Frequently Asked JAVA Interview Question and Answer Part 4

Hi Guys, In this post I'll share some interview questions and answers which i have faced or asked in interview panel. you can read my previous post related to JAVA QA JAVA QA PART 1 and JAVA QA PART 2 and JAVA QA PART 3.

Q1: How can you generate random numbers in Java?

Ans: Using Math.random() you can generate random numbers in the range greater than or equal to 0.1 and less than 1.0

·           Using Random class in package java.util

Q2. What is default switch case? Give example.

Ans: In a switch statement, default case is executed when no other switch condition matches. Default case is an optional case .
It can be declared only once all other switch cases have been coded.

In the below example, when score is not 1 or 2, default case is used.

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public class switchExample { int score=4; public static void main(String args[]) { switch (score) { case 1: system.out.println("Score is 1"); break; case 2: system.out.println("Score is 2"); break; default: system.out.println("Default Case"); } } }

Q3.  What’s the base class in Java from which all classes are derived?

Ans: java.lang.object

Q4. Can main() method in Java can return any data?

Ans: In java, main() method can’t return any data and hence, it’s always declared with a void return type.

Q5. What are Java Packages? What’s the significance of packages?

Ans: In Java, package is a collection of classes and interfaces which are bundled together as they are related to each other. Use of packages helps developers to modularize the code and group the code for proper re-use. Once code has been packaged in Packages, it can be imported in other classes and used.

Q6.  Can we declare a class as Abstract without having any abstract method?

Ans: Yes we can create an abstract class by using abstract keyword before class name even if it doesn’t have any abstract method. However, if a class has even one abstract method, it must be declared as abstract otherwise it will give an error.

Q7. What’s the difference between an Abstract Class and Interface in Java?

Ans: The primary difference between an abstract class and interface is that an interface can only possess declaration of public static methods with no concrete implementation while an abstract class can have members with any access specifiers (public, private etc) with or without concrete implementation.

Another key difference in the use of abstract classes and interfaces is that a class which implements an interface must implement all the methods of the interface while a class which inherits from an abstract class doesn’t require implementation of all the methods of its super class.

A class can implement multiple interfaces but it can extend only one abstract class.

Q8. What are the performance implications of Interfaces over abstract classes?

Ans:  Interfaces are slower in performance as compared to abstract classes as extra indirections are required for interfaces. Another key factor for developers to take into consideration is that any class can extend only one abstract class while a class can implement many interfaces.

Use of interfaces also puts an extra burden on the developers as any time an interface is implemented in a class; developer is forced to implement each and every method of interface.

Q9. Does Importing a package imports its sub-packages as well in Java?

Ans: In java, when a package is imported, its sub-packages aren’t imported and developer needs to import them separately if required.

For example, if a developer imports a package university.*, all classes in the package named university are loaded but no classes from the sub-package are loaded. To load the classes from its sub-package ( say department), developer has to import it explicitly as follows:

Import university.department.*

Q10. Can we declare the main method of our class as private?

Ans: In java, main method must be public static in order to run any application correctly. If main method is declared as private, developer won’t get any compilation error however, it will not get executed and will give a runtime error.

Q11.  How can we pass argument to a function by reference instead of pass by value?

Ans:  In java, we can pass argument to a function only by value and not by reference.

Q12. How an object is serialized in java?

Ans: In java, to convert an object into byte stream by serialization, an interface with the name Serializable is implemented by the class. All objects of a class implementing serializable interface get serialized and their state is saved in byte stream.

Q13. When we should use serialization?

Ans: Serialization is used when data needs to be transmitted over the network. Using serialization, object’s state is saved and converted into byte stream .The  byte stream is transferred over the network and the object is re-created at destination.

Q14. Is it compulsory for a Try Block to be followed by a Catch Block in Java for Exception handling?

Ans: Try block needs to be followed by either Catch block or Finally block or both. Any exception thrown from try block needs to be either caught in the catch block or else any specific tasks to be performed before code abortion are put in the Finally block.

Q15. Is there any way to skip Finally block of exception even if some exception occurs in the exception block?

Ans:  If an exception is raised in Try block, control passes to catch block if it exists otherwise to finally block. Finally block is always executed when an exception occurs and the only way to avoid execution of any statements in Finally block is by aborting the code forcibly by writing following line of code at the end of try block:

1	System.exit(0);

Q16. When the constructor of a class is invoked?

Ans: The constructor of a class is invoked every time an object is created with new keyword.

For example, in the following class two objects are created using new keyword and hence, constructor is invoked two times.

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public class const_example { const_example() { system.out.println("Inside constructor"); } public static void main(String args[]) { const_example c1=new const_example(); const_example c2=new const_example(); } }

Q17. Can a class have multiple constructors?

Ans: Yes, a class can have multiple constructors with different parameters. Which constructor gets used for object creation depends on the arguments passed while creating the objects.

Q18. Can we override static methods of a class?

Ans: We cannot override static methods. Static methods belong to a class and not to individual objects and are resolved at the time of compilation (not at runtime).Even if we try to override static method,we will not get an complitaion error,nor the impact of overriding when running the code.

Q19. In the below example, what will be the output?

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public class superclass { public void displayResult() { system.out.println("Printing from superclass"); } } public class subclass extends superclass { public void displayResult() { system.out.println("Displaying from subClass"); super.displayResult(); } public static void main(String args[]) { subclass obj=new subclass(); obj.displayResult(); } }

Ans: Output will be:

Displaying from subclass

Displaying from superclass

Q20. Is String a data type in java?

Ans: String is not a primitive data type in java. When a string is created in java, it’s actually an object of Java.Lang.String class that gets created. After creation of this string object, all built-in methods of String class can be used on the string object.

Q21.  In the below example, how many String Objects are created?

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String s1="I am Java Expert"; String s2="I am C Expert"; String s3="I am Java Expert";

Ans: In the above example, two objects of Java.Lang.String class are created. s1 and s3 are references to same object.

Q22. Why Strings in Java are called as Immutable?

Ans: In java, string objects are called immutable as once value has been assigned to a string, it can’t be changed and if changed, a new object is created.

In below example, reference str refers to a string object having value “Value one”.

1	String str="Value One";

When a new value is assigned to it, a new String object gets created and the reference is moved to the new object.

1	str="New Value";

Q23. What’s the difference between an array and Vector?

Ans: An array groups data of same primitive type and is static in nature while vectors are dynamic in nature and can hold data of different data types.

Q24. What is multi-threading?

Ans: Multi threading is a programming concept to run multiple tasks in a concurrent manner within a single program. Threads share same process stack and running in parallel. It  helps  in performance improvement of any program.

Q25. Why Runnable Interface is used in Java?

Ans: Runnable interface is used in java for implementing multi threaded applications. Java.Lang.Runnable interface is implemented by a class to support multi threading.

Q26. What are the two ways of implementing multi-threading in Java?

Ans: Multi threaded applications can be developed in Java by using any of the following two methodologies:

1. By using Java.Lang.Runnable Interface. Classes implement this interface to enable multi threading. There is a Run() method in this interface which is implemented.

2. By writing a class that extend Java.Lang.Thread class.

Cheers :)