Top 50 Java Interview Questions and Answers That You Must Know

Introduction#

Programming languages(JAVA, C, C++, Python) help us to communicate with computers. A paradigm (basically in programming) is a pattern that guides how programming languages are structured to solve different problems. OOP (Object Oriented Programming) is a programming paradigm, based on the usage of objects.

List of Top 50 Mostly Asked Java Questions for Interview with Proper Answers (2025)#

All questions are suitable for freshers and experienced everyone, and all answers are suitable for interview purposes for 2025.

Top 20 mostly asked Java Interview Questions on Core Java#

1. What is Java?#

Java is a high-level and platform-independent programming language, designed by James Gosling at Sun Microsystems. Java follows the Object-Oriented Programming Paradigm. Java allows Primitive Datatypes. For writing, compiling, and debugging Java programs, we need JDK (including JRE, JVM, and Development tools).

2. Features of Java?#

Java has so many strong and good features, making it more versatile and powerful for solving any problems and building diverse applications. All features are understandable for all types of students.

Let’s talk about the Features of Java :

1. Platform Independent: No need to depend on any platform for code execution. Java follows the ‘Write once, Run anywhere’ rules. Java-written codes (.java files) are converted into Java bytecodes (.class flies) by the Java compiler (javac). These bytecodes are interpreted and ready to execute on any platform by the JVM.
2. Object Oriented: A design around objects (representation of real-world entities). ****By following the OOP paradigm Java enables reusable, modular, flexible, and secure code.
3. Simple and Familiar: Syntax of Java is as easy, clean, and understandable as C, and C++ languages. If you are familiar with these languages (C, and C++) then it will be easy to learn Java also. These make Java simple and familiar.
4. Secure: Java does not allow the Pointer concept (inside pointer we directly access the memory). Java uses references that have been designed to prevent memory stolen. For these Java is a more secured language.
5. Robust: Java offers automatic memory management, and automatic garbage collection to prevent unnecessary memory leaks. To prevent errors, Java uses comprehensive exception handling mechanism. These make Java robust.
6. Multithreaded: Java ****supports multithreading to execute multiple threads concurrently. This optimizes CPU usage.
7. High-Performance: Just In Time (JIT) compiler translates Java bytecodes to native machine code (Machine understandable code) at the time of run-time. This process is a faster way for application execution. These make Java a high-performance language.
8. Distributed: Java is distributed because it allows applications to run on multiple computers or servers of a network with some features like Remote Method Invocation, Enterprise JavaBeans, etc.
9. Network-centric: For real-time communication with private networks, Java provides built-in networking support to create networking-based applications. That makes Java as a network-centric.
10. Versatile: Java is versatile for the usage of so many different fields ( Android application development, Web application development, IoT devices, gaming, etc.).
11. Dynamic: Java supports dynamic memory allocation and allows the loading of all classes at the run-time, which makes Java dynamic.
12. Extensible: Java supports external models and libraries which makes Java extensible.

3. What is JVM?#

Java Virtual Machine (JVM) is a process virtual machine. It provides a runtime environment (Java Runtime Environment) to execute the byte code of a single application independently.

JVM is a runtime engine. Java’s Write Once Run Anywhere (WORA) behavior is enabled by using JRE, Java Runtime Environment is a core component of JVM for executing the java byte code.

At the time of compilation, the source code (.java files) converts to the Java byte code (.class files) with the use of the javac compiler. This byte code is processed to execute with the help of multiple stages of execution of JVM.

JVM is responsible for making Java Platform-Independent, Secure, High-performance, and Robust.

4. Difference between JVM, JRE, and JDK?#

Java Virtual Machine (JVM) is a process virtual machine. It provides a runtime environment (Java Runtime Environment) to execute the byte code of a single application independently.

Java Runtime Environment(JRE) is a core component of JVM for executing the java byte code.

Java Development Kits (JDK) is used to develop, compile, and run Java applications. JDK is a combined version of JRE + Developments Tools and JRE is a Combined version of JVM + Libraries + Others tools.

