Curated interview practice questions with answers. These are practice scenarios, not a verified list of past interview questions.
About Barclays hiring for Java roles
Barclays is a British universal bank with retail, corporate and investment banking businesses. Its India service and technology centres in Pune, Chennai and Noida support global technology, operations, risk and analytics. It hires for software engineers, test engineers, data analysts, business analysts and risk analysts.
For Java roles, this Barclays practice set focuses most on Spring Boot and Microservices (7), Multithreading and Concurrency (5), Collections Framework (4), Strings and Exception Handling (4). It has 5 easy, 21 medium, 4 hard questions, so you can start easy and work up to the harder ones.
Company type
Banking & finance
Headquarters
London, United Kingdom
Main India offices
Pune, Chennai, Noida, Mumbai
Barclays Java interview process
Online assessment (aptitude + technical)
Technical round
Managerial round
HR round
Typical pattern based on public candidate experiences. Rounds vary by role, team and year.
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01
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.
volatile guarantees visibility and ordering: a write to a volatile field is immediately visible to other threads, and it prevents reordering around that access under the Java Memory Model. It does not make compound operations like count++ atomic, because that is a read, modify and write. Use it for flags such as a stop signal, and use AtomicInteger or locks for counters.
03
Sort a list of employees by department, then by salary descending.
Chain comparators.
employees.sort(
Comparator.comparing(Employee::department)
.thenComparing(Employee::salary,
Comparator.reverseOrder()));
// Streams version returning a new list
List<Employee> sorted = employees.stream()
.sorted(Comparator.comparing(Employee::department)
.thenComparing(Employee::salary, Comparator.reverseOrder()))
.toList();
List.sort is stable, so equal keys keep their order.
Transient: a new object not yet linked to a persistence context. Managed (persistent): attached to the context, so changes are tracked and flushed automatically through dirty checking. Detached: was managed but the context closed or it was evicted; changes are not tracked until merge(). Removed: scheduled for deletion on flush. persist, merge, detach and remove move entities between these states.
05
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.
06
A query is fast in development but takes 8 seconds in production. How do you investigate?
Production has more data, so check the execution plan with EXPLAIN ANALYZE on production-like data. Look for full table scans, missing indexes on filter or join columns, functions on indexed columns and poor statistics. Check if Hibernate generates the SQL you expect. Add a suitable composite index, rewrite the query or paginate, and verify the plan improves before release.
07
How does JWT based authentication work in a Spring Boot API?
After login, the auth server issues a signed JWT with claims like user id, roles and expiry. The client sends it in the Authorization: Bearer header. The API, configured as an OAuth2 resource server in Spring Security, verifies the signature with the public key, checks expiry and builds the security context, so no session is needed. Keep tokens short-lived and use refresh tokens for renewal.
Count characters with an array for lowercase letters.
public static boolean isAnagram(String a, String b) {
if (a.length() != b.length()) return false;
int[] count = new int[26];
for (int i = 0; i < a.length(); i++) {
count[a.charAt(i) - 'a']++;
count[b.charAt(i) - 'a']--;
}
for (int c : count) if (c != 0) return false;
return true;
}
O(n) time; use a HashMap for Unicode input.
09
What is the difference between an abstract class and an interface in modern Java?
An abstract class can have state (instance fields), constructors and any access modifier, and a class can extend only one. An interface has no instance state but, since Java 8, can have default and static methods, and since Java 9 private methods; a class can implement many. Use an interface for a capability contract and an abstract class to share code and state among closely related classes.
10
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.
Single Responsibility: a class has one reason to change. Open/Closed: open for extension, closed for modification. Liskov Substitution: subclasses must work wherever the parent is expected. Interface Segregation: prefer small, focused interfaces. Dependency Inversion: depend on abstractions, not concrete classes. Spring dependency injection is a practical example of dependency inversion.
12
A team member compares two Integer values with == and the check fails only for large numbers. Explain.
Integer.valueOf caches objects for values from -128 to 127, so autoboxed small numbers share the same object and == returns true. Larger values create new Integer objects, so == compares two different references and returns false. The fix is to use equals() or compare unboxed int values. Static analysis tools like SonarQube flag this pattern.
13
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.
14
What is the N+1 select problem in Hibernate and how do you fix it?
N+1 happens when you load N parent entities with one query and then a lazy association triggers one extra query per parent, giving N+1 queries. Fix it with a JOIN FETCH in JPQL, an @EntityGraph on the repository method, or batch fetching with @BatchSize or hibernate.default_batch_fetch_size. Detect it by enabling SQL logging or using a query count assertion in tests.
15
Write a Java method to check whether a string is a palindrome, ignoring case and non-letters.
Use two pointers from both ends.
public static boolean isPalindrome(String s) {
int i = 0, j = s.length() - 1;
while (i < j) {
char a = s.charAt(i), b = s.charAt(j);
if (!Character.isLetterOrDigit(a)) { i++; continue; }
if (!Character.isLetterOrDigit(b)) { j--; continue; }
if (Character.toLowerCase(a) != Character.toLowerCase(b)) return false;
i++; j--;
}
return true;
}
Time O(n), space O(1).
16
You have to process tasks in order of priority. Which collection do you use?
