Java Interview Questions And Answers 2021

Q 1) What is JAVA? 

 Java is a high-level programming language and is platform-independent. 

Java is a collection of objects. It was developed by Sun Microsystems. There are a lot of applications, websites and Games that are developed using Java. 

 

 

 

Q 2) What are the features in JAVA? 

 Features of Java: 

• Oops concepts 
• Object-oriented 
• Inheritance 

• Encapsulation 
• Polymorphism 
• Abstraction 
• Platform independent: A single program works on different platforms without any modification. 
• High Performance: JIT (Just In Time compiler) enables high performance in Java. JIT converts the bytecode into machine language and then JVM starts the execution. 

• Multi-threaded: A flow of execution is known as a Thread. JVM creates a thread which is called main thread. The user can create multiple threads by extending the thread class or by implementing Runnable interface. 

 

Q 3) How does Java enable high performance? 

Java uses Just In Time compiler to enable high performance. JIT is used to convert the instructions into bytecodes. 

 

Q 4) What are the Java IDE’s? 

 Eclipse and NetBe  are the IDE’s of JAVA. 

 

 

 

Q 5) What do you mean by Constructor? 

 The points given below explain what a Constructor is in detail: 

• When a new object is created in a program a constructor gets invoked corresponding to the class. 
• The constructor is a method which has the same name as class name. 
• If a user doesn’t create a constructor implicitly a default constructor will be created. 
• The constructor can be overloaded. 
• If the user created a constructor with a parameter then he should create another constructor explicitly without a parameter. 

 

Q 6) What is meant by Local variable and Instance variable? 

  Local variables are defined in the method and scope of the variables that have existed inside the method itself. 

An instance variable is defined inside the class and outside the method and scope of the variables exist throughout the class. 

 

Q 7) What is a Class? 

 All Java codes are defined in a class. A Class has variables and methods. 

Variables are attributes which define the state of a class. 

Methods are the place where the exact business logic has to be done. It contains a set of statements (or) instructions to satisfy the particular requirement. 

 

Q 8) What is an Object? 

 An instance of a class is called object. The object has state and behavior. 

Whenever the JVM reads the “new()” keyword then it will create an instance of that class. 

 

 

 

Q 9) What is meant by Method Overriding? 

 Method overriding happens if the sub class method satisfies the below conditions with the Super class method: 

• Method name should be same 
• Argument should be same 
• Return type also should be same 

The key benefit of overriding is that the Sub class can provide some specific information about that sub class type than the super class. 

 

Q 10)What is the difference between an Inner Class and a Sub-Class? 

 An Inner class is a class which is nested within another class. An Inner class has access rights for the class which is nesting it and it can access all variables and methods defined in the outer class. 

A sub-class is a class which inherits from another class called super class. Sub-class can access all public and protected methods and fields of its super class. 

 

Q11) What are the various access specifiers for Java classes? 

 In Java, access specifiers are the keywords used before a class name which defines the access scope. The types of access specifiers for classes are: 

1. Public :Class,Method,Fieldis accessible from anywhere. 
2. Protected:Method,Fieldcan be accessed from the same class to which they belong or from the sub-classes,and from the class of same package,but not from outside. 
3. Default:Method,Field,classcan be accessed only from the same package and not from outside of it’s native package. 
4. Private:Method,Fieldcan be accessed from the same class to which they belong. 

 

Q12)  What’s the purpose of Static methods and static variables? 

 When there is a requirement to share a method or a variable between multiple objects of a class instead of creating separate copies for each object, we use static keyword to make a method or variable shared for all objects. 

 

Q13) What is data encapsulation and what’s its significance? 

 Encapsulation is a concept in Object Oriented Programming for combining properties and methods in a single unit. Encapsulation helps programmers to follow a modular approach for software development as each object has its own set of methods and variables and serves its functions independent of other objects. Encapsulation also serves data hiding purpose. 

 Q14) What is a singleton class? Give a practical example of its usage. 

A singleton class in java can have only one instance and hence all its methods and variables belong to just one instance. Singleton class concept is useful for the situations when there is a need to limit the number of objects for a class. The best example of singleton usage scenario is when there is a limit of having only one connection to a database due to some driver limitations or because of any licensing issues. 

 

 

 Q15) What are Loops in Java? What are three types of loops? 

 Looping is used in programming to execute a statement or a block of statement repeatedly. There are three types of loops in Java: 

1) For Loops 

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

2) While Loops 

While loop is used when certain statements need to be executed repeatedly until a condition is fulfilled. In while loops, condition is checked first before execution of statements. 

3) Do While Loops 

Do While Loop is same as While loop with only difference that condition is checked after execution of block of statements. Hence in case of do while loop, statements are executed at least once. 

 

Q16) What is an infinite Loop? How infinite loop is declared? 

 An infinite loop runs without any condition and runs infinitely. An infinite loop can be broken by defining any breaking logic in the body of the statement blocks. 

 

Q 17) What is the difference between continue and break statement? 

 break and continue are two important keywords used in Loops. When a break keyword is used in a loop, loop is broken instantly while when continue keyword is used, current iteration is broken and loop continues with next iteration. 

 

Q18) What is the difference between double and float variables in Java? 

In java, float takes 4 bytes in memory while Double takes 8 bytes in memory. Float is single precision floating point decimal number while Double is double precision decimal number. 

 

Q 19) What is Final Keyword in Java? Give an example. 

  In java, a constant is declared using the keyword Final. Value can be assigned only once and after assignment, value of a constant can’t be changed. 

