Java Interview Questions

• General Questions
• OOP Questions
• Algorithms and Data Structures / Collections / Generics
• Streams And Lambdas
• Threads
• Design Patterns
• Java EE
• Spring
• Generic WEB
• SQL
• Exercises

General Questions

• What are the key memory segments inside JVM ? Explain HEAP, STACK.
• What is a Memory Leak ? How can a memory leak appear in garbage collected language ?
• Is Java pass-by-value or pass-by-reference ?
• Write a Java method to swap the values of two integer values (Is it possible ?):

public void swap(int x, int y) { /** code here **/ }

• What is a primitive type in Java ? What are the main primitive types ?
• What is auto-boxing/unboxing ?
• What is implicit casting ? What is explicit casting ?
• If developing an e-commerce site, what is the recommend type to use for the price if performance is not an issue. What is performance is an issue ?
• Explain the usage of the following keywords: strictfp, native.
• Explain the usage of the following keyword: final.
• Can you give example of a final class from the Java Standard library ?
• What is the difference between == and equals() ?

OOP Questions

• What is a constructor ?
• What is the default constructor ?
• Explain the the concept of Inheritance. Why doesn't Java supports the concept of Multiple Inheritance ?
• What is Polymorphism ?
• What is Overloading ? What is Overriding ?
• What is the output if we execute the following code:

public abstract class Engineer {
    public static String name() {
        return "Engineer";
    }
    public String favorite() {
        return "Math and Physics";
    }
    public static void main(String[] args) {
        Engineer e2 = new ElectricalEngineer();
        Engineer e3 = new SoftwareEngineer();
        System.out.format("%s %s \n", e2.name(), e2.favorite());
        System.out.format("%s %s", e3.name(), e3.favorite());
    }
}
class ElectricalEngineer extends Engineer {
    public static String name() { return "Electrical Engineer" }
    public String favorite() { return "Electricity"; }
}
class SoftwareEngineer extends Engineer {
    public static String name() { return "Software Engineer"; }
    public String favorite() { return "Java"; }
}

• Can a static method be overwritten ?
• Explain the concept of Encapsulation.
• How would you encapsulate the following class:

public class Circle {
  double radius;
  double area;
}

• What is immutability ?
• How can we write an Immutable class ?
• Explain of the concept of Marker Inteface.
• Explain the concept of Serialization.
• Explain how the keyword transient works.
• Explain the concept behind the Cloneable interface and how does it work.
• What is the output if we execute the following code:

public class Constructors {
    public static void main(String[] args) {
        new Dog();
    }
}
class Animal {

    public Animal() { System.out.println("1"); }

    public Animal(String s) {
        this();
        System.out.println("2");
    }
}
class Dog extends Animal {
    public Dog() {
        super("3");
        System.out.println("4");
    }
}

• What is the output if we execute the following code:

public class Ternary {
    public static void main(String[] args) {
        Integer x = 1987;
        Integer y = 1987;
        System.out.println(x == y ? "A" : "B");
    }
}

• What is the output if we execute the following code:

public class Strings {
    public static final String DELOITTE = new String("Deloitte");
    public static void main(String[] args) {
        String s1 = "Deloitte";
        String s2 = new String(s1);
        String s3 = new String("Deloitte");
        String s4 = "Deloitte";
        System.out.println(s1==s2);
        System.out.println(s1==s3);
        System.out.println(s1==s4);
        System.out.println(s1==DELOITTE);
        System.out.println(s2==DELOITTE);
        System.out.println(s3==DELOITTE);
    }
}

• What is a Java Interface ?
• Does an interface extends or implements another interface ?
• What is a Java Abstract Class ?
• Does a Java Abstract Class extends or implements another Java Abstract Class ?
• Explain the usage of the default keyword (Java 8 onwards).
• With the introduction of the default keyword are there any reasons to use Abstract Classes instead of Interfaces ?
• Unchecked Exceptions vs. Checked Exceptions.
• What are the differences between Exceptions and an Errors ? Are there any similitudes ?
• Name 3 Unchecked Exceptions.
• Name 3 Checked Exceptions.
• What are Java Annotations ?

