Top 20 Core Java Interview Questions and Answers

primitive data types

This question was mainly asked to see if the developer is familiar with the risk of memory leak, which sub-string can create. Public and private access specifiers are methods and instance variables known as members. Public members are visible in the same package as well as outside whereas private members are visible in the same classes only and not for other classes in the same packages. A block of code used to initialize an object is called a constructor. The constructor must share the same name as the class and it has no return type.

Java Vs .NET: Difference between Java and DotNet – Simplilearn

Java Vs .NET: Difference between Java and DotNet.

Posted: Sun, 26 Feb 2023 08:00:00 GMT [source]

The life cycle of a thread includes five stages, as mentioned below. Write a Java program to rotate arrays 90 degree clockwise by taking matrices from user input. A class requesting its dependencies through its function Object() is the most typical instance of dependency injection. Since the client cannot be constructed without the required dependencies, this guarantees that it is always in a correct state.

Can we make the abstract methods static in Java?

Constructors are called when a class instance is created such as creating an object using the new() keyword. When a class does not contain a constructor, a default constructor, provided by the Java compiler, is automatically called. The constructor can be defined as the special type of method that is used to initialize the state of an object.

14 Essential Hadoop Interview Questions and Answers to Know – Solutions Review

14 Essential Hadoop Interview Questions and Answers to Know.

Posted: Thu, 17 Mar 2022 07:00:00 GMT [source]

DriverManager java interview questions for senior developer manages the registered drivers. It can be used to register and unregister drivers. It provides factory method that returns the instance of Connection. Most of the times, web applications are accessed using HTTP protocol and thats why we mostly extend HttpServlet class.

Q30. What is the difference between a local variable and an instance variable?

If there is no constructor in a class, then the compiler automatically creates a default constructor. There are multiple benefits of using blocks in Java. But the main benefit was that we could use a static block to execute the program without the main method. It could be executed without the main method in older versions of JDK. But after the release of JDK 1.7 main method is a must to run a java program. Programmers can access variables and methods inside a class by creating objects, which are instances of the class.


Operator, it will force JVM to create new string in heap . JVM stands for Java Virtual Machine and it is responsible for running Java program in JRE. It is an interpreter as it runs code line by line. It is also responsible for converting the bytecode into native code which is machine readable.

Q What is the difference between ArrayList and vector?

We need to implement Comparable interface to support sorting of custom objects in a collection. The Comparable interface has compareTo method which is used by sorting methods and by providing this method implementation, we can provide default way to sort custom objects collection. We can get runtime binding in composition where inheritance binds the classes at compile time. So composition provides flexibility in the invocation of methods. From Java 8 onwards, interfaces can have default method implementations. So to handle diamond problem when a common default method is present in multiple interfaces, it’s mandatory to provide implementation of the method in the class implementing them.

  • There is no distance because the first element is at the beginning of the array.
  • Reflection is a runtime API for inspecting and changing the behavior of methods, classes, and interfaces.
  • We use the isDaemon() method to check if a thread is a daemon or not.
  • If you create a similar method with the same return type and same method arguments in child class then it will hide the superclass method; this is known as method hiding.
  • The name Qualifier is replaced by Predicate and the function boolean isWorldCupQualifier; is replaced by boolean test;.

If you don’t want to execute the finally block, you need to call the System.exit() method explicitly in the finally block. It cannot be shared between many threads without proper synchronization code.It is synchronized. Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects. Other programming languages like C, C++ are compiled into platform specific binary files with .exe extension.

Due to its ease of use and simplicity, is used almost everywhere in the IT sector. This means that the professionals who are skilled in Java are always required. If you are someone who is likely to attend a job interview that would test you on your Java skills, then please go through the list of Java interview questions and answers provided herein. You may also refer to them just to add on to your knowledge.

This is very must know question for all the interviewees, you will most probably face this question in almost every time you go for interview. When more than one thread try to access same resource without synchronization causes race condition. Difference between starting thread with run() and start() method. JVM is the virtual machine on which java code executes. Checks for referential equality of object, it checks whether two references are referring to same object or not.

Basically all these are just short cut, you should be good on what are you doing. You automatically gain experience while working in Java and should be able to answer any Java question up-to your level. Mostly Immutable classes are also final in Java, in order to prevent subclasses from overriding methods, which can compromise Immutability.

Running state- This stage is when the thread receives a CPU moving from the runnable to the running state. The purpose of encapsulation is to maintain code and protect the code from others. After going to this blog, I realized some important questions I missed earlier. This is Puja Bhardwaj, a creative writer, and content strategist. I’m passionate about storytelling through written and visual content, and market that content for cultivating a committed audience. I come to the table with 5 years of content writing and marketing experience (in the agency, in-house, and freelance writing).

An abstract class is a class that may have the usual flavours of class members (private, protected, etc.), but has some abstract methods. A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class.


When all user threads have completed their execution, the JVM stops itself. If the JVM detects a functioning daemon thread, it stops the thread and then shuts down the entire system. The JVM is unconcerned about whether or not the Daemon thread is executing.

object class

In the Synchronous programming model, assigning tasks to individual threads and making them available for other activities as soon as the allocated task is completed occurs. Asynchronous programming allows numerous threads to work on the same task at the same time, making the individual threads as useful as possible. We use data abstraction when we want to hide details from the user. Data Abstraction is the property that allows the user to see only the essential details. We use interfaces and abstract classes to implement abstraction in Java.

Immutable object is object whose state can not be changed once created. You can take String object as example for immutable object. Constructor should not have any return type else it will be same as method. Inheritance allows to inherit properties and methods of parent class, so you can reuse all methods and properties.

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