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Java Telephonic Interview Questions & Answers

30 questions for the telephonic interview stage — 17 easy, 13 medium.

About the telephonic interview round

The telephonic or screening round is a 15-30 minute call with a recruiter or engineer. It checks your basics, your CV and whether you can explain Java concepts clearly without a screen to point at.

Tips for the telephonic interview round

Keep your CV in front of you and be ready to explain every line on it.
Answer in short, clear sentences; there is no whiteboard, so structure matters.
Take the call somewhere quiet and confirm the time in advance.
Prepare one question to ask the interviewer at the end.

Solved — telephonic interviewStart practicing

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What is exception chaining?

Strings and Exception HandlingMedium

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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.

You need to remove items from an ArrayList while looping over it. How do you do it safely?

Collections FrameworkEasy

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Removing inside a for-each loop throws ConcurrentModificationException. Use list.removeIf(item -> item.isExpired()), which is clear and efficient. Alternatives are an explicit Iterator calling iterator.remove(), or looping backwards by index. For large lists removeIf is best because it shifts elements once instead of on every removal.

A team member compares two Integer values with == and the check fails only for large numbers. Explain.

Java Basics and JVMMedium

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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.

Can you override a static or private method in Java?

OOP in JavaEasy

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No. Private methods are not visible to subclasses, so a method with the same name in a child is a new method. Static methods belong to the class; a static method with the same signature in a child hides the parent one, and which one runs depends on the reference type, not the object. The @Override annotation on such methods gives a compile error.

What is the difference between checked and unchecked exceptions?

Strings and Exception HandlingEasy

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Checked exceptions extend Exception (but not RuntimeException) and must be caught or declared with throws, for example IOException and SQLException. Unchecked exceptions extend RuntimeException, such as NullPointerException and IllegalArgumentException, and need no declaration. Use checked exceptions for recoverable conditions callers must handle and unchecked ones for programming errors. Errors like OutOfMemoryError should not be caught.

Does the finally block always execute?

Strings and Exception HandlingMedium

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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.

Why is PreparedStatement preferred over Statement?

JDBC, Hibernate and JPAEasy

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PreparedStatement sends the SQL with placeholders separately from the parameter values, so user input is never treated as SQL, which prevents SQL injection. The database can also cache and reuse the execution plan, and it handles types like dates correctly. Statement with string concatenation is unsafe and slower for repeated queries, so it should not be used with user input.

An API needs to find the price band for any amount, for example the nearest lower slab. Which structure fits?

Collections FrameworkMedium

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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.

What is the difference between a static and an instance variable?

Java Basics and JVMEasy

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A static variable belongs to the class, so there is only one copy shared by all objects, and it is created when the class is loaded. An instance variable belongs to each object and gets a separate copy per instance. A counter of created objects should be static, while a customer's name should be an instance field.

How does HashSet work internally?

Collections FrameworkEasy

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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.

What is the difference between ArrayList and LinkedList?

Collections FrameworkEasy

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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.

What is the contract between equals() and hashCode()?

OOP in JavaMedium

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If two objects are equal by equals(), they must return the same hashCode(). Unequal objects may share a hash code, but fewer collisions give better performance. If you override equals() but not hashCode(), HashMap and HashSet break: an equal key lands in a different bucket and cannot be found. Use the same fields in both methods, or use records, which generate both.

Compare String, StringBuilder and StringBuffer.

Strings and Exception HandlingEasy

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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.

Write a simple REST controller in Spring Boot for creating and fetching products.

Spring Boot and MicroservicesEasy

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Use constructor injection and validation. @RestController @RequestMapping("/api/products") class ProductController { private final ProductService service; ProductController(ProductService service) { this.service = service; } @PostMapping ResponseEntity<Product> create(@Valid @RequestBody ProductRequest req) { Product p = service.create(req); return ResponseEntity.created(URI.create("/api/products/" + p.id())).body(p); } @GetMapping("/{id}") Product get(@PathVariable Long id) { return service.get(id); } }

How do you handle a situation where you made a wrong technical decision?