Algorithms and Data Structures / Collections / Generics

• Explain the O(n) Notation (Big O).
• How does a Stack data-structure works ? Are there any standard Java Stack implementations ?
• Recursively calculate the sum of numbers from a List<Integer> (don't use for/do/while loops).
• Why is not possible to use primitives as generic types ?
• Explain the concept of Type Erasure.
• Explain the following data structures: List, Map, Queue, Set.
• Name a few implementations for each interface:

List	Set	Map<K,V>	Queue

• What is the output if we run the following code:

import java.util.HashSet;
import java.util.Set;

class A {
    @Override
    public int hashCode() {
        return 0;
    }
}

public class HashTest {
    public static void main(String[] args) {
        Set<A> set = new HashSet<>();

        set.add(new A());
        set.add(new A());
        set.add(new A());

        System.out.println(set.size());
    }
}

• Explain how a HashMap is implemented. What is the relationship between equals() and hashCode().
• What are hash collisions ?
• What are the operations for which a LinkedList is more efficient than an ArrayList ?
• What is the difference between CopyOnWriteArrayList, Vector and ArrayList ?
• What are the key differences between a HashMap and ConcurrentHashMap ?
• How does a WeakHashMap works ? What are the main differences between a WeakHashMap and a HashMap ?
• Does a Set accepts null as an element ?
• What is the difference between an Iterator and ListIterator ?
• Are there any Immutable Collection Classes ?
• What is a RingBuffer ?
• What happens if we run the following code:

public static void main(String[] args) {
        List<String> list = new LinkedList<>();

        list.add("A");
        list.add("C");
        list.add("D");

        Iterator<String> it = list.iterator();

        System.out.println(it.next());

        list.add(1, "B");
        
        System.out.println(it.next());
        System.out.println(it.next());
        System.out.println(it.next());
}

Streams and Lambdas

• What is a Functional Interface ?
• Can you please explain what is a Predicate, Consumer, Function, Supplier ?
• What is the difference between a Stream and an Iterator ?
• Using the filter() method write a method that returns only the positive numbers from a List<Integer>.
• What is a parallelStream() ? How is different from a standard stream() ?
• Find out the max element from a List<Integer> using the reduce() method.
• Find out the sum of elements from a List<Integer> using the reduce() method.
• What is the output if we run the following code:

    public static void main(String[] args) {
        final List<String> l = new LinkedList<>();

        l.add("A");
        l.add("B");
        l.add("C");

        l.stream().forEach(e -> {
            l.add("Z");
            System.out.println(e);
        });
    }

• What is the output if we run the following code:

    public static void main(String[] args) throws InterruptedException {

        List<String> list = new LinkedList<>();

        list.add("A");
        list.add("B");
        list.add("C");
        list.add("D");
        list.add("E");
        list.add("F");
        list.add("G");
        list.add("H");

        ListIterator<String> iterator = list.listIterator();

        for (int i = 0; i < list.size(); i++) {
            CompletableFuture.runAsync(() -> System.out.println(iterator.next()));
        }

        Thread.sleep(2000);
    }

Threads

• What is a Thread ?
• Is the ++ operator thread-safe ?
• How can we implement a Thread ?
• How synchronized keyword works ?
• How is the method thread.join() working ?
• Explain the concept of Thread Starvation ?
• Explain the concept of Thread Pool ?
• What can you tell about the Executor Interface ?
• What is a Semaphore in Java ?
• What is the output if we execute the following code:

    public static void main(String[] args) throws ExecutionException, InterruptedException {

        CompletableFuture<Void> cf1 = CompletableFuture.runAsync(() -> {
            System.out.println("B");
        });

        CompletableFuture<Void> cf2 = CompletableFuture.runAsync(() -> {
            System.out.println("A");
        });

        cf1.get();
        cf2.get();
    }

Design Patterns

• Singleton
• Builder
• Factory
• Visitor

Java EE

• What is a Servlet ?
• What are the main methods of a HttpServlet ?
• What are the main bean scopes ? Explain the following annotations: @RequestScoped, @SessionScoped, @ApplicationScoped, @Dependent, @Conversation .
• What is the main difference between a Servlet and Filter ?

