Curated interview practice questions with answers. These are practice scenarios, not a verified list of past interview questions.
About Infosys hiring for Java roles
Infosys is an Indian multinational IT services and consulting company offering digital transformation, cloud, application management, testing and data services to global clients. It is known for its Mysuru training campus where new hires go through foundation programmes. In India it recruits for roles such as systems engineer, test engineer, data analyst and data engineer, often through campus and off-campus drives.
For Java roles, this Infosys practice set focuses most on Collections Framework (6), OOP in Java (5), Java Basics and JVM (5), Strings and Exception Handling (4). It has 13 easy, 16 medium, 1 hard questions, so you can start easy and work up to the harder ones.
Typical pattern based on public candidate experiences. Rounds vary by role, team and year.
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HashSet is backed by a HashMap. Each element you add becomes a key in that map with a shared dummy object as the value. So uniqueness depends entirely on the element's hashCode() and equals(), and operations are O(1) on average. LinkedHashSet uses a LinkedHashMap to keep insertion order, and TreeSet uses a TreeMap to keep elements sorted.
Declare the class final, make all fields private and final, set them only in the constructor, and provide no setters. For mutable fields like a List or Date, make defensive copies in the constructor and in getters, or use List.copyOf. Immutable objects are thread safe and make good map keys. Java records give a concise way to create shallowly immutable data classes.
03
What is the difference between composition and inheritance, and which should you prefer?
Inheritance is an is-a relation where the subclass reuses parent code, but it creates tight coupling and fragile hierarchies. Composition is a has-a relation where a class holds another object and delegates work, such as a Car having an Engine. Prefer composition because it is flexible, allows changing behaviour at runtime and is easier to test. Use inheritance only for a true is-a relationship.
04
What is the difference between method overloading and method overriding?
Overloading means several methods in the same class share a name but differ in parameter list; it is resolved at compile time. Overriding means a subclass provides its own version of a parent method with the same signature; it is resolved at runtime through dynamic dispatch. Return type alone cannot overload a method, while overriding allows a covariant return type.
05
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.
06
A log file shows a NullPointerException only for some users. How do you debug it?
Start with the stack trace; since Java 14 helpful NPE messages name exactly which variable was null. Reproduce with the failing user's data, which often has an optional field missing. Fix the root cause by validating input at the boundary, using Optional for values that may be absent and annotating with @NonNull. Add a unit test for that data case so it does not return.
07
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.
The heap stores all objects and is shared by threads; it is managed by the garbage collector. Each thread has its own stack holding frames with local variables and partial results. Metaspace (native memory since Java 8) stores class metadata. There are also the program counter register and native method stacks. StackOverflowError comes from the stack and OutOfMemoryError usually from heap or metaspace.
09
Exceptions are being swallowed with empty catch blocks in your codebase. How would you fix this?
Empty catches hide failures and make production issues invisible. I would find them with static analysis like SonarQube, then decide per case: handle the error properly, rethrow wrapped with context, or at least log with the stack trace. I would add a global exception handler for APIs and a team rule in code review and lint config to block new empty catch blocks.
10
A getter returns an internal List and callers modify it, causing bugs. How do you fix this?
This breaks encapsulation. Return an unmodifiable view with Collections.unmodifiableList(list) or a copy with List.copyOf(list), so callers cannot change internal state. Also copy the list passed into the constructor or setter. If the class should be fully immutable, store List.copyOf in a final field. Expose specific methods like addItem() when controlled changes are needed.
11
Which JVM flags would you set to start a production service with a 4 GB heap, G1 and a heap dump on OOM?
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.
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.
14
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.
15
Join a list of names into a comma separated string with brackets using streams.
HashMap stores entries in an array of buckets. It computes the key's hashCode(), spreads the bits and picks a bucket index. Collisions are stored as a linked list in the bucket, and since Java 8 a bucket with more than 8 entries (when the table has at least 64 buckets) becomes a red-black tree, making the worst case O(log n). When size passes capacity times load factor 0.75, it resizes to double.
17
Your application hangs and CPU is near zero. How do you find out why?