 

Q 20) How can you generate random numbers in Java? 

• 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 

 

 

 

 

Q21) What’s the base class in Java from which all classes are derived? 

• java.lang.object 

 

Q 22) Can main() method in Java can return any data? 

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

 

Q23) What are Java Packages? What’s the significance of packages? 

 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. 

 

Q24)Can we declare a class as Abstract without having any abstract method? 

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. 

 

Q25) What’s the difference between an Abstract Class and Interface in Java? 

  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.  

 

Q26) What are the performance implications of Interfaces over abstract classes? 

•  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. 

 

Q27) Does Importing a package imports its sub-packages as well in Java? 

 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.* 

 

Q28) How an object is serialized in java? 

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. 

 

Q 29) When we should use serialization? 

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. 

 

Q 30) Is it compulsory for a Try Block to be followed by a Catch Block in Java for Exception handling? 

 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. 

 

Q 31) Is there any way to skip Finally block of exception even if some exception occurs in the exception block? 

 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: 

• System.exit(0); 

 

Q 32) Can a class have multiple constructors? 

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. 

 

Q 33) Can we override static methods of a class? 

 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. 

 

Q 34) What is multi-threading? 

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. 

 

Q36) Why Runnable Interface is used in Java? 

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

 

Q37) What are the two ways of implementing multi-threading in Java? 

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

1. By usingJava.Lang.RunnableInterface. 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 extendJava.Lang.Threadclass. 

 

Q38) When a lot of changes are required in data, which one should be a preference to be used? String or StringBuffer? 

Since StringBuffers are dynamic in nature and we can change the values of StringBuffer objects unlike String which is immutable, it’s always a good choice to use StringBuffer when data is being changed too much. If we use String in such a case, for every data change a new String object will be created which will be an extra overhead. 

 

Q39) What’s the purpose of using Break in each case of Switch Statement? 

Break is used after each case (except the last one) in a switch so that code breaks after the valid case and doesn’t flow in the proceeding cases too. 

If break isn’t used after each case, all cases after the valid case also get executed resulting in wrong results. 

 

Q40) How garbage collection is done in Java? 

In java, when an object is not referenced any more, garbage collection takes place and the object is destroyed automatically. For automatic garbage collection java calls either System.gc() method or Runtime.gc() method. 

 

Q41) How we can execute any code even before main method? 

If we want to execute any statements before even creation of objects at load time of class, we can use a static block of code in the class. Any statements inside this static block of code will get executed once at the time of loading the class even before creation of objects in the main method. 

 

Q42) How objects of a class are created if no constructor is defined in the class? 

Even if no explicit constructor is defined in a java class, objects get created successfully as a default constructor is implicitly used for object creation. This constructor has no parameters. 

 

Q43) In multi-threading how can we ensure that a resource isn’t used by multiple threads simultaneously? 

In multi-threading, access to the resources which are shared among multiple threads can be controlled by using the concept of synchronization. Using synchronized keyword, we can ensure that only one thread can use shared resource at a time and others can get control of the resource only once it has become free from the other one using it. 

 

Q44) Can we call the constructor of a class more than once for an object? 

 Constructor is called automatically when we create an object using new keyword. It’s called only once for an object at the time of object creation and hence, we can’t invoke the constructor again for an object after its creation. 

 

Q45) There are two classes named classA and classB. Both classes are in the same package. Can a private member of classA can be accessed by an object of classB? 

Private members of a class aren’t accessible outside the scope of that class and any other class even in the same package can’t access them. 

 

Q 46) What is the difference between Error and Exception? 

An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors you cannot repair them at runtime. Though error can be caught in the catch block but the execution of application will come to a halt and is not recoverable. 

While exceptions are conditions that occur because of bad input or human error etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving the user feedback for entering proper values etc. 

 

Q47) How can you handle Java exceptions? 

There are five keywords used to handle exceptions in Java:  

1. try 
2. catch 
3. finally 
4. throw 
5. throws 

Q48) What are the differences between Checked Exception and Unchecked Exception? 

Checked Exception 

• The classes that extend Throwable class except RuntimeException and Error are known as checked exceptions.  
• Checked exceptions are checked at compile-time. 
• Example: IOException, SQLException etc. 

Unchecked Exception 

• The classes that extend RuntimeException are known as unchecked exceptions.  

• Unchecked exceptions are not checked at compile-time. 
• Example: ArithmeticException, NullPointerException etc. 

 

Q 49) How to create a custom Exception? 

To create you own exception extend the Exception class or any of its subclasses. 

• class New1Exception extends Exception { }               // this will create Checked Exception 
• class NewException extends IOException { }             // this will create Checked exception 
• class NewException extends NullPonterExcpetion { }  // this will create UnChecked exception 

 

Q 50) What are the important methods of Java Exception Class? 

Exception and all of it’s subclasses doesn’t provide any specific methods and all of the methods are defined in the base class Throwable. 

String getMessage() – This method returns the message String of Throwable and the message can be provided while creating the exception through it’s constructor. 

String getLocalizedMessage() – This method is provided so that subclasses can override it to provide locale specific message to the calling program. Throwable class implementation of this method simply use getMessage() method to return the exception message. 

Synchronized Throwable getCause() – This method returns the cause of the exception or null id the cause is unknown. 

String toString() – This method returns the information about Throwable in String format, the returned String contains the name of Throwable class and localized message. 

void printStackTrace() – This method prints the stack trace information to the standard error stream, this method is overloaded and we can pass PrintStream or PrintWriter as an argument to write the stack trace information to the file or stream.