JDBC, Hibernate and JPAEasy

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Situation: I chose eager loading for a reporting entity to save time, and later pages became slow. Task: own and fix it. Action: I told my lead, measured the impact, switched associations to lazy with targeted fetch joins and added query count tests. Result: performance recovered within a sprint and I now review fetch strategy in every design discussion.

Explain the main memory areas of the JVM.

Java Basics and JVMMedium

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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.

What is the difference between composition and inheritance, and which should you prefer?

OOP in JavaMedium

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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.

What are the four pillars of OOP? Explain with Java examples.

OOP in JavaEasy

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Encapsulation hides data behind methods, for example private fields with getters and setters in a BankAccount class. Abstraction exposes only what is needed through abstract classes or interfaces, like a Payment interface. Inheritance lets a class reuse another, such as SavingsAccount extends Account. Polymorphism lets one reference behave differently, like a Payment variable calling pay() on a UpiPayment or CardPayment.

What is a functional interface?

Java 8+ Features and StreamsEasy

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A functional interface has exactly one abstract method, so it can be implemented with a lambda or method reference. It may also have default and static methods. Examples are Runnable, Comparator, and those in java.util.function: Function, Predicate, Consumer, Supplier and BiFunction. The @FunctionalInterface annotation makes the compiler enforce the single abstract method rule.

What is the difference between List.of(), Arrays.asList() and Collections.unmodifiableList()?

Collections FrameworkMedium

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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.

What is autoboxing and what pitfall can it cause?

Java Basics and JVMMedium

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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.

Objects used as HashMap keys cannot be found after insertion. What could be wrong?

OOP in JavaMedium

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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.

What are the primitive data types in Java and their sizes?

Java Basics and JVMEasy

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Java has eight primitives: byte (1 byte), short (2), int (4), long (8), float (4), double (8), char (2 bytes, UTF-16 code unit) and boolean (true or false, size JVM dependent). Primitives store values directly and have default values when used as fields, for example 0 for int and false for boolean. Each has a wrapper class such as Integer.

What is the difference between final, finally and finalize?

Java Basics and JVMEasy

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final is a keyword: a final variable cannot be reassigned, a final method cannot be overridden and a final class cannot be extended. finally is a block that runs after try-catch whether or not an exception occurs, used for cleanup. finalize() was a method called before garbage collection; it is deprecated for removal and should be replaced with try-with-resources or Cleaner.

What are method references? Give the four types.

Java 8+ Features and StreamsEasy

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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.

What is the difference between HashMap, LinkedHashMap and TreeMap?

Collections FrameworkEasy

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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.

What does effectively final mean for lambdas?

Java 8+ Features and StreamsMedium

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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.

What is the difference between static and dynamic binding?

OOP in JavaMedium

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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.

What are the ways to create a thread in Java?

Multithreading and ConcurrencyEasy

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You can extend Thread and override run(), or implement Runnable or Callable and pass it to a Thread or an ExecutorService. Callable returns a result and can throw checked exceptions, giving a Future. Modern code rarely creates threads directly; it uses executors, CompletableFuture or, from Java 21, virtual threads with Thread.ofVirtual().start(task).

You have to process tasks in order of priority. Which collection do you use?

Collections FrameworkMedium

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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.

Frequently asked questions

What happens in the Java telephonic interview round?

The telephonic or screening round is a 15-30 minute call with a recruiter or engineer. It checks your basics, your CV and whether you can explain Java concepts clearly without a screen to point at.

How do I clear the telephonic interview round?

Keep your CV in front of you and be ready to explain every line on it. Answer in short, clear sentences; there is no whiteboard, so structure matters. Take the call somewhere quiet and confirm the time in advance.

Which round comes next?

A typical order is online assessment, telephonic screening, one or two technical rounds, a managerial round and HR. Use the other round pages to practise each stage.

Practise every interview round

Online Assessment questions Technical Round questions Managerial Round questions Rapid-Fire questions HR & behavioural questions Full mock interview

More ways to practise

Company-wise Java questions Experience-wise Java questions Role-wise Java questions City-wise Java questions Most asked Java questions Scenario-based questions Practical / hands-on questions HR & behavioural questions Java mock interview Online assessment questions Technical round questions Managerial round questions Java topic sheets