Low CPU with a hang suggests threads are blocked, often a deadlock or waiting on a slow resource. Take several thread dumps with jcmd pid Thread.print. The JVM reports detected deadlocks at the end of the dump. Otherwise look for many threads BLOCKED on the same lock or WAITING on a connection pool or socket read, then fix lock order, add timeouts or increase the pool.
18
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.
19
What is the difference between HashMap, LinkedHashMap and TreeMap?
HashMap gives O(1) average operations with no ordering guarantee. LinkedHashMap keeps insertion order, or access order if configured, which helps build an LRU cache. TreeMap keeps keys sorted by natural order or a Comparator with O(log n) operations and supports range methods like floorKey and headMap. HashMap allows one null key; TreeMap does not allow null keys with natural ordering.
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.
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.
22
Tell me about a time you optimised a slow database query.
Situation: the customer search page took 6 seconds at peak. Task: bring it under 500 ms. Action: I captured the SQL, ran EXPLAIN ANALYZE and found a full scan due to LOWER(email) on an unindexed column and an N+1 on addresses. I added a functional index, fetched addresses with an entity graph and added pagination. Result: response time dropped to 180 ms and database CPU fell by a third.
23
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.
24
What is the difference between List.of(), Arrays.asList() and Collections.unmodifiableList()?
List.of() returns a truly immutable list; add, set and remove throw exceptions and nulls are not allowed. Arrays.asList() returns a fixed-size list backed by the array: set() works and writes through to the array, but add() and remove() throw. Collections.unmodifiableList() is a read-only view, so changes to the original list are still visible through it.
25
What is the difference between map() and flatMap()?
map() transforms each element into exactly one new element, so a Stream of orders mapped to totals stays one-to-one. flatMap() transforms each element into a stream and flattens all of them into one stream, which is useful for one-to-many cases, such as turning a list of orders into a single stream of all items with orders.stream().flatMap(o -> o.items().stream()).
26
What is the difference between intermediate and terminal operations in streams?
Intermediate operations like filter, map, sorted and distinct return a new stream and are lazy; nothing runs until a terminal operation is called. Terminal operations like collect, forEach, reduce, count and findFirst trigger processing and produce a result. Laziness lets streams fuse steps into a single pass and short-circuit, so findFirst on filtered data stops early.
The JVM first interprets bytecode, then profiles which methods run often. The Just In Time compiler converts these hot methods into optimised native code at runtime. HotSpot uses tiered compilation: C1 compiles quickly with light optimisation and C2 applies aggressive optimisations like inlining and escape analysis. This is why Java apps get faster after a warm-up period.
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
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.
The string pool is a special area in the heap where string literals are stored once and reused. String a = "hi" and String b = "hi" point to the same object. new String("hi") creates a separate heap object, and intern() returns the pooled copy. Pooling saves memory because many strings in apps repeat, such as status codes and keys.
Tips to crack the Infosys Java interview
Practise pseudocode output questions and puzzle-style reasoning, which commonly appear in Infosys online tests and HackWithInfy style contests.
Revise SQL joins, normalisation and one programming language well enough to write and explain simple programs on paper.
Mention willingness to learn new technologies and relocate, since Infosys emphasises its training programme and flexible project allocation.
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; Infosys interviewers usually ask follow-ups on your first answer.
Frequently asked questions
How many rounds are there in the Infosys Java interview?
Candidates usually go through 4 stages: Online assessment (aptitude, reasoning + pseudocode); Coding test; Technical interview; 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 Infosys Java interview?
This practice set leans on Collections Framework (6), OOP in Java (5), Java Basics and JVM (5), Strings and Exception Handling (4). Revise those first, then practise explaining one or two of your own projects end to end, because most Infosys interviewers ask you to walk through real work.
Are these the exact questions asked at Infosys?
No. These are curated practice questions that match the skills and difficulty usually tested in it services interviews like Infosys's. They are not a leaked or verified list of past questions.
Where does Infosys hire Java professionals in India?
Infosys has large teams in Bengaluru, Mysuru, Pune, Hyderabad, Chennai. Openings and locations change often, so check the Infosys careers page and filter by city and experience.