Java Interview Question and Answer

1. What are the main features of Java?

Ans. Java has exclusive features that make it very powerful. Some of the notable Java features are:

• It is an object-oriented language, and because of this, it can be extended easily.
• It is Platform independent having the ability to Write Once and Run Anywhere (WORA)
• Secured programming language because it has no explicit pointer and runs inside a virtual machine sandbox
• Interpreted, the Java compiler generates byte-codes, rather than native machine code.
• Robust in terms of memory management, automatic garbage collection,
• It is a high performing language when compared to other traditional interpreted programing language because it uses the Just in Time compiler
• It is multi-threaded, and it shares a common memory area to deal with multiple tasks in a single go.

2. Why Java is not purely an object-oriented language?

Ans. To be a purely object-oriented language, Java must satisfy all the aspects of object-oriented. But because of the primitive data types, it is not purely an object-oriented language. Coders can work using primitive data types – byte, short, int, char, Boolean, and many more.

For example, both types will return the same value:

int i = 10; – Primitive Type

integer i = 10; Object Type

Another factor in Java is that we can always directly represent the static data without instantiation. Further, in Java, one can communicate with objects without actually calling their methods.

3. What is Data Encapsulation? What are the advantages of Encapsulation?

Ans. Data Encapsulation is the process of wrapping code and data into a single unit. It prevents outer classes from accessing and changing the fields and methods of a class. It is one of the four fundamental object-oriented programming (OOP) concepts and helps to achieve data hiding. Encapsulation can be implemented in Java by following the steps below: 

• Declare the variables of a class as private so that they cannot be accessed directly from outside the class.
• Provide public setter and getter methods to set and get (modify and view) the values of the variables.

The following are the advantages of encapsulation in Java: 

• The main advantage of data encapsulation is the ability to modify implemented code without breaking the code of others.
• The encapsulated code is more flexible and easy to change with new requirements.
• It prevents the other classes from accessing the private fields.
• It improves the maintainability of the application.
• Encapsulated code is easy to test for unit testing.

4. What are primitive data types?

Ans. There are 8 data types (Boolean, byte, char, long, float, double, short, and int) that serve as the building blocks of data manipulation in Java.

5. Explain the difference between == and equals?

Ans. equals() and == operator are used to compare objects and find the equality between two objects, but there is some difference between both of them:

• equals() is a method while == is an operator
• == operator is used for reference comparison ( check whether both objects are in same memory) while equals() method is used for content comparison( compare values in an object)
• == never throws NullPointerException

enum1 Color { RED, BLUE };

Color nothing = null;

if (nothing == Color.RED);      // runs fine

if (nothing.equals(Color.RED)); // throws NullPointerEx

• == is subject to type compatibility check at the compile time

enum1 Color { RED, BLUE};

enum1 Chiral { LEFT, RIGHT };

if (Color.RED.equals(Chiral.LEFT)); // compiles fine

if (Color.RED == Chiral.LEFT);      // DOESN’T COMPILE!!! Incompatible types!

Coding example:

Output:

false

True

6. What are the differences between C++ and Java?

Ans. Differences between C++ and Java are:

7. What are the major differences between JDK, JRE, and JVM?

Ans. The difference between JDK, JRE, and JVM are:

8. What do you understand about JVM?

Ans. JVM stands for Java Virtual Machine. JVM is a virtual machine that enables the computer to run the Java program. The Java code is compiled by JVM to be a Bytecode, which is machine-independent and close to the native code.

9. What are the different types of memory areas allocated by JVM?

Ans. Different types of memory areas allocated by JVM are:

• Stack
• Class
• Program counter register
• Native method stack
• Heap

10. Is the JVM platform independent?

Ans. No, JVM is not platform-independent as it is not written in Java.

11. What is a Class in Java?

Ans. In Java, a class represents a defined common set of properties and methods to all objects of the same type. Generally, a class includes components like a modifier, class name, superclass, interface, and body. For real-time java applications, several types of classes are used.

Few ways to create a Class in Java:

• New keywords
• forName method
• Clone () method

12. What are wrapper classes?

Ans. Wrapper classes are used to convert or wrap Java primitives into the reference objects.

Features of java wrapper classes:

• Wrapper classes convert numeric strings into numeric values.
• They are used to store primitive data into the object.
• All wrapper classes use typeValue() method. It returns the object value and its primitive type.
• The wrapper classes use the valueOf() method.

13. What is a pointer? Does Java support pointer?

Ans. A pointer helps to directly access the memory location using the address. Java does not have the concept of a pointer because improper handling of pointers results in memory leaks and other related problems. This makes Java a powerful language than C or C++,

14. What is an immutable object?

Ans. An immutable object cannot be modified once created. Software developers rely on immutable objects for creating simple and reliable codes.

