36 practice questions with answers for Java interviews in Indore.
Java jobs in Indore
Indore is one of central India's fastest-growing IT hubs, with IT-services campuses and product start-ups in its Super Corridor and IT parks. Companies here hire many freshers and value hands-on project work.
Below are 36 questions that match the mix of service, product and domain employers hiring java developers in Indore.
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01
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.
02
Implement a producer-consumer using BlockingQueue.
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.
04
Read a large text file line by line efficiently and count lines containing ERROR.
Stream the file lazily so it never loads fully into memory.
Path p = Path.of("app.log");
try (Stream<String> lines = Files.lines(p)) {
long errors = lines
.filter(l -> l.contains("ERROR"))
.count();
System.out.println(errors);
}
try-with-resources closes the file handle. BufferedReader.readLine() in a loop is an equally good option.
Exception chaining wraps a low level exception in a higher level one while keeping the original as the cause, for example throw new OrderException("save failed", sqlEx). The full stack trace shows both, with Caused by lines. It lets each layer speak its own language without losing the root cause, which is essential when debugging production logs.
06
Objects used as HashMap keys cannot be found after insertion. What could be wrong?
Most likely the class overrides equals() but not hashCode(), or neither, so equal keys land in different buckets. Another cause is a mutable key: a field used in hashCode() was changed after insertion, so the lookup goes to a different bucket. The fix is to implement both methods on the same fields and use immutable keys such as String, records or final classes.
07
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.
08
When would you choose ArrayDeque over Stack and LinkedList?
Stack is a legacy synchronized class built on Vector, so it is slow and exposes index methods that break stack rules. LinkedList allocates a node per element. ArrayDeque is a resizable circular array with no per-element allocation, so push, pop, offer and poll are fast amortised O(1). Use ArrayDeque for both stacks and queues in single-threaded code; it does not accept null.
Autoboxing is automatic conversion between a primitive and its wrapper, such as int to Integer, and unboxing is the reverse. A common pitfall is a NullPointerException when unboxing a null Integer. Another is comparing Integer objects with ==, which works only for cached values from -128 to 127. Boxing inside tight loops also creates extra garbage and slows code.
10
What is the difference between static and dynamic binding?
Static binding is resolved at compile time and applies to private, static and final methods and to overloaded methods, because the compiler knows exactly which method to call. Dynamic binding is resolved at runtime based on the actual object type and applies to overridden instance methods. Static methods are hidden, not overridden, so the reference type decides which one runs.
11
What is the difference between wait() and sleep()?
wait() is an Object method called inside synchronized code; it releases the monitor and waits until notify(), notifyAll() or a timeout. sleep() is a Thread static method that pauses the current thread for a time but keeps any locks it holds. wait() is for coordination between threads, while sleep() simply delays. Modern code prefers Condition, BlockingQueue or CountDownLatch.
12
A team member chains Optional.get() everywhere and gets NoSuchElementException. What do you suggest?
get() on an empty Optional throws, so it brings back the same problem Optional was meant to solve. Suggest orElseThrow with a clear domain exception, orElse or orElseGet for defaults, and map or flatMap to chain safe transformations. Static analysis can flag get() without isPresent. Since Java 10 orElseThrow() without arguments is the clear name for the unchecked version.
13
Why does Java not support multiple inheritance of classes?
Multiple inheritance of classes creates the diamond problem: if two parents have the same method with different code, the child cannot know which one to use, and state duplication becomes messy. Java avoids this by allowing one superclass but many interfaces. If two interfaces provide the same default method, the class must override it and can call a specific one with InterfaceName.super.method().
14
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.
15
What is the first level and second level cache in Hibernate?
The first level cache is the persistence context itself; it is per session or EntityManager, always on, and returns the same object for repeated finds by id in one transaction. The second level cache is shared across sessions and optional, backed by providers like Ehcache or Infinispan through JCache. Use it for read-mostly reference data, and be careful about stale data in clustered setups.
16
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.
17
Run two tasks in parallel with CompletableFuture and combine their results.
The simplest safe options are an enum or the holder idiom.
public class Config {
private Config() {}
private static class Holder {
static final Config INSTANCE = new Config();
}
public static Config getInstance() {
return Holder.INSTANCE;
}
}
The JVM loads Holder lazily and class initialisation is thread safe. An enum singleton also resists reflection and serialization attacks.
Method references are short forms of lambdas that just call an existing method. Types: static method (Integer::parseInt), instance method of a specific object (System.out::println), instance method of an arbitrary object of a type (String::toUpperCase) and constructor (ArrayList::new). They make stream code clearer when the lambda only forwards its arguments.
20
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.
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.
22
An API listing 100 orders fires 101 SQL queries. How do you fix it?
