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

Interview Questions and Answers - Order By Rating

Q1441. What is the difference between public class and class ?

Ans. Class without any access specifier has the default scope i.e it can be accessed by any class within same package.

Class declared public can be accessed from anywhere.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java access specifier access modifiers public class class}

Try 1 Question(s) Test

Frequently asked to fresh graduates and less experienced.

Q1442. Why do we write public static void main ? Can we use some other syntax too for main ?

Core Java

Ans. 1. public is the access modifier that makes the method accessible from anywhere, static is the keyword that makes it accessible even without creating any object, void means it doesn't return anything , String args[] is the array of argument that the method receives.

2. If we use main without the string args , it will compile correctly as Java will treat it as just another method. It wont be the method "main" which Java looks for when it looks to execute the class and hence will throw

Error: Main method not found in class , please define the main method as:
public static void main(String[] args)

3. Main is not a keyword but a special string that Java looks for while initiating the main thread.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java main method Asked in 4 Companies basic frequent}

Try 1 Question(s) Test

Q1443. Why every object constructor automatically call super() in Object before its own constructors?

Core Java

Ans. Derived object carries the body of its class as well as the body of the parent class. Its body ( member elements ) is initialized using its own class constructor whereas the body ( member elements ) carried from the parent class are initialized using super class constructor. So In order to initialize the elements of the parent class before its own elements are even initialized, super is called.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops constructor super inheritance object oriented programming (oops) oops concepts inheritence Asked in 1 Companies intermediate}

Q1444. Will this code compile ?

public class BuggyBread1{
abstract public void test();
}

Core Java

Ans. No. It will give the compilation error saying "The abstract method test in type BuggyBread1 can only be defined by an abstract class".

We need to declare the class abstract for it to have any abstract method.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops abstract class abstract methods java compilation error}

Q1445. Will this Code compile ?

abstract public class BuggyBread1{
abstract public void test(){};
}

Core Java

Ans. No. This will give a compilation error saying "Abstract methods do not specify a body".

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java abstract classes abstract methods java compilation error java coding java code coding yes-no}

Q1446. How is Abstraction implemented in Java ?

Core Java

Ans. Abstraction is provided in Java by following ways -

Coding to the ( Interfaces / Abstract Classes ) or contracts

By Encapsulating details within classes and exposing the minimal Door ( few public methods )

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops concepts abstraction interfaces abstract class encapsulation object oriented programming (oops) oops concepts Asked in 3 Companies basic frequent}

Q1447. Give an Example for Builder Pattern ?

Core Java

Ans. String str = new StringBuilder().append("Buggy").append("Bread").toString();

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java builder pattern design pattern stringbuilder string Asked in 3 Companies}

Try 1 Question(s) Test

Q1448. What are some different logging levels for log4j ?

Log4j

Ans. DEBUG
ERROR
ALL
WARN
INFO
FATAL
OFF
TRACE
TRACE_INT
WARN

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java log4j log4j level logging error logging Asked in 10 Companies}

Q1449. Is this Polymorphism ?

Map<String, List<String>> inventoryManagerCountMap = new HashMap<String, ArrayList<String>>();

Core Java

Ans. No. This will result in compilation error as Polymorphism cannot be performed on Object types.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java polymorphism object oriented programming (oops) oops concepts}

Rarely asked as it was introduced with Java 8.

Q1450. Difference between DoubleSummaryStatistics , IntSummaryStatistics and LongSummaryStatistics ?

Core Java

Ans. They all does the same task i.e to compute statistical information on the stream of data. They differ by the way they store the statistical information as they expect a different data type of the values being used.

IntSummaryStatistics and LongSummaryStatistics expect non floating point values and hence stores the statistical information like min,max and sum as non floating values ( int or long ) whereas DoubleSummaryStatistics stores these information as floating value.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java java8 java 8 lambda expressions doublesummarystatistics intsummarystatistics longsummarystatistics summarystatistics}

Not frequently asked as it was introduced with Java 8.

Q1451. What is StringJoiner ?

Core Java

Ans. StringJoiner is a util method to construct a string with desired delimiter. This has been introduced with wef from Java 8.

Sample Code

StringJoiner strJoiner = new StringJoiner(".");

strJoiner.add("Buggy").add("Bread");

System.out.println(strJoiner); // prints Buggy.Bread

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java java8 java 8 string stringjoiner}

Not frequently asked as it was introduced with Java 8.

Q1452. What is the use of Optional ?

Core Java

Ans. Optional is a good way to protect application from runtime nullPointerException in case the the absent value has been represented as null. So basically Optional class provides the type checking during compile time and hence will never result in NPE.

