Lesson 23 of 31By Ravikiran A S
Last updated on Mar 12, 202144354Java is the most widely used programming language in the current IT industry. One major reason for the vast number of beginners and professionals in the field of programming is the career potential that Java knowledge comes with. This article is dedicated to the same purpose. Here is a complete guide on how to help you crack the most frequently asked Java Interview questions.
This article is divided into three levels based on the complexity of the questions. It is as shown below.
We get two major things along with the Java Download file.
JDK - Java Development Kit
JRE - Java Runtime Environment
JDK |
JRE |
Abbreviation for JavaDevelopment Kit |
Abbreviation for Java Runtime Environment |
JDK is a dedicated kit for solely software development |
JRE is a set of software and library designed for executing Java Programs |
Unlike JVM, JDK is Platform Dependent |
Unlike JVM, JRE is also Platform Dependent |
JDK package is a set of tools for debugging and Developing |
JRE Package is one that only supports files and libraries for a runtime environment |
JDK package will be provided with an installer file |
JRE Package does not get an installer but has only a runtime environment |
A classloader in Java is a subsystem of Java Virtual Machine, dedicated to loading class files when a program is executed; ClassLoader is the first to load the executable file.
Java has Bootstrap, Extension, and Application classloaders.
Java has five significant types of memory allocations.
Yes, the program will successfully execute if written so. Because, in Java, there is no specific rule for the order of specifiers
Neither the Local Variables nor any primitives and Object references have any default value stored in them.
public class Simplilearn
{
public static void main (String args[])
{
System.out.println(100 + 100 +“Simplilearn");
System.out.println(“E-Learning Company" + 100 + 100);
}
}
The answers for the two print statements are as follows.
An Association can be defined as a relationship that has no ownership over another. For example, a person can be associated with multiple banks, and a bank can be related to various people, but no one can own the other.
A Copy Constructor in Java is a constructor that initializes an object through another object of the same class.
An empty interface in JavaJava is referred to as a Marker interface. Serializable and Cloneable are some famous examples of Marker Interface.
An ability to recreate an object entirely similar to an existing object is known as Object Cloning in Java. Java provides a clone() method to clone a current object offering the same functionality as the original object.
Java is not considered as a 100% object-oriented programming language because it still makes use of eight or more primitive data types like int, float double, etc.
In Java, when you declare primitive datatypes, then Wrapper classes are responsible for converting them into objects(Reference types).
In Java, when you make the constructor of a class private, that particular class can generate only one object. This type of class is popularly known as a Singleton Class.
The package is a collective bundle of classes and interfaces and the necessary libraries and JAR files. The use of packages helps in code reusability.
Java Virtual Machine takes care of memory management implicitly. Java's primary motto was to keep programming simple. So, accessing memory directly through pointers is not a recommended action. Hence, pointers are eliminated in Java.
For instance, variables are declared inside a class, and the scope is limited to only a specific object.
A local variable can be anywhere inside a method or a specific block of code. Also, the scope is limited to the code segment where the variable is declared.
A collection of strings in Java's Heap memory is referred to as Java String Pool. In case you try to create a new string object, JVM first checks for the presence of the object in the pool. If available, the same object reference is shared with the variable, else a new object is created.
An Exception in Java is considered an unexpected event that can disrupt the program's normal flow. These events can be fixed through the process of Exception Handling.
The term final is a predefined word in Java that is used while declaring values to variables. When a value is declared using the final keyword, then the variable's value remains constant throughout the program's execution.
When the main method is not declared as static, then the program may be compiled correctly but ends up with a severe ambiguity and throws a run time error that reads "NoSuchMethodError."
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JDK is an abbreviation for Java Development Kit. It is a combined Package of JRE and Developer tools used for designing Java Applications and Applets. Oracle has the following variants.
Access Specifiers are predefined keywords used to help JVM understand the scope of a variable, method, and class. We have four access specifiers.
Yes, A constructor can return a value. It replaces the class's current instance implicitly; you cannot make a constructor return a value explicitly.
The term "this" is a particular keyword designated as a reference keyword. The "this" keyword is used to refer to the current class properties like method, instance, variable, and constructors.
The term "super" is a particular keyword designated as a reference keyword. The "super" keyword refers to the immediate parent class object.
The process of creating multiple method signatures using one method name is called Method Overloading in Java. Two ways to achieve method overloading are:
No, Java does not support the Overloading of a static method. The process would throw an error reading "static method cannot be referenced."
Binding is a process of unifying the method call with the method's code segment. Late binding happens when the method's code segment is unknown until it is called during the runtime.
The Dynamic method dispatch is a process where the method call is executed during the runtime. A reference variable is used to call the super-class. This process is also known as Run-Time Polymorphism.
Delete Function is faster in linked lists as the user needs to make a minor update to the pointer value so that the node can point to the next successor in the list
The life cycle of a thread includes five stages, as mentioned below.
Although they look similar, there is a massive difference between both.
The life cycle of an applet involves the following.
Compile-time type safety is provided by using generics. Compile-time type safety allows users to catch unnecessary invalid types at compile time. Generic methods and classes help programmers specify a single method declaration, a set of related methods, or related types with an available class declaration.
