36 practice questions with answers for Java interviews in Visakhapatnam.
Java jobs in Visakhapatnam
Visakhapatnam (Vizag) has a developing IT market with delivery centres, start-ups and IT parks around Rushikonda. Many local candidates also apply to Hyderabad and remote roles, so a mix of service and product styles is common.
Below are 36 questions that match the mix of service, product and domain employers hiring java developers in Visakhapatnam.
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01
Using streams, find the employee with the highest salary in each department.
Group by department with a maxBy downstream collector.
Map<String, Optional<Employee>> top = employees.stream()
.collect(Collectors.groupingBy(
Employee::department,
Collectors.maxBy(Comparator.comparingInt(Employee::salary))));
To get values without Optional, wrap with collectingAndThen(maxBy(...), Optional::get), which is safe because each group has at least one element.
02
A counter shared between threads shows wrong totals. How do you fix it?
count++ is not atomic; two threads can read the same value and both write the same result, so updates are lost. Use AtomicInteger.incrementAndGet(), which uses CAS, or LongAdder for very high contention, as it spreads updates over cells. A synchronized method also works but is slower. volatile alone does not fix it because it gives visibility, not atomicity.
03
Run two tasks in parallel with CompletableFuture and combine their results.
Since the calls are independent, run them in parallel with CompletableFuture.supplyAsync on a dedicated executor or with virtual threads, then join with allOf. Total latency becomes roughly the slowest call, about 300 ms. Add a timeout per call with orTimeout and a fallback for failures so one slow service does not block the whole response. Structured concurrency is another option in newer Java.
05
Find the first non-repeating character in a string.
Count in one pass, then scan again in order.
public static char firstUnique(String s) {
int[] cnt = new int[128];
for (char c : s.toCharArray()) cnt[c]++;
for (char c : s.toCharArray()) {
if (cnt[c] == 1) return c;
}
return '_';
}
Two passes, O(n) time and O(1) extra space for ASCII input.
06
Write a sealed interface with records and a switch that computes area.
The switch needs no default because the type set is closed.
sealed interface Shape permits Circle, Rect {}
record Circle(double r) implements Shape {}
record Rect(double w, double h) implements Shape {}
static double area(Shape s) {
return switch (s) {
case Circle c -> Math.PI * c.r() * c.r();
case Rect(double w, double h) -> w * h;
};
}
The JVM runs bytecode and handles memory, garbage collection and JIT compilation. The JRE used to be the JVM plus core libraries needed to run programs. The JDK adds development tools such as javac, jar, javadoc and jshell. Since Java 11, vendors ship only the JDK, and jlink can build a slim custom runtime for deployment.
08
What is the difference between ArrayList and LinkedList?
ArrayList is backed by a resizable array, so get(i) is O(1) and appends are amortised O(1), but inserting in the middle shifts elements. LinkedList is a doubly linked list with O(n) random access but O(1) add or remove at the ends or through an iterator. In practice ArrayList is faster for almost all cases because of better CPU cache use; use ArrayDeque for queues.
09
Write a minimal Dockerfile for a Spring Boot application.
A multi-stage build keeps the image small.
FROM eclipse-temurin:21-jdk AS build
WORKDIR /app
COPY . .
RUN ./mvnw -q package -DskipTests
FROM eclipse-temurin:21-jre
WORKDIR /app
COPY --from=build /app/target/*.jar app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-XX:MaxRAMPercentage=75", "-jar", "app.jar"]
Spring Boot can also build images without a Dockerfile using mvn spring-boot:build-image.
10
Why must you be careful when initial capacity of a HashMap is too small?
If a HashMap starts small and grows large, it resizes many times; each resize allocates a new array and rehashes every entry, which costs CPU and creates garbage. When the expected size is known, use HashMap.newHashMap(n) in Java 19 or later, or set capacity to n divided by 0.75. Too large a capacity wastes memory, so size it to real data.
11
A stream pipeline that updates a shared ArrayList in forEach gives random results with parallel(). Why?
ArrayList is not thread safe, and parallel streams run forEach on many threads, so additions race, get lost or throw exceptions. Side effects in streams are the problem. Replace forEach with collect(Collectors.toList()) or toList(), which handles merging per thread safely. Keep lambdas stateless and avoid modifying external state inside stream operations.
finally runs after try and catch in almost every case, including when the try block returns or throws. It does not run if the JVM exits through System.exit(), the process is killed or the thread dies abruptly. Avoid returning from finally, because that silently overrides the try block's return value and swallows exceptions.