5. Why java is Platform Independent language?#

At the time of compilation, the source code (.java files) converts to the Java byte code with the use of the javac compiler. Java’s Write Once Run Anywhere (WORA) behavior is enabled by using JRE, Java Runtime Environment is a core component of JVM for executing the java byte code. JVM is responsible for making Java Platform-Independent.

6. Why java is Object Oriented Language?#

Java is an Object-Oriented Programming (OOP) language because it is based on objects and classes and follows four OOP principles:

1. Encapsulation – Data hiding using private fields and access through methods.
2. Abstraction – Hides implementation details using abstract classes & interfaces.
3. Inheritance – Reusability of code using extends (class) & implements (interface).
4. Polymorphism – Same method behaves differently in different classes (overloading & overriding).

7. Why java is not fully Object-oriented?#

Java is not 100% Object-Oriented because:

• It supports primitive data types (int, char, float, etc.), which are not objects.
• It allows static methods and variables, which belong to a class rather than an object.

8. What is Constructor? Why do we need Constructors?#

Constructors look like methods or functions. Constructors have no return type. Constructors are automatically called when an object is instantiated.

Why do we need Constructors?

• Initialization: Constructors allow setting initial values for object attributes during object creation.
• Object Creation Process: They ensure that objects are properly set up before they are used, preventing errors due to uninitialized data.
• Custom Initialization: You can define multiple constructors with different parameters to provide flexibility in object creation.

9. What is the difference between == and equals() in Java?#

== : is used to compare object references or primitive values.

public class EqualsOperator {
    public static void main(String[] args) {

        int l1 = 5;
        int l2 = 5;
        boolean b2 = (l1==l2);  //true

         }
}

equals() : Compares the specified object with this list for equality.

public class EqualMethod {
    public static void main(String[] args) {

        List<Integer> l1 = new ArrayList<>(5);
        List<Integer> l2 = new ArrayList<>(5);

        boolean b1 = l1.equals(l2);   //true
        
        }
}

10. Explain the difference between String, StringBuilder, and StringBuffer#

String is immutable and not thread-safe, ideal for fixed text.

StringBuilder is mutable, non-thread-safe, and offers better performance in single-threaded scenarios.

StringBuffer is also mutable but thread-safe, making it suitable for multi-threaded environments, though it’s slower than StringBuilder due to synchronization overhead.

11. What is the purpose of static keyword?#

The static keyword in Java is used to define class-level variables, methods, and blocks that belong to the class rather than individual instances. Static variables are shared among all instances of a class, while static methods can be called directly on the class and can only access static members. Static blocks are used for initializing static variables, and static nested classes can exist independently of the outer class's instances.

12. What is the String pool in Java?#

The String Pool in Java is a memory area in the Heap that stores string literals to optimize memory usage by reusing identical string values. When a string literal is created, Java checks the pool to see if it already exists and reuses it; otherwise, it adds the new string to the pool. Strings created using new are stored outside the pool.

13. What is garbage collection in Java?#

Garbage Collection (GC) automatically removes unused objects from memory, preventing memory leaks and optimizing performance. It uses a mark-and-sweep algorithm and calls finalize() before object removal. GC ensures efficient memory management, reducing manual effort and improving application stability. Calling the gc method suggests that the Java Virtual Machine expends effort toward recycling unused objects to make the memory they currently occupy available for reuse by the Java Virtual Machine. The call System.*gc*() is effectively equivalent to the call: Runtime.*getRuntime*().gc().

14. What is the role of the this keyword in Java?#

this is a way to refer to and interact with the current object, improving code clarity and flexibility.

this refers to the current instance of the class. It helps differentiate instance variables from local variables, invoke methods and constructors of the current object, and return the current object itself from a method. We mostly use to remove any Ambiguity in variable.

class Car {
    String model;
    
    //constructors
    Car(String model) {
        this.model = model;  // 'this.model' refers to the instance variable
    }
    
    //methods
    Car getCar() {
        return this;  // Returns the current object
    }
    
     void start() {
        System.out.println("Car started");
    }

    void drive() {
        this.start();  // Calls the current class method
    }
    
    
    
}

15. What is the difference between a constructor and a method?#

Both are defined as functions, but they are declared differently.