Example:

Output:

ABC

101

15. What are the local variables?

Ans. A variable declared within the body of a method is a local variable. These variables need to be initialized before use.

Syntax

16. What are instance variables?

Ans. A variable declared inside a class, but outside a method is called an instance variable. These variables don’t need to be initialized before use, as they automatically initialized to their default values.

Key features of instance variables:

• They are declared in the class, i.e., outside the method.
• Instance variables are created when the object is created, and it also destroys the purpose.
• Access modifiers are used in the instance variables.
• They are visible to all the methods, constructors, and blocks in the class.
• Default values are given to the instance variables. If the value is numeric, it will be ‘0’, and if the value is boolean then it will be ‘FALSE.’

Example:

17. What is object cloning?

Ans. It is a way to create an exact copy of an object. Clone() is defined in the object class.

18. Can you make an array volatile?

Ans. Yes, you can make an array volatile but only the reference, which is pointing to an array.

19. Which operator is considered to be with the highest precedence?

Ans. Postfix operators.

20. Can a source file have more than one class declaration?

Ans. Yes, a source file can have more than one class declaration.

21. What is the difference between throw and throws?

Ans. A throw is used to trigger an exception, while throws is used in the declaration of exception.

22. What is the JAR file?

Ans. A JAR (Java Archive files) holds java classes in a library.

23. Define the JIT compiler?

Ans. JIT stands for Just-In-Time Compiler. It optimizes the performance of Java applications at run time or execution time. It does so by compiling bytecode into native machine code that can be sent directly to a computer’s processor (CPU). It is enabled by default and is activated when a java method is called. JIT compiler is a part of JVM. When a method has been compiled, the JVM calls the compiled code of that method directly instead of interpreting it. 

JIT compilation requires processor time and memory usage. The JIT compilation includes two approaches to translate code into machine code namely AOT (Ahead-of-Time compilation) and interpretation.  

How does JIT work?

1. The Java source code (.java) converts to byte code (.class) occurs with the help of the javac compiler.
2. The .class files are loaded at run time by JVM. These are converted to machine understandable code with the help of an interpreter.
3. Once the JIT compiler is enabled, the JVM analyzes the method calls in the .class files and compiles them to get more efficient and native code. It also ensures that the prioritized method calls are optimized.
4. Now, the JVM executes the optimized code directly instead of interpreting the code again. This aids in improving the performance and speed of the execution.

24. Is Empty .java file name a valid source file name?

Ans. Yes, the Empty .java file name is a valid source file name as Java allows us to save java file by .java only. First, we need to compile it by using javac .java and run by using java classname.

Example:

25. What is the output of the following Java program?

Ans. Output:

40Javatpoint

Javatpoint2020

26. What is the output of the following Java program?

Ans. Here, the above code will give the compile-time error because there is a need for integer value in the second part of for loop where we are putting ‘0’.

27. Give an example of a ternary operator?

Ans. Example:

28. Name the modifiers allowed for methods in an interface?

Ans. Public and abstract modifiers.

29. Define a constant variable.

Ans. A constant variable should be declared as final and static. E.g.:

30. How can you implement the singleton pattern?

Ans. We can implement a singleton pattern by following common concepts:

• A private static variable of the same class, that is, the only instance of the class
• Private constructor to restrict instantiation of the class from other classes
• Public static method that returns the instance of the class

31. Give one difference between the continue and break statement?

Ans. The continue statement is used to end the current loop iteration while when the break statement is used inside a loop, the loop gets terminated and return at the next statement.

32. What is a reflection in Java programming? Why is it useful?

Ans. It is used to describe code that is used to inspect other code in the same system and make it dynamic and tied together at runtime.

Example:

If you want to call a ‘doSomething’ method in java on an unknown type object. Java static system does not support this until the object confirms a known interface. If you are using reflection, then it takes your code to the object, and it will find the method ‘doSomething.’ After that, we can call the method whenever we want.

Method method_N = foo.getClass().getMethod(“doSomething”, null);

method_N .invoke(foo, null);

33. Is it possible to overload the methods by making them static?

Ans. No, it is not possible to overload the methods by just applying the static keyword to them (the number of parameters and types are the same).

Example:

Output

Demo.java:7: error: method consume(int) is already defined in class Demo

static void consume(int x)

^

Demo.java:15: error: non-static method consume(int) cannot be referenced from a static context

Demomenu.consume(20);

^

2 error

34. What are the types of statements supported by JDBC?

Ans. The different types of statements supported by JDBC:

• Statement: Executes static queries and for general access to the database
• CallableStatement: Offers access to stored procedures and runtime parameters
• PreparedStatement: Provides input parameters to queries when they are executing

35. Mention what will be the initial value of an object reference, which is defined as an instance variable?

Ans. In Java, all object references are initialized to null.