Use PriorityQueue, a binary heap where poll() always returns the smallest element by natural order or by a Comparator, such as Comparator.comparingInt(Task::priority). Insert and poll are O(log n) and peek is O(1). For multiple threads use PriorityBlockingQueue. Iterating a PriorityQueue does not give sorted order; only repeated poll() does.
17
Use CountDownLatch to wait for three worker threads to finish.
The main thread waits until the count reaches zero.
CountDownLatch latch = new CountDownLatch(3);
for (int i = 0; i < 3; i++) {
new Thread(() -> {
try { doWork(); }
finally { latch.countDown(); }
}).start();
}
latch.await(30, TimeUnit.SECONDS);
System.out.println("all done");
countDown in finally makes sure the latch is released even on error.
18
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.
19
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.
20
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.
21
Describe a time you had to learn a new technology quickly.
Situation: I joined a project using reactive Spring WebFlux with no prior experience. Task: contribute within two weeks. Action: I studied Project Reactor basics, built a small practice service, paired with a senior on my first ticket and kept notes on common operators. Result: I delivered my first feature in the second week and later wrote the team guide on avoiding blocking calls in reactive code.
22
How does ExecutorService work and why use a thread pool?
ExecutorService manages a pool of worker threads and a task queue. You submit Runnable or Callable tasks and get Futures, and workers reuse threads instead of creating one per task, which saves creation cost and limits concurrency. Configure core size, max size, queue and rejection policy through ThreadPoolExecutor. Always call shutdown() and awaitTermination() when done.
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.
24
A thread pool fills up and tasks get rejected during traffic spikes. How do you handle it?
First check why tasks are slow, since slow tasks fill pools. Then choose a policy: a bounded queue with CallerRunsPolicy applies backpressure by running the task in the submitting thread; otherwise return a fast 503 so clients retry. Tune pool and queue sizes from load tests. Never use an unbounded queue, because it hides the problem until memory runs out.
25
Clients report duplicate orders when they retry after a timeout. How do you prevent it?
Make the create order API idempotent. The client sends an Idempotency-Key header; the server stores the key with the result in a table with a unique constraint or in Redis. A retry with the same key returns the saved response instead of creating a new order. Database unique constraints on business keys add another safety net.
26
Bind typed configuration properties with validation in Spring Boot.
Use a record with @ConfigurationProperties.
@ConfigurationProperties(prefix = "payment")
@Validated
public record PaymentProps(
@NotBlank String baseUrl,
@Min(100) int timeoutMs) {}
// application.yml
// payment:
// base-url: https://pay.internal
// timeout-ms: 2000
Enable it with @ConfigurationPropertiesScan on the main class. The app fails at startup if values are invalid.
27
You need to add a new payment method to a system that has a big if-else on payment type. How would you redesign it?
This breaks the Open/Closed principle. I would create a PaymentProcessor interface with a pay() method and one implementation per type, such as UpiProcessor and CardProcessor. A factory or a Spring map of beans keyed by type picks the right one. Adding a new method then means adding one new class without touching existing tested code, and each processor can be unit tested alone.
28
What is Spring Boot and how is it different from the Spring Framework?
Spring Framework provides dependency injection, MVC, transactions and data access but needs manual configuration. Spring Boot builds on it with auto-configuration, starter dependencies, an embedded server such as Tomcat and production features through Actuator, so you can run a service with java -jar. Spring Boot 3 requires Java 17 or later and uses Jakarta EE packages instead of javax.
29
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.
30
Tell me about a time you solved a concurrency bug.
Situation: our wallet service sometimes showed wrong balances under load. Task: find the race and fix it without slowing the API. Action: I reproduced it with a JMeter test, found a read-then-update pattern in two concurrent requests, and replaced it with an atomic SQL update plus optimistic locking with a version column. Result: zero balance mismatches across festival traffic and a reusable load test for future changes.
Tips to crack the Barclays Java interview
Learn retail and investment banking basics like accounts, payments, trade lifecycle and regulatory reporting.
Revise testing concepts and automation tools if applying for QA roles, as banking systems need heavy regression testing.
Read Barclays' values of Respect, Integrity, Service, Excellence and Stewardship for behavioural answers.
Keep a one-minute story ready for every project on your CV: problem, your part, tools, result.
Practise the hard questions on this page aloud; Barclays interviewers usually ask follow-ups on your first answer.
Frequently asked questions
How many rounds are there in the Barclays Java interview?
Candidates usually go through 4 stages: Online assessment (aptitude + technical); Technical round; Managerial round; HR round. The exact rounds change with the role, team, location and your experience, so confirm the process with the recruiter.
Which topics should I prepare for the Barclays Java interview?
This practice set leans on Spring Boot and Microservices (7), Multithreading and Concurrency (5), Collections Framework (4), Strings and Exception Handling (4). Revise those first, then practise explaining one or two of your own projects end to end, because most Barclays interviewers ask you to walk through real work.
Are these the exact questions asked at Barclays?
No. These are curated practice questions that match the skills and difficulty usually tested in banking & finance interviews like Barclays's. They are not a leaked or verified list of past questions.
Where does Barclays hire Java professionals in India?
Barclays has large teams in Pune, Chennai, Noida, Mumbai. Openings and locations change often, so check the Barclays careers page and filter by city and experience.