Constructor:

• The constructor name is the same as the classes. Constructors have no return type, not even void.
• Constructors are used to initialize objects when they are created.
• Constructors are automatically called when an object is instantiated.

public class Animal{

   int a;

   //default constructor
   Animal(){
      System.out.println("This is a Constructor");
   }
   
   //parameterized constructor
   Animal(int a){
      a=this.a;
   }
   public static void main(String[] args) {
       Animal obj1 = new Animal();
       Animal obj2 = new Animal(8);
      }
}

Method:

• The method can have any name, following the class naming conventions.
• Methods can return values (any type, including void).
• Methods are used to perform specific actions or behaviors.
• Methods must be explicitly called by an object or class.

class Car {
    String model;

    // Method
    void startEngine() {
        System.out.println("Engine started!");
    }
}

public class Main {
    public static void main(String[] args) {
        Car car1 = new Car();
        car1.startEngine();  // Method is called here
    }
}

16. How does Optional work in Java 8?#

Optional<T> is one of the many containers newly introduced by JAVA 8 in its java.util package. This class is all about the situations where a value may be present or absent; hence, there will be no more null. This would make for minimal use, or no use at all, of NullPointerException.

17. What is dependency injection in Java?#

Dependency Injection (DI) can be defined as a design pattern in Java in which the concerned object receives its other associated dependencies from outside its domain instead of creating them internally. This loose coupling makes the code very maintainable, testable, and scalable.

Types of Dependency Injection

Constructor Injection (Injection through Constructor) – Dependencies are injected through the constructor.

public class Service {
    private Repository repository;
    public Service(Repository repository) {
        this.repository = repository;
    }
}

Setter Injection – It is the injection of dependencies through setter methods.

public class Service {
    private Repository repository;
    public void setRepository(Repository repository) {
        this.repository = repository;
    }
}

Field Injection – Injecting dependencies directly into fields by using annotation (Spring Framework).

@Autowired
private Repository repository;

18. What are the advantages of using Java Generics?#

Generics are Parameterized Typed. By using generics, it is possible to create classes, interfaces, and methods that work for different data types. You can use Generics methods, interfaces, and fields for multiple different data types.

• Type Safety – Prevents ClassCastException by enforcing type checks at compile-time.
• Code Reusability – Generic classes and methods can work with multiple data types.
• Elimination of Type Casting – Reduces the need for explicit casting, making code cleaner.
• Improved Readability & Maintainability – Enhances code clarity and reduces errors.
• Better Performance – Compile-time checks improve efficiency by avoiding runtime overhead.

19. Java 17 features#

Oracle introduced Java 17, the LTS Java version, in September 2021 to improve and upgrade the application’s stability, performance, and garbage collection to enhance security and API. Java 17 resolves these issues with Sealed Classes, Pattern Matching, Records, ZGC, and better API security.

20. What is the Collectors class in Java 8?#

The Collectorsclass in Java 8 is part of the package java.util.stream, and it gives static factory methods to collect stream elements into various data structures such as lists, sets, and maps and to perform accumulation operations on them.

Top 20 mostly asked Java Interview Questions on OOPs And Collection Framework#

1. OOPs Concept: 4pillars of OOPs#

OOP (Object Oriented Programming) is a programming paradigm, based on the usage of objects. Java, C++, etc. are an Object-Oriented Programming (OOP) language because it is based on objects and classes and follows four OOP principles:

• Encapsulation: Restricting direct access to data and allowing controlled access via methods. To achieve encapsulation, we use different access modifiers (to get access publicly and privately…), these make code more secure and reusable. Using getter setter to get the access of private properties.
  • Examples: Daily wearing Bags: The books inside a bag are private (protected from direct access). You can only access them using a zipper (getter method) or place books inside using an opening (setter method).

class Bags {
    private String book;   // Private property (data hiding)
    
    public String getBook() { return book; }    //you can get a private variable
    
    public void setBook(String bookName) { this.book = bookName; }   //you can set the value of a private variable
}

• Abstraction: Hiding implementation details and showing only essential features. It improves code simplicity and usability.
  • Examples: A TV : You use a remote to change channels (essential functionality).You don’t need to know how internal circuits work.