The Externalizable interface helps with control over the process of serialization. An "externalisable" interface incorporates readExternal and writeExternal methods.
The Daemon thread can be defined as a thread with the least priority. This Daemon thread is designed to run in the background during the Garbage Collection in Java.
The setDaemon() method creates a Daemon thread in Java.
Enumeration or enum is an interface in Java. Enum allows the sequential access of the elements stored in a collection in Java.
Java is designed to adapt to an evolving environment. Java programs include a large amount of runtime information that is used to resolve access to objects in real-time.
Yes, we can execute any code, even before the main method. We will be using a static block of code when creating the objects at the class's load time. Any statements within this static block of code will get executed at once while loading the class, even before creating objects in the main method.
The finalize method is called the Garbage collector. For every object, the Garbage Collector calls the finalize() method just for one time.
No, "this" and "super" keywords should be used in the first statement in the class constructor. The following code gives you a brief idea.
public class baseClass {
baseClass() {
super();
this();
System.out.println(" baseClass object is created");
}
public static void main(String []args){
baseClass bclass = new baseClass();
}
}
JSP is an abbreviation for Java Servlet Page. The JSP page consists of two types of text.
JDBC is an abbreviation for Java Database Connector.
JDBC is an abstraction layer used to establish connectivity between an existing database and a Java application
Directives are instructions processed by JSP Engine. After the JSP page is compiled into a Servlet, Directives set page-level instructions, insert external files, and define customized tag libraries. Directives are defined using the symbols below:
start with "< %@" and then end with "% >"
The various types of directives are shown below:
It includes a file and combines the content of the whole file with the currently active pages.
Page Directive defines specific attributes in the JSP page, like the buffer and error page.
Taglib declares a custom tag library, which is used on the page.
Objects that inherit the "Observable class" take care of a list of "observers."
When an Observable object gets upgraded, it calls the update() method of each of its observers.
After that, it notifies all the observers that there is a change of state.
The Observer interface gets implemented by objects that observe Observable objects.
A session is essentially defined as the random conversation's dynamic state between the client and the server. The virtual communication channel includes a string of responses and requests from both sides. The popular way of implementing session management is establishing a session ID in the client's communicative discourse and the server.
Spring is essentially defined as an application framework and inversion of control containers for Java. The spring framework creates enterprise applications in Java. Especially useful to keep in mind that the spring framework's central features are essentially conducive to any Java application.
Java Cryptography Architecture gives a platform and provides architecture and application programming interfaces that enable decryption and encryption.
Developers use Java Cryptography Architecture to combine the application with the security applications. Java Cryptography Architecture helps in implementing third party security rules and regulations.
Java Cryptography Architecture uses the hash table, encryption message digest, etc. to implement the security.
The Java Persistence API enables us to create the persistence layer for desktop and web applications. Java Persistence deals in the following:
Authentication options are available in Servlets: There are four different options for authentication in servlet:
Usernames and passwords are given by the client to authenticate the user.
In this, the login form is made by the programmer by using HTML.
It is similar to basic authentication, but the passwords are encrypted using the Hash formula. Hash Formula makes digest more secure.
It requires that each client accessing the resource has a certificate that it sends to authenticate itself. Client Authentication requires the SSL protocol.
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FailFast iterators and FailSafe iterators are used in Java Collections.
FailFast iterators do not allow changes or modifications to the Java Collections, which means they fail when the latest element is added to the collection or an existing element gets removed from the collection. The FailFast iterators tend to fail and throw an exception called ConcurrentModificationException.
Ex: ArrayList, HashMap
Whereas, on the other hand, FailSafe iterators allow changes or modifications to be done on the Java Collections. It is possible, as the FailSafe iterators usually operate on the cloned copy of the collection. Hence, they do not throw any specific exception.
Ex: CopyOnWriteArrayList
The string can be reversed by using the following program.
package simplilearnJava;
public class StringReverse {
public static void main(String args[]) {
String str = "Simplilearn";
String reverse = new StringBuffer(str).reverse().toString();
System.out.printf("Actual Word: %s, Word after reversing %s", str, reverse);
}
public static String reverse(String source) {
if (source == null || source.isEmpty()) {
return source;
}
String reverse = "";
for (int i = source.length() - 1; i >= 0; i--) {
reverse = reverse + source.charAt(i);
}
return reverse;
}
}
Expected Output:
Actual Word: Simplilearn, Word after reversing nraelilpmiS
The Square root of a number can be found by using the following program.