For ex -

List> intList = new ArrayList>();
intList.add(Optional.empty());
intList.add(Optional.of(new Employee("abc")));
intList.add(Optional.of(new Employee("xyz")));
intList.add(Optional.of(new Employee("123")));
System.out.println(intList.get(0).getName());

So Now , even when the first list element is empty, this code will never throw an NullPointerException.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java java8 java 8 optional}

Q1453. Have you ever used StringTokenizer ?

Core Java

Ans. No, Alternatively I have used String.split("\\s") for achieving the same function.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java string stringtokenizer}

Q1454. What is ConcurrentLinkedDeque ?

Core Java

Ans. It's a Collections concrete class that provides implementation of a Double Ended queue that allows concurrent insertion, removal, and access operations that can be executed safely across multiple threads.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections queue deque concurrent collections}

Q1455. What is CopyOnWriteArrayList ?

Ans. Its a type of ArrayList in which all Write operations , i.e add and set are performed by creating a new copy. This array never changes during the lifetime of the iterator, so it never throws ConcurrentModificationException

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections list arraylist copyonwritearraylist ConcurrentModificationException Asked in 1 Companies}

Q1456. What are the advantages and disadvantages of CopyOnWriteArrayList ?

Core Java

Ans. This collections class has been implemented in such a manner that it can never throw ConcurrentModificationException. As it performs update and write operations by creating a new copy of ArrayList, It's slower compared to ArrayList.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections list arraylist copyonwritearraylist advantages-disadvantages ConcurrentModificationException Asked in 4 Companies Expert}

Q1457. Name few Concurrent Collection Classes ?

Ans. ConcurrentHashMap
ConcurrentLinkedDeque
ConcurrentLinkedQueue
ConcurrentMap
ConcurrentNavigableMap
ConcurrentSkipListMap
ConcurrentSkipListSet

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections concurrent collections}

Q1458. Name few Collections Map implementations ?

Ans. AbstractMap
ConcurrentHashMap
ConcurrentSkipListMap
EnumMap
HashMap
IdentityHashMap
LinkedHashMap
SystemFlavorMap
TreeMap
WeakHashMap

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections map}

Q1459. What is ConcurrentModificationException ?

Core Java

Ans. This is the exception that is thrown when we try to modify the non concurrent collection class while iterating through it.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections concurrentmodificationexception exception concurrency Asked in 14 Companies intermediate}

Q1460. What is ArrayIndexOutOfBoundException ?

Core Java

Ans. Exception thrown by the application is we try to access an element using an index which is not within the range of array i.e lower than 0 or greater than the size of the array.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java exceptions arrayindexoutofboundexception Asked in 1 Companies}

Q1461. how can we generate JavaDoc documentation for our code ?

Eclipse

Ans. In Eclipse, Go to File -> Export -> Java -> JavaDoc

Select the projects , other properties and output directory for which JavaDoc and Click Finish.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java javadoc eclipse}

Q1462. Can we generate Java Documentation using Export -> JavaDoc using a jar file ?

Eclipse

Ans. Yes, If the jar contains the source code. No, If it contains only classes.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java javadoc eclipse}

Q1463. Can we initialize member variables within static block ?

Ans. Static block is like static method that gets executed upon class loading. The way static method allows accessing member variables ( using object references only ), the same static block can also access and initialize member variables.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java static block}

Q1464. Difference between jar , war and ear ?

Java EE

Ans. Jar is Java Archieve i.e compressed Class or Class / Java files.

War comprises of compressed Servlet class files,JSP FIles,supporting files, GIF and HTML files.

Ear comprise of compressed Java and web module files ( was files ).

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java j2ee jar web development war ear build management release management Asked in 12 Companies basic frequent}

Q1465. Name few interfaces that extends Collection Interface ?

Ans. http://www.buggybread.com/2015/02/java-collections-interfaces-that.html

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections collection interface}

Q1466. Name few classes that extends Number Class ?

Ans. http://www.buggybread.com/2015/02/java-util-classes-that-inherit-number.html

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java data types number class}

Q1467. Name few classes that extend Thread class ?

Core Java

Ans. http://www.buggybread.com/2015/02/java-threads-classes-that-inherit.html

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java threads multithreading thread class Asked in 1 Companies}

Q1468. Name few classes that extends Collections class ?

Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-inherit.html

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections collections class}

Q1469. Name few Date and Time Classes you have used in your projects ?

Ans. http://www.buggybread.com/2015/02/java-util-date-time-classes-and.html

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java util java.util.date time}

Q1470. Name few Calendar related classes and Interfaces ?

Ans. http://www.buggybread.com/2015/02/java-util-calendar-classes-and.html

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java util java.util calendar}

Search Interview Questions

Help us and Others Improve. Please let us know the questions asked in any of your previous interview.