abstract class TV {
    abstract void changeChannel();  // Abstract method (no implementation)
}

class SmartTV extends TV {
    @Override
    void changeChannel() {  
        System.out.println("Switching to next channel...");
    }
}

• Inheritance: Enabling one class to inherit properties and behavior from another. It reduces code duplication and makes maintenance easier.
  • Examples: Family Inheritance: A child inherits features from parents, like eye color and habits. The child can also have their own unique features.

class Parent {
    void nature() {
        System.out.println("Parent is kind and hardworking.");
    }
}

class Child extends Parent {
    void hobby() {
        System.out.println("Child loves painting.");
    }
}

public class FamilyTest {
    public static void main(String[] args) {
        Child c = new Child();
        c.nature();  // Inherited from Parent
        c.hobby();   // Child's own method
    }
}

• Polymorphism: Allowing methods to perform differently based on the object type (overloading & overriding) improves code flexibility.
  • Examples: A Person in Different Roles: A teacher in class (explains topics). The same person at home (cooks, relaxes).

class Person {

    void cook() {
        System.out.println("Person is cooking at home");
    }
    
    void cook(String CookingItems) {   // Method Overloading - Compile-time Polymorphism
        System.out.println("Person is cooking " + CookingItems + " at home");
    }
    
    
}
class Teacher extends Person {
    @Override
    void cook() {    // Method Overriding - Runtime Polymorphism
        System.out.println("Person is cooking at school");
    }
}

2. What is the difference between abstraction and encapsulation?#

• Abstraction focuses on hiding implementation details and exposing only the essential features of an object. It simplifies complexity by allowing the user to interact with a system without worrying about the inner workings. It is achieved using abstract classes and interfaces. The goal is to show what an object does, not how it does it. An example of abstraction is an ATM: users can withdraw money without needing to know the internal workings of the ATM.
• Encapsulation, on the other hand, is about protecting data and restricting access to it. This is done by making fields private and providing controlled access through public methods like getters and setters. The focus here is on how the data is accessed and modified, ensuring the internal state of an object is protected from unauthorized access. A good example of encapsulation is a bank account: the balance is private, and users can only interact with it through controlled methods like deposit and withdrawal.

3. What is an abstract class and how is it different from an interface?#

• An abstract class is a class that cannot be instantiated on its own and is meant to be extended by other classes. It can have both abstract (without implementation) and concrete (with implementation) methods, as well as instance variables and constructors. Abstract classes are used when you want to provide a common base with shared behavior and force subclasses to implement specific methods.
• An interface, on the other hand, defines a contract of methods that a class must implement. Before Java 8, interfaces could only contain abstract methods, but from Java 8 onwards, they can also contain default methods (with implementation), static methods, and private methods. Interfaces allow for multiple inheritance, meaning a class can implement multiple interfaces, while abstract classes allow single inheritance.

4. What is a Functional Interface in Java 8?#

A Functional Interface in Java 8 is an informative annotation type used to indicate that an interface type declaration is intended to be a functional interface as defined by the Java Language Specification. Conceptually, a functional interface has exactly one abstract method. Since default methods have an implementation, they are not abstract. These interfaces are intended to be used with lambda expressions or method references, enabling functional programming paradigms in Java.

@java.lang.FunctionalInterface
interface FunctionalInterface {
    int add(int a,int b);
    
}

5. What are lambda expressions in Java? What are the benefits of using Java 8 Lambda expressions?#

Lambda expressions in Java, introduced in Java 8, provide a concise way to represent methods that can be passed around as arguments. They consist of parameters, an arrow token (->), and a body. The benefits include more concise and readable code, support for functional programming, improved performance, cleaner code with functional interfaces, and easier parallel processing using streams. Lambda expressions enhance Java's ability to handle functional programming patterns, making the code more expressive and efficient.

public class Lambda {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
        numbers.stream().filter(n -> n % 2 == 0).forEach(System.out::println);

    }
}

6. Explain the Comparable and Comparator interfaces in Java.#

The Comparable interface is used to define a natural ordering of objects within the class itself by overriding the compareTo() method, typically for a single sorting criterion.