36. What is the difference between StringBuilder and StringBuffer?

Ans. StringBuffer

• Synchronized means two threads can’t call the methods of StringBuffer simultaneously.
• Less proficient than StringBuilder

StringBuilder

• Non-synchronized means two threads can call the threads of StringBuilder simultaneously
• More efficient than StringBuffer

37. Which are the four principle concepts upon which object-oriented design and programming rest?

Ans. Following are four principle concepts upon which object-oriented design and programming rest (A-PIE):

• Abstraction
• Polymorphism
• Inheritance
• Encapsulation

38. How is the queue implemented in Java?

Ans. A queue is known as the linear data structure similar to the stack data structure. It is an interface available in java.util package and used to store the elements with insertion and deletion operations. In this process, the first element is inserted from the one end called REAR (Tail), and the existing element is deleted from the other end called FRONT (Head). The whole operation of queue implementation is known as FIFO (First in first out).

39. Name the two environment variables that must be set to run any Java program.

Ans. PATH variable and CLASSPATH variable are the two environment variables that must be set to run any Java program.

• The PATH environment variable is used to specify the set of directories that contains execution programs.
• The CLASSPATH environment variable specifies the location of the classes and packages.

40. Explain the difference between the Runnable and Callable interface in Java.

Ans. Following are the differences between the Runnable and Callable interface in Java:

41. Explain the difference between the poll() and remove() method of a Queue interface in Java?

Ans. Following are the difference between the poll() and remove() method:

42. How do you avoid deadlock in Java?

Ans. We can achieve this by breaking the circular wait condition. For this, we need to arrange the code such that it imposes the ordering on acquisition and release of locks.

43. How can you implement to use an Object as a Key in HashMap?

Ans. To use any object as a Key in HashMap or Hashtable, it must implement equals and hashcode method in Java.

44. How do you convert bytes to a character in Java?

Ans. This is one of the most commonly asked Java interview questions; here is how you can frame the answer.

We can convert bytes to character or text data using character encoding. Incorrect choice of character encoding may change the meaning of the message as it interprets it differently.

Example:

Output:

byte value: 70

Converted char value: F

45. How do you define Destructors in Java?

Ans. Generally, a destructor is a method that removes an object from the computer’s memory. Java lacks a destructor element, and instead, they make use of a garbage collector for resource deallocation.

46. What is an Anonymous Class?

Ans. As the name suggests, an Anonymous Class is defined without a name in a single line of code using a new keyword.

47. What is the difference between Stack and Queue?

Ans. The stack is based on the Last in First out (LIFO) principle

A queue is based on FIFO (First In, First Out) principle.

48. What is a Package?

Ans. A package is a namespace of related classes and interfaces. They are similar to different folders on your computer.

49. Which package is imported by default?

Ans. Java.lang package is imported by default.

49. How to access Java package from another package?

Ans. We can access the package from outside the package via three ways –

• import package.*;
• import package.classname;
• fully qualified name

Example:

//save by X.java

package p1;

public class X{

public void msg(){System.out.println(“Hey!”);}

}

//save by Y.java

package myp1;

class Y{

public static void main(String args[]){

p1.X obj = new p1.X();

obj.msg();

}

}

Output:

Hey!

50. What is the default value of the local variable?

Ans. The class variables have default values. Local variables don’t have any default value.

51. Can a program be executed without main () method?

Ans. One can execute a program without a main method by using a static block.

52. What are the various access modifiers for Java sessions

Ans. Access modifiers are the keywords that specify the accessibility of a field, class, method, constructor, and other members.

There are four access modifiers in Java:

• Public: The class, method, or other members defined as Public can be accessed by any class or method.
• Protected: The access level is within the package and outside the package through child class.
• Default: The access level is only within the package.
• Private: This can be accessed within the class only.

53. How does Java handle integer overflows and underflows?

Ans. In java, overflow and underflow are handled by using low order bytes that can fit into the size of the type provided by the operation.

54. What is the output of the following Java program?

Ans.

Output

100

55. What is the difference between an object-based programming language and an object-oriented programming language?

Ans. Object-based programming language doesn’t support the features of OOPs except for inheritance. Object-oriented languages are java, C, and C++.

56. What is the name of the package used for matching with regular expressions?

Ans. Java.util.regex package

57. Name frequently used Java Tools.

Ans. Web developers know that Java is one of the most used commercial grade languages. The frequently used Java tools are:

• JDK (Java Development Kit)
• Eclipse IDE
• NetBeans
• JRat
• Junit

58. Explain the implementation of Binary Tree in Java?

Ans. A binary tree is a linear data structure similar to stack, queue, and lists. It consists of two children in a parent node.