Spring

• Explain the concept of Inversion of Control. What is the Spring IoC Container ?
• Explain the concept of Dependency Injection (in Spring).
• What are the main advantages and disadvantages for setter dependency injection vs constructor dependency injection ?
• What is a Spring Bean ?
• What are the main Spring Bean Scopes ? Explain singleton, prototype, request, session, global session.
• What are the Spring Stereotyping Annotations ? Explain the differences between: @Component, @Controller, @Repository, @Service.
• Explain how the following Spring Annotations are working: @Autowired, @Qualifier, @Required.
• Explain how the @Async annotations functions.
• Explain the concept of Spring Profiles.
• What is Spring Boot ?
• What is Spring Data ?
• What is Spring Integration ?
• What is Spring Batch ?
• What is Spring Security ?

Generic WEB

• What is JWT ? How does JWT works ?
• Where is recommended to store JWT tokens received from the Server on the client-side ?
• What are the main HTTP verbs ?
• What is the difference between POST and PUT ?
• What is the difference between POST and PATCH ?
• What is the difference between POST and GET ?
• You have to develop a REST API for a book store. This API needs to implement CRUD-like operations. How would you design the API ?

Operation	HTTP VERB	ENDPOINT
Add a book
Remove a book
List books with pagination
Edit a book
Get info for a book

SQL

Given the following SQL table:

id	full_name	mng_id
100	Patrick Read	101
101	Bradley Hayes	null
102	Kieran Bennett	101
103	George Barnes	101
104	Alex Griffiths	102
105	Issac Jacobs	102
106	Will Mack	104

Write an SQL query that returns the following result:

id	full_name	mng_full_name
100	Patrick Read	Bradley Hayes
...	............	.............

Write an SQL query that returns one row containing the manager with the most direct subalterns.

Generic Questions

• From a protocol perspective what is the difference between UDP and TCP ?
• What library would use to write a Scheduler in Java ?
• What are the main Maven alternatives ?
• What are the main JUnit alternatives ?
• How is Mockito used ? Are there any other alternatives to Mockito
• What is Continuous Integration (CI) ?
• Name other JVM-based languages ?
• What is Functional Programming ? Can you give a few examples of Functional Programming languages ?

Exercises

Pangrams (Simple)

Roy wanted to increase his typing speed for programming contests. So, his friend advised him to type the sentence "The quick brown fox jumps over the lazy dog" repeatedly, because it is a pangram. (Pangrams are sentences constructed by using every letter of the alphabet at least once.) After typing the sentence several times, Roy became bored with it. So he started to look for other pangrams. Given a sentence s, tell Roy if it is a pangram or not.

Input Format Input consists o a string s. Example:

We promptly judged antique ivory buckles for the next prize

Output Format Output a line containing “pangram” if s is pangram, otherwise print “not pangram”. Example:

pangram

Balanced Parentheses (Medium)

Given a sequence consisting of parentheses, determine whether the expression is balanced. A sequence of parentheses is balanced if every open parentheses can be paired uniquely with a closed parentheses that occurs after the former. Also, the interval between them must be balanced. You will be given three types of parentheses: (, {, and [.

{[()]} - This is a balanced parentheses.
{[(])} - This is not a balanced parentheses.

Example:

Input

3
{[()]}
{[(])}
{{[[(())]]}}

Ouput

YES
NO
YES