This is the N+1 problem from a lazy association loaded in a loop or by JSON serialisation. Fetch what the endpoint needs in one query with JOIN FETCH or @EntityGraph(attributePaths = "items"), or use a DTO projection query. For collections with pagination, fetch ids first, then fetch associations in a second query to avoid in-memory paging. Add a test that asserts the query count.
23
Tell me about a time a production incident happened because of your code.
Situation: a release I made caused failed payments for some users. Task: restore service and find the cause fast. Action: I rolled back within ten minutes, then found I had not handled a null bank code from one partner, which threw an exception. I fixed it, added validation and a test with that partner's data and shared a short write-up. Result: no repeat and the team added null checks to our review checklist.
24
Tell me about a time you found and fixed a difficult production bug in a Java application.
Situation: our order service crashed with OutOfMemoryError every few days. Task: find the root cause without long downtime. Action: I enabled heap dumps on OOM, analysed the dump in Eclipse MAT and found a static HashMap used as a cache that was never cleared. I replaced it with a bounded Caffeine cache with expiry and added a heap usage alert. Result: no crashes in six months and memory stayed flat.
25
Your service throws OutOfMemoryError: Java heap space in production. How do you investigate?
First capture evidence: enable -XX:+HeapDumpOnOutOfMemoryError and check GC logs to see if heap grows steadily, which signals a leak, or spikes, which signals a large request. Analyse the heap dump in Eclipse MAT or VisualVM and look at the dominator tree for big retainers like static maps or caches. Fix the leak or bound the cache, then increase -Xmx only if the real workload needs it.
26
An API needs to find the price band for any amount, for example the nearest lower slab. Which structure fits?
TreeMap is ideal because it keeps keys sorted and offers navigation methods. Store slab start amounts as keys and the band as value, then floorEntry(amount) returns the largest slab not above the amount in O(log n). ceilingKey, headMap and tailMap help with similar range queries such as tax slabs or delivery charges.
27
What is the difference between @Component, @Service, @Repository and @Controller?
All four mark a class as a Spring bean for component scanning. @Service and @Component are functionally the same, with @Service showing business logic intent. @Repository also enables translation of persistence exceptions into Spring DataAccessException. @Controller handles web requests and returns views, and @RestController adds @ResponseBody so return values are written as JSON.
28
Tell me about a microservice you designed end to end.
Situation: our monolith's notification code slowed every release. Task: extract it into its own service. Action: I designed a Spring Boot service consuming Kafka events, with its own database for templates and delivery logs, retries with backoff, idempotent consumers and Actuator metrics in Grafana. Result: it handles 20 lakh messages a day, deploys independently and notification incidents dropped sharply.
Optional is a container that may or may not hold a value, used mainly as a method return type to show that a result can be absent, for example findById. Use map, filter, orElse, orElseGet and orElseThrow instead of calling get() blindly. Do not use Optional for fields, parameters or collections, and prefer orElseGet when the default is costly to create.
30
What is garbage collection and how does the JVM decide an object is garbage?
Garbage collection automatically frees memory used by objects that are no longer reachable. The JVM starts from GC roots such as local variables on thread stacks, static fields and active threads, and marks every object reachable from them. Anything not marked is garbage. Because reachability is used rather than reference counting, circular references between unreachable objects are still collected.
31
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.
32
Write a Java solution for the Two Sum problem using a HashMap.
Store each number's index and look up its complement.
public int[] twoSum(int[] nums, int target) {
Map<Integer, Integer> seen = new HashMap<>();
for (int i = 0; i < nums.length; i++) {
Integer j = seen.get(target - nums[i]);
if (j != null) return new int[]{j, i};
seen.put(nums[i], i);
}
return new int[0];
}
One pass, O(n) time and O(n) space.
33
Write code that runs 10,000 blocking tasks using virtual threads.
Use the virtual-thread-per-task executor in Java 21.
try (var ex = Executors.newVirtualThreadPerTaskExecutor()) {
for (int i = 0; i < 10_000; i++) {
int id = i;
ex.submit(() -> {
Thread.sleep(Duration.ofMillis(200));
return id;
});
}
} // close() waits for all tasks
All tasks finish in about 200 ms because blocked virtual threads free their carriers.
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
36
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.
Few big employers listed in Indore?
Many Indore candidates also apply to teams in Pune and to remote or hybrid roles. Practise the Pune set as well.
Which companies hire Java professionals in Indore?
Companies with large offices in Indore include many IT services and product firms. Use their company pages below to practise employer-specific sets.
Are Java interviews in Indore different from other cities?
The questions depend more on the company than the city. Indore's mix of employers decides the style: Companies here hire many freshers and value hands-on project work
How do I find Java jobs in Indore?
Shortlist companies with Indore offices, keep your CV focused on Java projects, apply on company career pages and job portals with the Indore filter, and practise the questions on this page before each interview.