By the below code, we can add the starting node (Root) of the binary tree:

59. What is the Executor Framework in Java? How is it different from the Fork Join Framework?

Ans. The Executor framework is used to manage various threads with the help of a group of components. It is used to run the runnable objects without building new threads and use the existing threads.

Example:

Executor Framework is different from the Fork Join Framework:

Fork Join Framework is created to execute ForkJoinTask. It is the lighter version of FutureTask, whereas Executor Framework is created to offer a thread pool that executes the offered task with the use of multiple pooled threads.

There are some ThreadPoolExecutor accessible by JDK API, which is a tread pool of the background thread. Executors.newCachedThreadPool is an unbounded thread used to spawn new threads and reclaim old threads

60. What advantage does the Java layout manager offer over the traditional windowing system?

Ans. Java uses a layout manager to layout components across all windowing platform. They do not have consistent sizing and positioning so that they can provide platform-specific differences among the windowing system.

61. Explain map interface in Java programming?

Ans. A map is an object used for mapping between a key and a value. It does not contain a duplicate key, and each key can map to one value. A map interface is not a subtype of the collection interface. So it acts differently from other collection types.

62. Which Classes Of Exceptions may be caught by a Catch Clause In Java Programming?

Ans. A catch clause can catch any exception that is assigned to the throwable type, including the error and exception types.

63. What do three dots in the method parameters mean? What do the three dots in the following method mean?

Ans. The given code describes that it can receive multiple String arguments.

Syntax:

64. Name the Container Method used to cause Container to be laid out and redisplayed in Java Programming?

Ans. ‘validate()’ Container method is used to cause a container to be laid out and redisplayed in Java Programming.

65. What is a classloader in Java?

Ans. Classloader is a class in java that is used to load other class files from the network, file system, and other sources. Java code in classes is compiled by the javac compiler and executed by JVM. Java stores three built-in classloaders:

• Bootstrap ClassLoader
• Extension ClassLoader
• System/Application ClassLoader

66. What is runtime polymorphism or dynamic method dispatch?

Ans. Runtime polymorphism or dynamic method dispatch is the process in which the overridden methods are resolved at runtime, not at compile time.

67. Does a class inherit constructors of its superclass in Java programming?

Ans. No, A class does not inherit constructs of its superclass

68. How are the elements of a Gridbaglayout arranged in Java programming?

Ans. In Gridbaglayout, elements are arranged in the form of a grid, whereas elements are present in different sizes and occupy the space according to their sizes. It can cover one or more rows and columns of various sizes.

69. What are the common data structures and algorithms used in Java?

Ans. The following most common data structures and algorithms used in Java:

Linear Data Structures

• Arrays
• Linked List
• Stacks
• Queues

Hierarchical Data Structures

• Binary Trees
• Heaps
• Hash Tables

70. What do you mean by loops? What are the three types of loops in Java?

Ans. In Java, Loops execute a set of statements or a block repeatedly until a particular condition is satisfied. The three types of Loops in Java are:

For Loops

For loops execute statements repeatedly for a given number of times. These are used when the number of times to execute the statements is known to the programmer.

While Loops

It is used when certain statements need to be executed repeatedly until a condition is fulfilled. The condition is checked first before the execution of statements.

Do While Loops

It is the same as the While loop with a difference that condition is checked after execution of a block of statements. Thus, statements are executed at least once.

71. How to generate random numbers in Java?

Ans. We can generate random numbers in Java using the following:

• random method
• util.Random class
• ThreadLocalRandom class

72. What is the significance of Java packages?

Ans. A Java package is a set of classes and interfaces that are bundled together in a way that they are related to each other. Java Packages provide multiple benefits, such as protection against name collisions; help organize source code; hide implementation for multiple classes; make searching/locating and usage of classes, control access, and annotations easy.

73. What is an infinite loop?

Ans. The infinite loop is an instruction in which the loop has no exit function, so it repeats endlessly. The infinite loop will terminate automatically when the application ends.

Example:

74. What is the output of the following java code?

Ans. Output: Compile time error

If statement should have the argument of boolean type

75. How can you prevent a method from being overridden?

Ans. Use the final modifier on the method declaration to prevent a method from being overridden.

76. Give an example of a class variable declaration?

Ans. Class variables are declared with static modifiers.

77. What environment variables are required to run java programs?

Ans. PATH and CLASSPATH, JAVA_HOME, and JRE_HOME are needed to run a simple java application and used to find JDK binaries that are used to run java programs on platforms like Windows and Linux.

Environment variables are used in the programs to know which directory is used to install files and where to store installed files.

78. What does JAXB stand for?

Ans. JAXB means Java API for XML binding. It is a fast and suitable way to bind Java representation to incorporate XML data in Java applications.

