

Core Java

Ans. LinkedList maintains the insertion order in collection.

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Q273. Find different combination of given String and trace the output

Core Java

Ans. private void permutation(String prefix, String sufix)
{
int ln = sufix.length();
if(ln == 0) {
System.out.println(prefix);
} else {
IntStream.range(0, ln).forEach(i->permutation(prefix sufix.charAt(i), sufix.substring(0,i) sufix.substring(i 1, ln)));
}
}

call:permutation("", "abcdef");

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Q274. Why do we need to specify import statement ? Don't you think Java could have been designed in such a way to automatically import everything that's in the class path ?

Core Java

Ans. Java could design it in such a manner but would result in ambiguity if there are multiple files with the same name. The only way to get over it to show the message when you use any such class to provide explicit import with the package prefix. The other problem could be that Java might have to change the early import to Late import and check what's being used to decide what needs to be imported as otherwise you will see errors regarding the ambiguous imports with duplicate file name.

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Q275. Which of the OOP's feature is most important ?

Core Java

Ans. Encapsulation facilitate hiding and restricted access and hence more of a security feature. Encapsulation is definitely a great feature as when applications expand criss cross communication between objects / modules could lead to blunders.

Inheritance facilitates code reuse.

Polymorphism comprise of method overloaded ( which to me is negligible usage ) and method overriding. Method overriding is of great usage as it facilitates concept of interfaces and plugin development.

So it’s Security / Organization vs

Code Reuse / Support for other features like overriding vs

Contracts / Plugin Development facilitating the creation of frameworks / libraries.

Which is more important may vary from application to application , its scale , its use , its sensitivity etc.

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Q276. Can we have a private default method ?

Core Java

Ans. No. The whole idea of default method is to provide a default definition in case the implementing class intend to provide definition for only some of the methods. Making any of the interface method private would restrict it from being implemented by the implementing class. So default method is an option to provide implementation and not restricting a new definition.

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Q277. What is the relation between classes in Java ?

Core Java

Ans. There are different types of relationship between classes

Inheritance - Also called is-a relationship, Child class object carries the body of the Parent class when initiated. Moreover there are certain privileges attach to method overriding to the classes related this way. This relationship exist for code reuse, method overriding and interfacing ( through abstract class ).

Composition - Also called has-a relationship. Class objects carries a reference to other class objects when instantiated. This relationship exist for work flow / work delegation.

Inner / Nested class - It’s a relationship that encapsulates the inner class or it’s object within the outer class. This relationship exist for encapsulation / security.

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Q278. Find Fibonacci first n numbers using recursion ?

Core Java

Ans. public int fibonacci(int n) {
if(n == 0)
return 0;
else
if(n == 1)
return 1;
else
return fibonacci(n - 1) fibonacci(n - 2);
}

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Q279. Have you ever faced problems with rounding decimal places and what were the reasons ?

Core Java

Ans. 1. The problem with double (x*100)/100 doesn't return exact x but few fractions lesser than x and then if you are using floor rounding , it makes a big difference

2. Rounding only after getting a result vs rounding each outcome of 2 operand make difference

3. Usage of inappropriate Rounding mode and Rounding scale.

4. Results with double and BigDecimal

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Q280. Can we have multiple inheritance in java using abstract class ?

Core Java

Ans. No

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Q281. Does use of Final class enforces composition over Inheritance in Java ?

Core Java

Ans. Yes, to a certain extent. But the objective for Final class could be beyond just enforcing composition as certain classes might have been created without inheritance or composition in mind.

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Q282. What could be the reasons for restricting inheritance using final class ?

Core Java

Ans. 1. Enforcing composition over inheritance
2. Restricting overriding of certain methods
3. Final methods are faster than regular instance methods
4. Enforcing Immutability

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Q283. Write code to create a folder if it doesn't exist.

Core Java

Ans. File folder = new File(path);

if(!folder.exists()){
try {
folder.mkdir();

} catch (Exception e) {}
}

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Q284. Difference between Arrays and ArrayList ?

Core Java

Ans. Both Arrays and ArrayLists are used to store elements. Elements can be either primitives or objects in case of Arrays, but only objects can be stored in Arraylist. Array is a fixed length data structure while arraylist is variable length collection class. Once created, you cannot change the size of the arrays, but arraylists can dynamically resize itself when needed.Another notable difference between Arrays and Arrayslist is that arary is part of core java programming and array list is part of collection classes

Sample Code for arrays arraylist

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Q285. Can we declare an array without assigning the size of an array?

Core Java

Ans. No, It will throw compile time error saying "must provide either dimension expressions or an array initializer"

Alternatively we can provide array initializer like

String[] strArray = new String[]{"Buggy","Bread"};

which will initialize it to size 2 with values as "Buggy" and "Bread"

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Q286. What is meant by "unreachable code" ? Can you write a code segment explaining this ?

Core Java

Ans. Unreachable code is a code segment that can never be executed in any case. For example -

int x = 5;
return;
System.out.println(x);

In this code , 2nd line will return the control out of the method and hence 3rd line will never get executed and hence is unreachable code. This will be caught at compile time only.

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Q287. Why doesn't this code give compile time error when we are clearly trying to add an integer element to an array of Strings

Object[] strArray = new String[2];
strArray[0] = 5;

Core Java

Ans. Because the reference is of Object Array and the resolution of what it holds is identified at the runtime.

String[] strArray = new String[2];
strArray[0] = 5;

would have given the compile time array as the reference is of String array.

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Q288. 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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Q289. 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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Q290. 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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Q291. why string pool concept for only String but not for StringBuffer ?

Core Java

Ans. As String is immutable so by maintaining pool area we can refer the object to same area if they are with same content. But when it comes to string buffer or builder then due to mutable property if we do so then all the objects referring to that area will from now has new content.. So as to avoid this pool is present only in string.

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Q292. why string pool concept for only String but not for StringBuffer ?

Core Java

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

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Q293. Difference between List and LinkedList ?

Core Java

Ans. A List is an child interface of collection interface in java where as Linked list is and implementation class of List interface which has doubly linked as a underlying data structure

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Q294. What is concurrent interface ?

Core Java

Ans. ConcurrentMap is an interface and it is a member of the Java Collections Framework. It represents a Map that is capable of handling concurrent access to it without affecting the consistency of entries in a map. ConcurrentMap interface present in java.util.concurrent package.

HashMap operations are not synchronized, while Hashtable provides synchronization. Though Hashtable is thread-safe, it is not very efficient. To solve this issue, the Java Collections Framework introduced ConcurrentMap in Java 1.5.

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Q295. Write a program to print next largest palindrome number?

Core Java

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

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Q296. Why cannot we initialize a final field within the constructor if the initialization is within try block ?

Core Java

Ans. What if the initialization throws an exception. In that case , it will let it move forward without initializing the final field. So it's a way to enforce that either the field is initialized or it fails completely.

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Q297. Can we reduce the visibility of the overridden method ?

Core Java

Ans. No

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Q298. Can static method access instance variables ?

Core Java

Ans. Though Static methods cannot access the instance variables directly, They can access them using instance handler.

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Very frequently asked. Usually asked in this format or as difference with interfaces / concrete classes.

Q299. What is an abstract class ?

Core Java

Ans. Abstract class is the class that is not supposed to be instantiated. The purpose of the class to only have extension to the derived class.

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Q300. Which interface does java.util.Hashtable implement?

Core Java

Ans. Java.util.Map

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