At startup Spring Boot reads auto-configuration classes listed in META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports. Each class uses conditions like @ConditionalOnClass, @ConditionalOnMissingBean and @ConditionalOnProperty, so it creates beans only when the library is on the classpath and you have not defined your own. Run with --debug to see the condition report showing what was applied and why.
String is immutable and thread safe by design. StringBuilder is mutable and not synchronized, so it is the fastest choice for building strings in a single thread, such as in loops. StringBuffer is mutable and synchronized, so it is thread safe but slower, and is rarely needed today. The compiler already optimises simple + concatenation outside loops.
15
How do you explain OOP concepts to a junior developer who is new to Java?
Situation: I mentored two interns who struggled with OOP. Task: help them design classes for a library app. Action: I used real objects they knew, like Book and Member, showed encapsulation with private fields, then wrote one interface with two implementations to show polymorphism, and reviewed their code daily. Result: within two weeks they designed modules on their own and their pull requests needed fewer review comments.
16
Show runtime polymorphism with a simple Java example.
A parent reference calls the child's overridden method at runtime.
abstract class Shape {
abstract double area();
}
class Circle extends Shape {
double r = 2;
double area() { return Math.PI * r * r; }
}
class Square extends Shape {
double s = 3;
double area() { return s * s; }
}
Shape sh = new Circle();
System.out.println(sh.area()); // Circle area
17
You must expose configuration that differs between dev, QA and production. How do you manage it?
Use Spring profiles with application-dev.yml and application-prod.yml, and bind settings to typed @ConfigurationProperties classes with validation. Keep secrets out of the repo by injecting them through environment variables, Kubernetes Secrets or a vault. Activate the profile with SPRING_PROFILES_ACTIVE in each environment. Use the same build artefact everywhere and change only configuration.
18
Write Java code to reverse an integer without converting it to a string.
Take the last digit with % 10 and build the result, checking for overflow.
public static int reverse(int x) {
long rev = 0;
while (x != 0) {
rev = rev * 10 + x % 10;
x /= 10;
}
if (rev > Integer.MAX_VALUE || rev < Integer.MIN_VALUE) return 0;
return (int) rev;
}
Negative numbers work because % keeps the sign in Java.
Situation: our team was still on Java 8 while LTS releases moved ahead. Task: make a case for upgrading. Action: I followed release notes and JEPs, tried records, pattern matching and virtual threads in a side project, and built a proof of concept moving one service to Java 21 with measured gains. Result: the team approved the upgrade and I ran an internal session on the new features.
20
An application gets StackOverflowError in a recursive method. What do you do?
StackOverflowError means the thread stack is full, usually because recursion has no proper base case or the input is very deep. First check the base case and the recursive step with the failing input. If recursion is correct but deep, such as walking a long linked structure, convert it to an iterative solution with an explicit stack. Raising -Xss is only a temporary workaround.
21
What is the difference between Collectors.toMap() and groupingBy()?
toMap() creates one entry per element using key and value functions and throws IllegalStateException on duplicate keys unless you give a merge function. groupingBy() collects all elements with the same key into a List by default, or applies a downstream collector like counting or summingInt. Use toMap for unique keys like id and groupingBy for categories like city.
22
What is the difference between synchronized and ReentrantLock?
synchronized is a built-in keyword that locks a monitor and releases it automatically at block exit. ReentrantLock is an explicit lock that adds tryLock with timeout, interruptible locking, fairness options and multiple Condition objects, but you must unlock in a finally block. Both are reentrant. Prefer synchronized for simple cases and ReentrantLock when you need its extra features.
23
Tell me about a time you improved performance by choosing a better data structure.
Situation: a report job matching 2 lakh transactions to customers took 40 minutes. Task: bring it under the 5 minute window. Action: profiling showed list.contains() inside a loop, which was O(n squared). I loaded customer ids into a HashSet and grouped transactions with a HashMap. Result: runtime dropped to 50 seconds and I shared the pattern in our team wiki.
24
A HashMap shared by many threads sometimes loses updates. How do you fix it?
HashMap is not thread safe, so concurrent put calls can overwrite each other. Replace it with ConcurrentHashMap and use atomic methods instead of get-then-put, for example counts.merge(key, 1, Integer::sum) or computeIfAbsent for lazy creation. Collections.synchronizedMap also works but locks the whole map, which hurts throughput under contention.
25
Group a list of orders by city and sum the amount per city.
Use groupingBy with a summing collector.
Map<String, Double> totals = orders.stream()
.collect(Collectors.groupingBy(
Order::city,
TreeMap::new,
Collectors.summingDouble(Order::amount)));
TreeMap keeps cities sorted. For money, store amounts as long paise or BigDecimal and use Collectors.reducing with BigDecimal::add.