79. What are the new features for Java 8?

Ans. Java 8 is full of really interesting features at both the language level and the JVM level. Some of the features that are an absolute must to know about are:-

• Parallel operations
• Concurrent accumulators
• Lambda Expressions
• Generic type changes and improvements
• Functional interfaces

80. What type of variables can a class consist of?

Ans. A class comprises of an instance variable, class variable, and local variable.

81. What are the differences between Heap and Stack Memory in Java?

Ans. The differences between heap and stack memory: 

82. Explain the final keyword in Java. 

Ans. final is used as a non-access modifier to restrict the user. A final variable can be used in several contexts, such as variable, method, and class. 

• final variable

When the final keyword is used with a variable then its value cannot be changed once assigned. It will be constant. 

• final method

When a method is declared final, then it cannot​ be overridden or hidden by subclasses.

• final class

When a class is declared as final, it cannot be extended (inherited).

Java OOPs – Constructor Interview Questions

83. What is a constructor?

Ans. A constructor is a block of code that enables you to create an object of the class. It is called automatically when a new instance of an object is created.

84. What is the use of the default constructor?

Ans. A constructor is used to initialize the object. E.g.:

85. Name some methods of an object class.

Ans. Methods of an object class are:

• clone(): Create and returns a copy of an object
• equals(): Compares the equality of two objects
• hashCode(): Returns a hash code value for the object
• getClass(): Returns the runtime class of the object
• finalize(): It is called by the garbage collector on the object
• toString(): Returns a string representation of the object
• notify(), notifyAll(), and wait(): Synchronize the activities of the independently running threads in a program

86. Why do we use the default constructor?

Ans. The default constructor is used to assign the default value to the objects. If there is no constructor in the class then the java compiler will automatically create a default constructor.

Example 1:

Output:

0 null

0 null

Reason: In the given code there is no constructor, so the compiler will automatically provide a default constructor. Where the default constructor provides the null values.

Example 2:

Output:

0 null

0 null

87. What is the output of the following Java program?

Output:

X = 10 y = 15

In this program, the data type of the variables x and y, i.e., byte gets promoted to int, and the first parameterized constructor with the two integer parameters is called.

88. What is the output of the following Java program?

Ans. Output: 0

Here, the variable a is initialized to 0 internally where a default constructor is implied in the class, the variable i is initialized to 0 since there is no constructor in the class.

89. Is it possible to initialize the final blank variable?

Ans. Yes, it is possible to initialize the final blank variable in the constructor, if it is not static. If it is a static blank final variable, it can be initialized only in the static block.

90. What is Polymorphism?

Ans. Polymorphism, an ability to perform single tasks in multiple ways, in Java is divided into two parts – Compile-time and Runtime polymorphism.

• Compile-time also called a static method is a process wherein the call during the overloading method is resolved at the same time of compilation.
• Runtime polymorphism also known as dynamic dispatch has a provision to support the overriding of methods. This is done during the runtime.

91. Mention the differences between the constructors and methods?

Ans. Following are the difference between constructors and methods:

92. What is the output of the following Java program?

Ans. Output

Men class constructor called

Child class constructor called

93. What is a Java copy constructor?

Ans. Java has a special type of constructor called the Copy Constructor in Java that is used for delivering a copy of specified objects. This is mostly used when a coder wants to copy something heavy to instantiate. Also, it is recommended that to detach both objects, use the deep copy. Further, it also offers full control over object creation.

94. Define the advantage of Copy constructor over Object.clone().

Ans. Java experts pick copy constructor over the Object.clone() because the copy constructor does not push to implement any specific interface but one can implement it as and when required. Similarly, it also allows modification in final fields.

95. Please introduce Java Spring Framework.

Ans. Frameworks is a large body of pre-existing codes. It is fast, efficient, and light-weighted. It is used to solve the coder’s specific problems very quickly. Java being a fast-paced programming language has widely used frameworks like JSF, Struts, and play!.

Among these, the Spring framework is one of the most powerful and light-weighted frameworks used for Enterprise Java (JEE). Spring is widely used for developing Java-based applications.

Features of Java Spring Framework

• Extensible XML configuration
• IoC container containing assembler code handling configuration management
• Spring MVC framework
• Support various dynamic languages
• Supports recognized system modules
• Compatible with various versions of Java
• Simple to use, testable,
• Loose coupling because of concepts like AOP

96. Define the Spring annotations that you use in projects.

Ans. Widely used Spring Annotations:

• @Controller
• @RequestMapping
• @Qualifier
• @Scope

97. What is a Java servlet?

Ans. Java developers use Servlets when they need to outspread the capabilities of a server. Servlets are used to extend the applications that are hosted by a web server and it is deployed to design a dynamic web page as well by using Java. These servlets run on JVM and resist attacks. Unlike CGI (Common Gateway Interface) the servlets are portable.