The Comparator interface, on the other hand, allows for multiple sorting orders by implementing the compare() method in a separate class or as a lambda expression. While Comparable is for default sorting, Comparator is used when different sorting strategies are needed.

7. What is the use of default methods in interfaces?#

Default methods in interfaces (introduced in Java 8) allow an interface to provide a method implementation. They also help resolve ambiguity in multiple inheritance scenarios.

8. What are the advantages of using the interface in Java?#

• Multiple Inheritance: They allow classes to implement multiple interfaces, supporting multiple inheritance.
• Loose Coupling: Interfaces promote decoupling between components, making the code more maintainable and testable.
• Abstraction: They define behavior without implementation details, achieving abstraction.
• Code Reusability: Multiple classes can implement the same interface, enabling code reuse.
• Flexibility: They allow easy changes to the implementation without affecting the rest of the code.
• Polymorphism: Interfaces enable polymorphism, allowing different classes to be treated uniformly.
• Functional Programming Support: Java 8 interfaces can have default methods supporting functional programming.

9. What are the different types of access modifiers in Java?#

To achieve data hiding we need access modifiers. Access modifiers in Java are used to control the visibility and accessibility of classes, methods, constructors, and variables.

public: We can access the public members from anywhere (Universally accessible).

public class Animal {
    public String name;
}

private: We can access the private members only from the same class. If you want to access it from outside, then you have to generate a getter-setter method (public method) inside that class where you define the private members.

class Animal {
    private String name;  // Not accessible outside the class
}

protected: We can access the protected members within the same package and by subclasses (including subclasses in other packages).

class Animal {
    protected void sound() {
        System.out.println("Animal makes a sound");
    }
}

default: We can access only within the same packages. When no access modifier is specified, it is considered default or package-private.

class Animal {
    void sound() {  // No modifier, so it's package-private
        System.out.println("Animal makes a sound");
    }
}

10. Explain the concept of polymorphism in Java.#

The term "polymorphism" is derived from the Greek words "poly" (meaning many) and "morph" (meaning form). It refers to the ability of an object to take on multiple forms.

It has two types:

1. Compile-time Polymorphism (Method Overloading): Same method name with different parameters, resolved at compile time.
2. Runtime Polymorphism (Method Overriding): A subclass redefines a superclass method, with the method to be called determined at runtime based on the object type.

Polymorphism enhances flexibility and code reusability, allowing methods to behave differently based on input parameters or object type.

11. What is the role of the super keyword in Java?#

The super keyword in Java is used to refer to the parent class members (methods, fields, constructors) from a child class. It allows calling overridden methods, accessing parent class constructors, and resolving conflicts between parent and child class members with the same name. It enables better control and interaction in an inheritance hierarchy.

class Animal {   //parent class
    
    String name;  //parent Class Fields:

    Animal(String name) {  //parent class constructor
        System.out.println("Animal name: " + name);
    }

    void sound() {  //parent class method
        System.out.println("Animal makes a sound");
    }
}

class Dog extends Animal {   //child class
    
    String name = super.name;  //access Parent Class Fields

    Dog(String name) {
        super(name); // calls the constructor of Animal class
        System.out.println("Dog created");
    }

    void sound() {
        super.sound(); // Calls the method from Animal class
        System.out.println("Dog barks");
    }
}

12. What is method overloading and method overriding?#

Method Overloading :

1. Same Class
2. Same Method name

class Animal {
    void sound() {
        System.out.println("Animal makes a sound");
    }
    
    void sound(String animalType) {
        System.out.println(animalType+" makes a sound");
    }
}

• Method Overriding:

1. Same method name
2. Same return type
3. Same parameter

different class (one is super/parent class another is/are sub/child class).