26
Write a custom unchecked exception and a Spring handler that returns HTTP 404.
Define the exception, then map it centrally.
public class NotFoundException extends RuntimeException {
public NotFoundException(String msg) { super(msg); }
}
@RestControllerAdvice
public class ApiErrors {
@ExceptionHandler(NotFoundException.class)
public ProblemDetail notFound(NotFoundException e) {
return ProblemDetail.forStatusAndDetail(
HttpStatus.NOT_FOUND, e.getMessage());
}
}
27
Print numbers 1 to 10 alternately from two threads (odd and even).
Use a shared lock with wait and notify.
class Printer {
int n = 1;
synchronized void print(boolean odd) throws InterruptedException {
while (n <= 10) {
if ((n % 2 == 1) == odd) {
System.out.println(Thread.currentThread().getName() + " " + n++);
notifyAll();
} else wait();
}
notifyAll();
}
}
Start one thread with print(true) and another with print(false).
28
How do you create a custom exception and when should you?
Extend RuntimeException for unchecked or Exception for checked, and provide constructors that accept a message and a cause so the root stack trace is kept. Create one when a domain error needs specific handling, such as InsufficientBalanceException, which a Spring @ControllerAdvice can map to an HTTP 422 response. Do not create exceptions for normal control flow.
29
Implement a producer-consumer using BlockingQueue.
Access order plus removeEldestEntry gives LRU in a few lines.
class LruCache<K, V> extends LinkedHashMap<K, V> {
private final int capacity;
LruCache(int capacity) {
super(16, 0.75f, true);
this.capacity = capacity;
}
@Override
protected boolean removeEldestEntry(Map.Entry<K, V> e) {
return size() > capacity;
}
}
get and put are O(1).
31
Explain the hierarchy of the Java Collections Framework.
The root interface is Iterable, then Collection, which has List (ordered, allows duplicates: ArrayList, LinkedList), Set (unique: HashSet, LinkedHashSet, TreeSet) and Queue/Deque (PriorityQueue, ArrayDeque). Map is separate and stores key-value pairs: HashMap, LinkedHashMap, TreeMap and ConcurrentHashMap. Java 21 added SequencedCollection and SequencedMap with getFirst() and getLast().
32
What is a ClassLoader and what is the delegation model?
A ClassLoader loads .class files into the JVM. The main loaders are Bootstrap (core classes), Platform (formerly Extension) and Application (classpath). In the parent delegation model a loader first asks its parent to load a class and only loads it itself if the parent cannot. This prevents user code from replacing core classes like java.lang.String.
33
What does the var keyword do and where can it not be used?
var, added in Java 10, lets the compiler infer the type of a local variable from its initializer, for example var list = new ArrayList<String>(). Java stays statically typed. var cannot be used for fields, method parameters or return types, and it needs an initializer that is not null. Use it when the type is obvious to keep code readable.
34
Tell me about a time you made a codebase more readable.
Situation: our reporting module had nested loops 200 lines long that new joiners feared. Task: make it maintainable without changing output. Action: I added snapshot tests on real report files, then refactored into small methods using streams, groupingBy and records for intermediate data, and reviewed each step with a teammate. Result: the module shrank by 40 percent and later changes took a day instead of a week.
A lambda can use a local variable from the enclosing method only if that variable is final or effectively final, meaning it is never reassigned after initialisation. Lambdas capture the value, not the variable, so this avoids confusing behaviour and races. To accumulate results, use a stream collector or an AtomicInteger instead of changing a captured local.
36
What is dependency injection and why is constructor injection preferred?
Dependency injection means the container creates objects and supplies their dependencies, instead of classes creating them, which reduces coupling and helps testing. Constructor injection is preferred because dependencies can be final, objects are never half-built, required dependencies are obvious and unit tests can pass mocks without Spring. Field injection with @Autowired hides dependencies and is harder to test.
Few big employers listed in Visakhapatnam?
Many Visakhapatnam candidates also apply to teams in Hyderabad and to remote or hybrid roles. Practise the Hyderabad set as well.
Which companies hire Java professionals in Visakhapatnam?
Companies with large offices in Visakhapatnam include many IT services and product firms. Use their company pages below to practise employer-specific sets.
Are Java interviews in Visakhapatnam different from other cities?
The questions depend more on the company than the city. Visakhapatnam's mix of employers decides the style: Many local candidates also apply to Hyderabad and remote roles, so a mix of service and product styles is common
How do I find Java jobs in Visakhapatnam?
Shortlist companies with Visakhapatnam offices, keep your CV focused on Java projects, apply on company career pages and job portals with the Visakhapatnam filter, and practise the questions on this page before each interview.