98. Define the steps of creating a servlet in Java.

Ans. Servlet follows a lifecycle having four stages:

• Loading
• Initializing
• Request Handling
• Servlet destroying

To go ahead with the process, the first and foremost thing is to create a servlet. Below is the process:

• Develop a structured directory
• Create the servlet
• Compile
• Add mappings into the web.xml file
• Deploy the project by initiating server
• Access it

99. What is a JSP in Java and are they better than servlets?

Ans. Java Server Pages (JSP) is a server-side technology that is used to create applications, dynamic web content, and independent web pages.

Yes, JSP is a better technology than servlets as they are very easy to maintain. Also, it doesn’t require any sort of recompilation or redeployment. JSP, being an extension to servlets, majorly covers most of its features.

100. What are the different methods of session management in Servlets?  

Ans. There are many ways to manage sessions in Java web applications written using Servlets. Some of the common ways are: 

1. User Authentication – In this, a user can provide authentication credentials from the login page. Then, we can pass the authentication information between server and client to maintain the session.
2. HTML Hidden Field – This allows us to create a unique hidden field in the HTML and when the user starts navigating. We can set its value unique to the user and keep track of the session.
3. Cookies – Cookies are small pieces of information sent by a web server in response header and it gets stored in the browser cookies. When a further request is made by the client, it adds the cookie to the request header and it can be utilized to keep track of the session.
4. URL Rewriting – In this, we can append a session identifier parameter with every request and response to keep track of the session.
5. Session Management API – It is built on top of the above methods for session tracking.

101. Explain the Collection Framework in Java.

Ans. The Java Collection framework provides an architecture to store and manage a group of objects. It defines several algorithms that can be applied to collections and maps. It enables the developers to access prepackaged data structures as well as algorithms to manipulate data.

The Java Collection Framework includes:

• Interfaces
• Classes
• Algorithm

102. What are the benefits of the Java Collection Framework?

Ans. The following are the benefits of the Java Collection Framework:

• Provides useful data structures and algorithms to reduce programming effort
• Increases the speed and quality of the program
• Promotes interoperability among unrelated APIs
• Reduces the effort required to learn, use, and design new APIs
• Encourages reusability of software

103. What is a Linked List? What are its different types?

Ans. A linked list is a linear data structure in which each element is considered as a separate object or entity in itself. Every element within a list consists of the data and the reference to the next node.

The different types of Linked Lists are:

• Singly Linked List: Each node stores two pieces of information – the address of the next node and the data.
• Doubly Linked List: Each node has two references – reference to the next node and the previous node.
• Circular Linked List: All nodes are connected to each other. It can be singly circular or doubly circular.

104. How will you convert an ArrayList to Array and an Array to ArrayList?

Ans. We can convert an Array into an ArrayList by using the asList() method.

Syntax:

An ArrayList can be converted into Array using the toArray() method.

Syntax:

105. What is the emptySet() method in the Java Collections framework?

Ans. The Collections.emptySet() returns the empty (immutable) Set whenever the programmer attempts to remove null elements.  It returns a serializable Set. The syntax of emptySet() is as follows:

Syntax:

106. What is the difference between Concurrent Collections and Synchronized Collections?

Ans. Concurrent collection and synchronized collection are used for thread safety. The difference between them is how they achieve thread-safety by their scalability, performance. The performance of concurrent collection is better than the synchronized collection as it holds the single portion of the map to attain concurrency.

107. What is CopyOnWriteArrayList? How is it different from ArrayList and Vector?

Ans. CopyOnWriteArrayList is used to implement a list interface in the array list. It is a thread-safe variant implemented for creating a new copy of the existing arrays.

ArrayList has a synchronized collection, whereas CopyOnWriteArrayList has a concurrent collection

108. What is a static keyword?

Ans. A Static Keyword is used for memory management to refer to the common property of all objects. If we apply the java static keyword with any method, it is called the static method. Static can be used to class nested with another class, initialization block, process, and variable.

Example:

Output:

Static block initialized.

from main

Value of x: 10

Value of y: 40

109. Define the static method?

Ans. Following are the pointers that define the static method:

• The static method is used to access and change the value of the static variable.
• No object is required to call the static methods.
• A static method is a part of a class, not an object.

Example:

Output:

Static block initialized.

from main

Value of x: 10

Value of y: 40

110. Can the non-static variable be accessed in a static context?

Ans. No, the non-static variable cannot be accessed in a static context.

111. Can you override the static method?

Ans. No, a static method cannot be overridden because they are resolved in compile-time and not runtime.

112. Explain what the static block is?

Ans. A static block is used to initialize the static data member in the program. It is executed before the main method.

Output:

static block is invoked

Hello Learners

113. Is it possible to execute a program without main() method?

Ans. Yes, it is possible to execute a program without main() method where one can execute the program using a static block.