package simplilearnJava;
import java.util.Scanner;
public class SRoot {
public static void main(String args[]) {
try (Scanner sc = new Scanner(System.in)) {
System.out.println("Input a number to find square root: ");
double square = sc.nextDouble();
double squareRoot = Math.sqrt(square);
System.out.printf("The square root is: %f \n", squareRoot);
}
}
}
Expected Output:
Input a number to find square root:
25
The square root is: 5
The program that finds the duplicate elements in a string is written below:
package simplilearnJava;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
public class FindDuplicate {
public static void main(String args[]) {
printDuplicateCharacters("Simplilearn");
}
public static void printDuplicateCharacters(String word) {
char[] characters = word.toCharArray();
Map<Character, Integer> charMap = new HashMap<Character, Integer>();
for (Character ch : characters) {
if (charMap.containsKey(ch)) {
charMap.put(ch, charMap.get(ch) + 1);
} else {
charMap.put(ch, 1);
}
}
Set<Map.Entry<Character, Integer>> entrySet = charMap.entrySet();
System.out.printf("List of duplicate characters in String '%s' %n", word);
for (Map.Entry<Character, Integer> entry : entrySet) {
if (entry.getValue() > 1) {
System.out.printf("%s: %d %n", entry.getKey(), entry.getValue());
}
}
}
}
Expected output:
List of duplicate characters in String 'Simplilearn.'
i: 2
l: 2
The following program can be implemented to remove duplicate elements in an ArrayList
package simplilearnJava;
import java.util.ArrayList;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;
public class ArrayDuplicate {
public static void main(String args[]) {
List<Integer> num = new ArrayList<Integer>();
num.add(1);
num.add(2);
num.add(3);
num.add(4);
num.add(5);
num.add(6);
num.add(3);
num.add(4);
num.add(5);
num.add(6);
System.out.println("Your list of elements in ArrayList : " + num);
Set<Integer> primesWithoutDuplicates = new LinkedHashSet<Integer>(num);
num.clear();
num.addAll(primesWithoutDuplicates);
System.out.println("list of original numbers without duplication: " + num);
}
}
Expected Output:
Your list of elements in ArrayList : [1, 2, 3, 4, 5, 6, 3, 4, 5, 6]
list of original numbers without duplication: [1, 2, 3, 4, 5, 6]
The following program can be used for word count.
package simplilearnJava;
import java.util.HashMap;
public class WordCount {
public static void main(String[] args) {
String str = "Hello World, Welcome to Simplilearn";
String[] split = str.split(" ");
HashMap<String, Integer> map = new HashMap<String, Integer>();
for (int i = 0; i < split.length; i++) {
if (map.containsKey(split[i])) {
int count = map.get(split[i]);
map.put(split[i], count + 1);
} else {
map.put(split[i], 1);
}
}
System.out.println(map);
}
}
Expected Output:
{Hello=1, Simplilearn=1, Welcome=1, to=1, World,=1}
The following program can be used to find the second biggest number in an array list.
package simplilearnJava;
public class NextHighest {
public static void main(String[] args)
{
int array[] = { 1, 2, 3, 4, 11, 12, 13, 14, 21, 22, 23, 24, 31, 32};
int high = 0;
int nextHigh = 0;
System.out.println("The given array is:");
for (int i = 0; i < array.length; i++)
{
System.out.print(array[i] + "\t");
}
for (int i = 0; i < array.length; i++)
{
if (array[i] > high)
{
nextHigh = high;
high = array[i];
}
else if (array[i] > nextHigh)
{
nextHigh = array[i];
}
}
System.out.println("\nSecond Highest is:" + nextHigh);
System.out.println("Highest Number is: " +high);
}
}
Expected Output:
The given array is:
1 2 3 4 11 12 13 14 21 22 23 24 31 32
Second Highest is:31
The highest number is: 32
System.out and System.err represent the monitor by default and thus can be used to send data or results to the monitor. System.out is used to display normal messages and results. System.eerr is used to display error messages. System.in represents InputStream object which by default represents standard input device, i.e., keyboard.
The simplest implementation that can be given is that of a List wherein one can place ordered words and perform a Binary search. The other implementation with a better search performance is HashMap where the key is used as the first character of the word and the value as a LinkedList.
Up another level, there are HashMaps like:
hashmap {
a (key) -> hashmap (key-aa , value (hashmap(key-aaa,value)
b (key) -> hashmap (key-ba , value (hashmap(key-baa,value)
z (key) -> hashmap (key-za , value (hashmap(key-zaa,value)
}
Up to n levels where n is the average size of the word in the dictionary.
Solution - Top 25 Most Frequently asked Pattern Programs in Java
With this, we have come to the end of this Java Interview Questions article. Moving ahead, we will look into the next crucial steps that you could pursue, to master Java.
Java Interview Questions are really important to go through before attending an interview. It helps you to put yourself on the safer side by getting you ready to be able to answer the questions asked in your interview. Now, the next step is to learn How to Become a Software Developer.
If you're looking for more in-depth knowledge about the Java programming language and information on how to get certified as a professional developer, explore our Java training and certification programs, which Simplilearn’s experienced industry experts offer in real-time. In particular, check out our Full Stack Java Developer Master’s Program today!
If you have any questions about this “Java Interview Questions” article, please leave them in the comments section, and our experts will answer them for you, at the earliest!
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Ravikiran A S works with Simplilearn as a Research Analyst. He an enthusiastic geek always in the hunt to learn the latest technologies. He is proficient with Java Programming Language, Big Data, and powerful Big Data Frameworks like Apache Hadoop and Apache Spark.
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