Method overriding occurs when a subclass provides its own implementation of a method that is already defined in its superclass. It is resolved at runtime (dynamic polymorphism).

class Animal {
    void sound() {
        System.out.println("Animal makes a sound");
    }
}

class AnimalTypes extends Animal {
    @Override
    void sound() {
        System.out.println("Dog barks");
    }
}

13. What is the difference between List, Set, and Map interfaces in Java?#

• List:
  • Represents an ordered collection of elements.
  • Allows duplicate elements and maintains the insertion order.
  • Common implementations: ArrayList, LinkedList.
  • Example: List<String> list = new ArrayList<>();
• Set:
  • Represents a collection of unique elements (no duplicates allowed).
  • Does not guarantee any specific order of elements.
  • Common implementations: HashSet, LinkedHashSet, TreeSet.
  • Example: Set<String> set = new HashSet<>();
• Map:
  • Represents a collection of key-value pairs, where each key is unique and maps to a specific value.
  • Allows fast lookup of values based on keys.
  • Common implementations: HashMap, LinkedHashMap, TreeMap.
  • Example: Map<String, Integer> map = new HashMap<>();

14. Explain the concept of exception handling in Java.#

The exception handling in Java is a way for developers to handle run-time errors without terminating the program. This is achieved by catching, i.e., capturing these exceptions and taking appropriate action.

Java exceptions are classified into the following types:

• Checked Exceptions – Handled by the programmer and checked during compile time (e.g., IOException, SQLException).
• Unchecked Exceptions – Occur mostly because of programming errors and are therefore neglected by the programmer (e.g., NullPointerException, ArrayIndexOutOfBoundsException).
• Errors – Serious conditions impossible to handle such as OutOfMemoryError, StackOverflowError.

15. What is the difference between final, finally, and finalize in Java?#

final – Prevents Modification

• Following its initialization, it cannot be modified.
• Can not be overridden by any subclass.
• Can not be inherited (subclassed).

finally – Ensures Code Execution

• After exception handling.
• Executed whether or not the exception has originated.
• Used file cleanup, connection release, etc.

finalize() – Called Before Object is Garbage Collected

• Defined in Object class.
• For cleanup processing before object destruction.
• Not recommended because the Garbage Collection is unpredictable.

16. What is a checked exception and an unchecked exception in Java?#

The exception handling in Java is a way for developers to handle run-time errors without terminating the program. This is achieved by catching, i.e., capturing these exceptions and taking appropriate action.

Checked exceptions are exceptions that must be handled at compile time using try-catch or throws . If the programmer doesn't apply it, then this causes the program not to compile. These exceptions are used in things like, for example, file not found (FileNotFoundException) or database error (SQLException).

try {
    FileReader file = new FileReader("test.txt"); // FileNotFoundException
} catch (FileNotFoundException e) {
    System.out.println("File not found!");
}

Unchecked exceptions shall demand no explicit handling because they occur at runtime. Since they are caused by programming mistakes—for example, trying to access a null reference (NullPointerException), division by zero (ArithmeticException), or an incorrect index value for an array (IndexOutOfBoundsException)—in addition to those cases where they occur.

int num = 10 / 0; // ArithmeticException

17. What is a try-catch-finally block in Java?#

try Block: Contains code that may throw an exception.

catch Block: Handles specific exceptions that occur in the **try**block.

finally Block: Executes always, whether an exception occurs or not.

public class FlowControlStatements {
    public static void main(String[] args) {
        try {
            int result = 10 / 0; // Exception occurs
        } catch (ArithmeticException e) {
            System.out.println("Error: Cannot divide by zero!");
        } finally {
            System.out.println("Finally block always executes.");
        }
    }
}

18. What is Thread in Java?#

A thread in Java is the smallest, concurrent unit of execution in a process. Java has built-in features for multithreading so that several threads can run together to boost the performance of the application.

How to Create a Thread?

By extending the Thread class: Override the run() method and call start() to begin execution.

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread is running...");
    }
}

public class Main {
    public static void main(String[] args) {
        MyThread t1 = new MyThread();
        t1.start(); // Starts the thread
    }
}

By implementing the Runnable interface: Implement the Runnable, give an instance to the Thread object, and call start().

class MyRunnable implements Runnable {
    public void run() {
        System.out.println("Thread is running...");
    }
}

public class Main {
    public static void main(String[] args) {
        Thread t1 = new Thread(new MyRunnable());
        t1.start();
    }
}

Thread Lifecycle in Java:

New-Newly Born (Created but not yet started).

Runnable—Ready to execute but waiting for CPU.

Running execution in progress.

Blocked/Waiting—Temporarily on Hold/Suspended.