114. What if the static modifier is removed from the signature of the main method?

Ans. If the static modifier is removed from the signature of the main method, then the program compiles. But, at runtime, it throws an error “NoSuchMethodError.”

115. Is it possible to declare the static variables and methods in an abstract class?

Ans. Yes, It is possible to declare static variables and methods in an abstract method. There is no need to make the object access the static context. We can access the static context declared inside the abstract class by using the name of the abstract class.

Example:

Output

Naukri

i = 2020

116. What is an abstract method?

Ans. It is a function that is declared without an implementation. It lies in the heart of inheritance when you define certain tasks functions that are dealt with within the child’s classes differently.

Syntax

117. What is the abstract class in Java? Is it similar or different when compared to C++?

Ans. The abstract class generally contains one or more than one abstract method, and these abstract methods are declared, but they do not contain implementation. In Java, to make a class abstract, a separate keyword is required ‘abstract.’ But in C++, if any class comprises at least one pure vital function, then the class automatically becomes abstract.

Example:

118. When should we use abstract classes?

Ans. We use interfaces when something in our design changes frequently.

119. What is the base class of all classes?

Ans. Java.lang.object

120. How “this” keyword is used in the class?

Ans. Following are the uses of this keyword in the class:

• Can be used as an argument in the method call.
• It is used as an argument in the constructor call.
• It is used to refer to the existing class instance variable.
• Can be used to assign current class methods.
• It can be used to return the existing class instance from the method.

121. Do we use this keyword to refer to static members?

Ans. Yes, we can use this keyword to refer to static members because this is used as a reference variable that refers to the current class object.

Example:

Output:

50

122. What do you mean by Super? What are the main uses of the super keyword?

Ans. The super keyword in Java is used to refer to the immediate parent class object as it is a reference variable. When there is any instance created of the subclass, then the instance of the parent class is created implicitly, which is referred to by a super reference variable.

Super () keyword is used to call the main methods and also access the superclass constructor. It is also used to differentiate between the methods with the same name of superclasses and subclasses. The following are the uses of the super keyword:

• The super keyword is used as the immediate parent class instance variable.
• It can be used to instance the immediate parent class method.
• Super() can be used to request a parent class constructor.

Output:

Student is created

Jack is created

123. Name the class considered as the superclass for all the classes?

Ans. The object class is considered as the superclass of all other classes in Java.

124. Why Java doesn’t support multiple inheritances yet multiple interface implementation is possible?

Ans. Multiple inheritances refer to the process by which one inherits the properties and behavior of multiple classes into one single class like in C++. But Java being a simple, robust, and secure language, it omits multiple inheritances usually called the Diamond Problem.

In Java 8, it supports default methods and that’s how a class can go ahead with the implementation of two or more interfaces.

125. How can you do constructor chaining using this keyword?

Ans. Constructor chaining is used to call one constructor from another constructor of the class as per the current class object. This keyword is used to perform constructor chaining within the same class.

Example:

Output

ID: 10121 Name:Ron age:22 address: Boston

126. Is it possible to use this() and super() both in a constructor?

Ans. No, because this() and super() must be the first statement in the class constructor.

Example:

Output:

Demo.java:5: error: the call to this must be the first statement in the constructor

127. How to restrict inheritance for a class?

Ans. We can restrict inheritance for a class by:

• Using the final keyword
• Making a constructor private
• Using the Javadoc comment (” // “)
• Making all methods final, so that we cannot override them

128. Explain the difference between aggregation and composition in java?

Ans. The aggregation offers a weak relationship in which both objects can exist independently whereas composition allows a strong relationship in which if an object owns another object and that object cannot exist without the owner object then it is said to be composition.

Example of Composition

Example of Aggregation

129. What is the difference between Method Overloading and Overriding?

Ans. The differences between Method Overloading and Overriding are: 

130. Is it possible to override private methods?

Ans. No, it is not possible to override the private methods because the scope of private methods is limited to the class, and we cannot access these methods outside of the class.

131. Explain what is the covariant return type?

Ans. In Java, it is possible to override any method by altering the return type if the return type of the subclass overriding method is the subclass type, which is known as a covariant return type. The covariant return type defines that the return type may vary in the same direction as the subclass.

Output:

Welcome to the covariant return type

132. What do you mean by method overriding?

Ans.

If the child class has the same method as declared in the parent class, this is called method overriding. It allows a subclass or child class to provide a specific implementation of a method that is already provided by one of its super-classes or parent classes.

Program to illustrate Java Method Overriding  

class Vehicle {

   public void displayInfo() {

      System.out.println(“It is a Vehicle.”);

   }

} 

class Car extends Vehicle {

   @Override

   public void displayInfo() {

      System.out.println(“It is a car.”);

   }

}

class Main {

   public static void main(String[] args) {

      Car c1 = new Car();

      c1.displayInfo();

   }

}

Output:

It is a car.

