Top 50 Core Java Interview Questions and Answers in 2023

Q1. What is Java?

Java is a high-level, object-oriented programming language that is widely used to develop web applications, mobile applications, and desktop applications.

Q2. What are the features of Java?

Java is known for its simplicity, object-oriented programming, platform independence, secure, multithreading, etc.

Q3. What is JVM?

JVM stands for Java Virtual Machine, it is an abstract machine that provides the runtime environment for Java bytecode to run.

Q4. What is a class?

A class is a blueprint for creating objects that defines the attributes and behavior of an object.

Q5. What is an object?

An object is an instance of a class that has its own state and behavior.

What is a constructor?

A constructor is a special method that is used to initialize objects.

What is inheritance?

Inheritance is a mechanism in Java that allows a class to inherit properties and behavior of another class.

What is an interface?

An interface is a blueprint for classes that define a set of methods and properties that a class must implement.

What is Polymorphism?

Polymorphism is the ability of an object to take on many forms. In Java, it can be achieved through method overloading and method overriding.

What is the difference between method overloading and method overriding?

Method overloading is when two methods have the same name but different parameters. Method overriding is when a subclass provides a specific implementation of a method that is already provided by its parent class.

What is the final keyword in Java?

The final keyword in Java is used to declare a variable or a method that cannot be changed or overridden.

What is the static keyword in Java?

The static keyword in Java is used to declare a method or a variable that belongs to the class and not to the object of the class.

What is a package in Java?

A package is a namespace that organizes a set of related classes and interfaces.

What is the difference between abstract class and interface?

An abstract class is a class that cannot be instantiated and can contain both abstract and non-abstract methods. An interface is a contract that defines a set of methods that a class must implement.

What is the difference between ArrayList and LinkedList?

ArrayList is an array-based implementation of the List interface, while LinkedList is a node-based implementation of the List interface. ArrayList is better for random access while LinkedList is better for sequential access.

What is the difference between a HashMap and a TreeMap?

HashMap is an unordered collection while TreeMap is an ordered collection. HashMap is faster for adding and removing elements while TreeMap is faster for iterating over elements in order.

What is a synchronized keyword in Java?

The synchronized keyword in Java is used to create a mutually exclusive block of code that can be accessed by only one thread at a time.

What is a thread in Java?

A thread is a lightweight process that can run concurrently with other threads.

What is the difference between a process and a thread?

A process is an executing program that has its own memory space, while a thread is a lightweight process that shares memory with other threads in the same process.

What is a deadlock in Java?

A deadlock is a situation where two or more threads are blocked and waiting for each other to release resources.

What is garbage collection in Java?

Garbage collection is the process of automatically freeing memory that is no longer being used by the program.

What is serialization in Java?

Serialization is the process of converting an object into a stream of bytes that can be stored in a file or transmitted over a network.

What is deserialization in Java?

Deserialization is the process of converting a stream of bytes back into an object.

What is a try-catch block in Java?

A try-catch block is used to handle exceptions that may occur during the execution of a program.

What is an exception in Java?

An exception is an event that occurs during the execution of a program that disrupts the normal flow of the program.

What is the difference between checked and unchecked exceptions?

Checked exceptions are checked at compile time and must be handled in the program or declared in the method signature. Unchecked exceptions are not checked at compile time and do not need to be handled or declared.

What is a stack trace in Java?

A stack trace is a list of method calls that shows the path of execution that led to an exception.

What is a package-private access modifier in Java?

The package-private access modifier is used to restrict access to members of a class to only the classes in the same package.

What is the difference between private and protected access modifiers?

Private members are only accessible within the same class, while protected members are accessible within the same class and its subclasses.

What is the difference between a StringBuilder and a StringBuffer?

StringBuilder is not thread-safe while StringBuffer is thread-safe. StringBuilder is faster than StringBuffer in a single-threaded environment.

What is the difference between a for loop and a while loop in Java?

A for loop is used when the number of iterations is known, while a while loop is used when the number of iterations is not known.

What is the ternary operator in Java?

The ternary operator is a shorthand way of writing an if-else statement in a single line.

What is the difference between a break statement and a continue statement in Java?

A break statement is used to exit a loop, while a continue statement is used to skip an iteration and continue with the next iteration.

What is the difference between an abstract class and an interface?

An abstract class can have both abstract and non-abstract methods, while an interface can only have abstract methods. A class can implement multiple interfaces but can only extend one abstract class.

What is the difference between a public and a private constructor in Java?

A public constructor can be accessed by any class, while a private constructor can only be accessed by the same class.

What is a static block in Java?

A static block is a block of code that is executed when a class is loaded into memory.

What is the difference between a final class and a static class in Java?

A final class cannot be subclassed, while a static class is a nested class that can be accessed without instantiating the outer class.

What is a lambda expression in Java?

A lambda expression is a shorthand way of writing anonymous inner classes that implement functional interfaces.

What is the difference between an inner class and a nested class in Java?

An inner class is a non-static nested class that can access the members of its outer class, while a nested class can be either static or non-static and cannot access the members of its outer class.

What is the diamond operator in Java?

The diamond operator is a shorthand way of specifying the type arguments in a generic class.

What is the difference between a method reference and a lambda expression in Java?

A method reference is a shorthand way of referring to an existing method, while a lambda expression is a shorthand way of writing anonymous inner classes that implement functional interfaces.

What is the default value of a Boolean in Java?

The default value of a Boolean is false.

What is the default value of an integer in Java?

The default value of an integer is 0.

What is the default value of a reference type in Java?

The default value of a reference type is null.

What is a stream in Java 8?

A stream in Java 8 is a sequence of elements that can be processed in parallel or sequentially. It provides a functional programming paradigm for processing data.

What is a lambda expression in Java 8?

A lambda expression in Java 8 is a shorthand way of writing anonymous functions that can be passed as arguments to methods or stored as variables.

What are the functional interfaces in Java 8?
Functional interfaces in Java 8 are interfaces that have only one abstract method. They are used to implement lambda expressions and method references.

What is the difference between a consumer and a supplier in Java 8?

A consumer is a functional interface that takes an argument and performs some action on it, while a supplier is a functional interface that does not take any arguments but produces a result.

What is the difference between a flatMap and a map in Java 8?
A map is used to transform each element of a stream into another element, while a flatMap is used to transform each element of a stream into a stream of elements, which are then concatenated into a single stream.

What is reflection in Java?

Reflection is a feature in Java that allows you to inspect and modify the runtime behavior of objects, classes, interfaces, and methods. It provides the ability to analyze and manipulate the structure of an object at runtime, without knowing its type at compile time.

Reflection is achieved through the use of Java's reflection API, which provides classes and methods for inspecting and manipulating objects at runtime. Some common use cases of reflection include creating new objects dynamically, invoking methods dynamically, and inspecting the metadata of classes and objects. However, it's worth noting that reflection can be slow and can introduce security vulnerabilities if used improperly.