Terminated—Execution is complete.

19. What is the difference between wait() and sleep() methods in Java?#

Both wait() and sleep() are different in Java: they allow execution to be paused.

The wait() Method

It is defined in the Object class. It is used for thread synchronization (waits for notification). The method releases the monitor lock while waiting. It must be called from inside synchronized blocks. The method can be resumed with notify() or notifyAll().

The sleep() Method

It is defined in theThread class. It is used to pause execution for a specified time. It does not release the lock; instead, it merely pauses execution. It can be interrupted from another thread using interrupt().

20. What is deadlock in Java, and how can it be prevented?#

A deadlock occurs in Java when two or more threads are holding locks on certain resources and are thereby waiting for one another to release those locks, forming a circular dependency. Thus, the threads stay blocked forever. A deadlock occurs when multiple threads hold locks and wait for each other indefinitely; it can either be prevented outright or resolved once it occurs.

How to Prevent Deadlock in Java?

• Always lock in the same order so that a circular dependency is not formed. Example: Lock A, then lock B.
• Use tryLock() in ReentrantLock to Avoid Blocking Forever: A thread can acquire a lock if it is free.
• Assign Locking Order: Always lock in a fixed and predefined order to avoid circular dependency formation.
• Use a Timeout Strategy: Lock.tryLock(timeout, TimeUnit.SECONDS) to avoid waiting indefinitely.
• Minimize the Locking Scope: Lock only the critical section to lessen contention.

Top 10 mostly asked Java Interview Questions on Advanced Java And APIs#

1. Explain Stream API in Java 8 and how it differs from Collection?#

Java 8 introduced the Stream API, which lets a person work functionally on the collections to get them processed efficiently. The operations allowed include filtering, mapping, reducing, and sorting the items using a pipeline without modifying the data source.

Key Features of Stream API:

Functional (using Lambda expressions).

Allowing parallel processing for better performance.

Doing lazy evaluation (operations only get executed when required).

Providing a clean and readable way to manipulate the data.

how does it differ from Collection?

The Stream API can work on collections in a functional, efficient, and parallel way. Unlike collections, however, streams don't carry data but act lazily and immutably on top of that data.

2. What is ExecutorService in Java?#

Mode of ExecutorService Java has more to offer in the form of high-level threading control. It also dwells in the domain of java.util.concurrent and abstracts the management of thread pools from arbitrary concurrency management into task submission.

ExecutorService proves to be a boon for concurrent and parallel extent in Java. Thread management becomes much simpler with it, and tasks or asynchronous functions can be simultaneously carried out in the thread pool, which ends up proving to be a performance and resource engine boost.

Thread pooling keeps management of activities to be performed without the constant overhead of instantiating threads to do it. Task submission can happen with either Runnable or Callable objects. It provides methods for shutting down gracefully where new tasks are not accepted while the currently executing tasks are still being completed.

3. What is the use of volatile keyword in Java?#

In Java, the volatile keyword guarantees that if one or more threads change the value of a variable, the latest value will always be seen by all threads. That is, value caching and optimization-related problems that arise in a multi-threaded environment are overcome.

4. What is the role of StackTrace in Java exception handling?#

The StackTrace in Java offers an elaborate description of the stack at the point where an exception occurs. It is used to debug and give error diagnoses. It'll eavesdrop on the chain of method calls leading to the exception, including class names, method names, and number of lines.

Role of StackTrace:

Debugging: Indicating the point at which the exception manifests itself to find it in the code.

Call Path: Displaying the sequence of methods leading to an exception.

Cause Analysis: Indicating the precise line in the source code where the exception occurred.

Logging: Useful for logging errors in production environments for later analysis.

Exception Traversal: For tracing the journey of exceptions over the methods.

5. Explain Java's memory model.#

In Java, the Memory Model defines how variables are stored in memory and how they are accessed by threads in a multithreaded environment concerning visibility, atomicity, and ordering.

Main Memory vs. Working Memory:

Main Memory: Shared among all threads and includes variables that are stored inside it.

Working Memory: Own private working memory copy of those variables kept by each thread.

Visibility:

When a thread changes the value of a variable, it guarantees that other threads will see the new value. You can enforce this by either adding the volatile keyword or using synchronized blocks.