133. What do you understand by thread-safe?

Ans. It is simply the program that behaves correctly when multiple simultaneous threads are using a resource.

134. Which Swing methods are thread-safe?

Ans. The thread-safe Swing methods are:

• Repaint
• Revalidate
• Invalidate

135. Explain the different states of a thread.

Ans. The different states of a thread are:

• Ready State: A thread is created.
• Running State: Thread currently being executed.
• Waiting State: A thread waiting for another thread to free certain resources.
• Dead State: A thread that has gone dead after execution.

136. Explain multithreading in Java.

Ans: Multithreading is a programming feature that allows multiple tasks to run simultaneously within a single program. In simple terms, it enables concurrent execution of two or more parts of a program for maximum utilization of CPU. Each program is called a thread. In Java, threads are light-weight processes within a process. They run in parallel and help in the performance improvement of any program.

137. How can you implement multithreading in Java?

Ans. We can implement multithreading in Java by using the following:

• By implementing the Runnableinterface.
• By writing a class that extends Java.Lang.Thread class.

138. What is thread-safety in Java? How do you achieve it?

Ans. Java provides a multi-threaded environment, multiple threads are created from the same object share object variable, and it will cause data inconsistency when threads are used to update and read shared data. JVM helps in improving the performance of applications by running bytecode in different worker threads.

Different methodologies to achieve thread-safety:

• Stateless implements
• Synchronized collections
• Thread-Local Fields
• Atomic objects
• Concurrent collections
• Immutable implementations

139. What is the objective of the Wait(), Notify(), And Notifyall() methods in Java programming?

Ans. In Java programming, the wait(), notify(), and notifyAll() methods are used to provide a systematic way for threads to communicate with each other.

140. What is synchronization in Java?

Ans. Synchronization is used to control the access of multiple threads in the shared resources to provide better results. It also helps to prevent thread interference and solve consistent problems.

There are two types of synchronization:

• Process Synchronization
• Thread Synchronization

Example:

Q141. When throws keyword can be used?

Ans. Providing information to the programmer that there may happen an exception so that the normal flow can be maintained, Throws keyword is used to declare an exception.

Syntax of java throws

142. Explain NullPointerException.

Ans. NullPointerException in Java is a RuntimeException. A special null value can be assigned to an object reference in Java. NullPointerException is thrown when a program attempts an object reference with the null value.

Null Pointer Exception is thrown in specific scenarios like:

• Method invoked using a null object.
• Accessing or modifying a null object’s field.
• When a null object is passed as an argument to a method
• Taking the length of a null array.
• Accessing or modifying the slots of a null array.
• Throwing a null object.
• Synchronizing a null object.

143. How can you avoid NullPointerException in Java?

Ans. We can avoid NullPointerException by the following:

• Use valueOf() over toString() where both return same result
• Avoid unwanted autoboxing and unboxing in the code
• Use null safe methods and libraries
• Avoid returning null from a method
• Use apache commons StringUtils for String operations
• Avoid passing of Null Parameters

144. How to avoid ConcurrentModificationException?

Ans. We can avoid the Exception by using Iterator and call remove():

Iterator<String> iter1 = myArrayList.iterator();

while (iter1.hasNext()) { String str = iter1.next();

if (someCondition) iter1.remove(); }

145. What is the purpose of garbage collection in Java, and when is it used?

Ans. Garbage collection is an automatic process. The memory is allocated to all the java programs. At the run time, there are some objects created at the heap and take the memory space. The garbage collectors delete these unwanted objects.

146. What is serialization in Java?

Ans. In this process, objects are converted into byte codes to store the object in memory space. The main task is to save the state of the object in order.

147. When we should use serialization?

Ans. Serialization is used when data is transmitted over the network. Serialization enables us to save the object state and convert it into a byte stream that can be sent over the network.  When the byte stream transfers over the network, the object is re-created at the destination.

148. When we use the Serializable vs. Externalization interface?

Ans. Following are the difference between Serializable and Externalization:

149. Explain JSON.

Ans. JSON stands for JavaScript Object Notation. It is a lightweight data-interchange format that is used as an alternative to XML. It is language-independent and is easy to understand. JSON uses JavaScript syntax, but its format is text only. The text can be read and used as a data format by any programming language.

150. What are the advantages of JSON over XML?

Ans. The advantages of JSON are:

• JSON is lighter and faster. The XML software parsing process can take a long time.
• The structure of JSON is straightforward, readable, and easily understandable.
• JSON objects and code objects match, enabling us to quickly create domain objects in dynamic languages.
• JSON uses a map data structure unlike the tree structure in XML.
• While JSON supports multiple data types—string, number, array, or Boolean – XML data are all strings.