Atomicity:

Operations on a single variable (like read or write) are atomic; that is, they cannot be interrupted. Compound operations (like increment, which reads, modifies, and writes) are not atomic automatically and generally require synchronization.

Ordering:

The happens-before order is what ensures that some operations will be visible to other threads in a strict order. For example, if a thread writes to a volatile variable, all other threads would be able to see it instantaneously.

6. What is a Singleton design pattern in Java?#

The singleton design pattern guarantees that there is only one instance of a class and provides a global point of access for that instance. It is implemented through:

• Private constructor prevents instantiation from outside the class.
• The static method (getInstance()) serves to provide the instance.
• This is lazy initialization that ensures that the instance is created only when required.

Pros:

• Gives a single-instance guarantee.
• Resource efficient.

Cons:

• Global state may lead to problems.
• Difficult to test and manage in multi-threaded environments.

7. Explain the Producer-Consumer problem in Java.#

The Producer-Consumer problem is associated with two types of threads: producers that create and insert data into the shared buffer and consumers that take the data from the buffer. The task is to make producers wait when the size of the buffer is full and consumers wait when the buffer runs empty.

The main points are:

• The producers add the data to the buffer and wait if it is full.
• The consumer consumes data from the buffer and waits if it is empty.
• Synchronize the threads using wait() and notify() to enable the distribution of mutual exclusion between them.

Common objective:

To maintain effective handling of their shared resources between multiple producers and consumers, keeping any possible data consistency from being compromised.

8. What is the difference between synchronized and concurrent collections in Java?#

Synchronized Collections:

• Thread-safe by synchronizing methods.
• Locks entire collection, which can cause performance issues.
• Examples: Vector, Hashtable, Collections.synchronizedList().

Concurrent Collections:

• Thread-safe with fine-grained locking.
• Allows multiple threads to operate concurrently with better performance.
• Examples: ConcurrentHashMap, CopyOnWriteArrayList, BlockingQueue.

9. How do you use the reduce() method in Java Streams?#

The reduce() method of the Java Streams framework is used to accumulate the stream elements into a single result. It takes a binary operator to combine the elements.

Syntax:

Optional<T> reduce(BinaryOperator<T> accumulator)

T reduce(T identity, BinaryOperator<T> accumulator)

The first variant returns an Optional type T, whereas the second variant returns a non-optional result with the help of an identity value.

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
int sum = numbers.stream().reduce(0, (a, b) -> a + b);  // Output: 15

10. Features of Java 8?#

Java 8 was critical in adding to the few basic features, which attested to the fact that the enhancement of the languages was realistic.

Lambda Expressions: Internalizes function-style syntax to enable passing behavior. Eg: (a, b) -> a + b.

Streams API: Functional-style processing of sequences (e.g., collections). Provides filter(), map(), reduce(), etc.

Functional Interfaces: Interfaces with only one abstract method that may use lambda expressions. Eg: @FunctionalInterface public interface MyFunction { void apply(); }.

Default Methods: Methods have a body for interfaces: Backwards compatible. Eg: default void newMethod() { ... }.

Method References: A shorthand for lambda expressions. Eg: String::toUpperCase.

Optional Class: It wraps values to effectively prevent NullPointerExceptions. Eg: Optional<String> name = Optional.of("John").

New Date and Time API: A more powerful API with which date/time manipulation has been made available (java.time). Eg: LocalDate.now().

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CompletableFuture: Asynchronous programming is thereby possible, creating a non-blocking operation:

Parallel Streams: Processing using multiple streams, hence more time-efficient.

Conclusion:#

The definitive article concerning the 50 most frequently asked Java interview questions covers everything from core Java to OOP and the collection framework. Candidates are fortified in their Java knowledge and problem-solving methodologies by elucidating pertinent concepts like JVM, JDK, JRE, platform independence, OOP principles, memory management, multithreading, exception handling, and Java 8+ features.

For an interview to be successful, coding practice and making optimized solutions go hand in hand with the latest updates on Java. With ample insight into all these concepts, you will be ready for every theoretical and coding challenge a Java interview throws at you.