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Java · Complete Question Bank
Practice the complete collection of Java interview questions including theory, coding, MCQs and real interview problems.
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154
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Scroll through every important Java question asked in real interviews. Includes MCQs, subjective questions, and coding prompts.
Correct Answer: Because Java compiles to bytecode executed by a JVM on each platform
Browse more Java questions, explore related subjects, or practice full interview sets to strengthen your preparation.
Correct Answer: JRE = JVM + libs; JDK = JRE + dev tools
Correct Answer: JVM Stack
Correct Answer: Converts hot bytecode paths to native machine code
Correct Answer: public static void main(String[] args)
Correct Answer: Autoboxing can introduce unintended null unboxing NPEs
Correct Answer: false
Correct Answer: final = non-override/const; finally = always-runs block; finalize = GC pre-cleanup hook
Correct Answer: Superclass static → Subclass static → Superclass instance (fields/blocks/ctor) → Subclass instance (fields/blocks/ctor)
Correct Answer: Binding data and methods that operate on that data into a single unit
Correct Answer: Single and multilevel (no multiple class inheritance)
Correct Answer: Inheritance → IS-A
Correct Answer: Using a covariant (more specific) return type
Correct Answer: All of the above
Correct Answer: Interfaces can have default and static methods
Correct Answer: Objects of subtype failing when used as base type
Correct Answer: Restrict which classes can extend/implement them
Correct Answer: All of the above
Correct Answer: To store interned String literals and reuse references
Correct Answer: In single-threaded concatenations requiring performance
Correct Answer: equals compares content, == compares references
Correct Answer: It resizes by allocating a larger array and copying elements
Correct Answer: No duplicates; ordering depends on implementation
Correct Answer: Override both equals and hashCode consistently
Correct Answer: Modifying the collection structurally outside the iterator during iteration
Correct Answer: Collections.unmodifiableList(list)
Correct Answer: ConcurrentHashMap
Correct Answer: Return type can be covariant
Correct Answer: super accesses parent members/constructors
Correct Answer: Override the method in the class and optionally call InterfaceName.super.m()
Correct Answer: final and private
Correct Answer: Overloading: same name, different params; Overriding: same signature, subclass
Correct Answer: Who can extend/implement them
Correct Answer: Be replaceable for its base type without breaking client expectations
Correct Answer: Object’s runtime type
Correct Answer: IOException: checked, NullPointerException: unchecked
Correct Answer: Throwable → Exception → RuntimeException
Correct Answer: finally may not execute if System.exit is invoked.
Correct Answer: Specific exceptions must come before general ones.
Correct Answer: It must implement AutoCloseable.
Correct Answer: They appear as suppressed exceptions on the primary exception.
Correct Answer: throws declares potential exceptions; throw actually raises one instance.
Correct Answer: Treat Errors as fatal conditions not meant to be caught routinely.
Correct Answer: Avoid using exceptions for expected branches; they are relatively expensive.
Correct Answer: Yes, it is valid; finally still runs for cleanup.
Correct Answer: It dynamically switches to a balanced tree (red-black) after a threshold.
Correct Answer: Load factor threshold is exceeded.
Correct Answer: If equals returns true, hashCode must be equal.
Correct Answer: Reads are non-blocking; updates use fine-grained synchronization.
Correct Answer: It maintains insertion or access order via a doubly-linked list.
Correct Answer: It disallows null keys because it must compare keys.
Correct Answer: It returns a read-only view; underlying list changes still appear.
Correct Answer: ArrayList: fail-fast | ConcurrentHashMap: weakly consistent (fail-safe style)
Correct Answer: Type erasure removes T at runtime, so the JVM does not know the component type.
Correct Answer: Use extends to produce, super to consume.
Correct Answer: peek and poll respect priority, but iteration order is unspecified.
Correct Answer: put: blocks until space is available
Correct Answer: It pools and reuses threads, provides task submission APIs, lifecycle management, and better error handling.
Correct Answer: newFixedThreadPool(8)
Correct Answer: Each worker steals tasks from others to balance load, improving parallelism for fine-grained tasks.
Correct Answer: It adds completion callbacks, non-blocking composition, and chaining of dependent stages.
Correct Answer: CAS (Compare-And-Set) with CPU atomics
Correct Answer: It supports fairness policies, tryLock with timeout, and interruptible lock acquisition.
Correct Answer: When reads dominate and can proceed concurrently while no writer holds the lock.
Correct Answer: put blocks if the queue is full; take blocks if the queue is empty.
Correct Answer: To control thread names, priority, daemon status, UncaughtExceptionHandler, and context class loader.
Correct Answer: Limiting concurrent access to a finite resource, like 10 database connections.
Correct Answer: It supports dynamic registration and multi-phase coordination with onAdvance hooks.
Correct Answer: shutdown(); awaitTermination(timeout); then shutdownNow() if needed.
Correct Answer: Rules for visibility and ordering of reads and writes between threads.
Correct Answer: A call to unlock on a monitor followed by a later lock of the same monitor by another thread.
Correct Answer: Visibility of writes to other threads and ordering around the volatile variable.
Correct Answer: By allocating non-escaping objects on the stack and eliminating locks that don’t escape.
Correct Answer: Most objects die young, so collecting the young generation frequently is cheap and reduces work on long-lived objects.
Correct Answer: G1 divides the heap into regions and performs mostly concurrent, incremental collections with predictable pause targets.
Correct Answer: Ultra-low pause times via concurrent relocation and load barriers.
Correct Answer: WeakReference → memory-sensitive caches that may be cleared eagerly
Correct Answer: Finalizers are unpredictable and can resurrect objects; prefer try-with-resources or java.lang.ref.Cleaner for non-memory resources.
Correct Answer: A JVM point where all threads reach a known state so the VM can perform actions like GC; TTSP spikes can occur due to long non-interruptible native calls or tight loops without polls.
Correct Answer: Merges identical char[] or byte[] backing arrays across different String instances to reduce heap usage.
Correct Answer: It has exactly one abstract method (SAM), regardless of default or static methods.
Correct Answer: Lambdas capture effectively final variables from the enclosing scope.
Correct Answer: objectRef::instanceMethod
Correct Answer: A source, zero or more intermediate operations, and one terminal operation
Correct Answer: map, filter, sorted → intermediate; collect, reduce, forEach → terminal
Correct Answer: When mapping each element to a stream or collection and you want a single flattened stream.
Correct Answer: distinct
Correct Answer: Prefer parallel streams when workloads are CPU-bound, data is large, operations are stateless, non-blocking, and order is not critical.
Correct Answer: Groups elements by a classifier into a Map with List values.
Correct Answer: When combining into mutable containers like lists, maps, or StringBuilder.
Correct Answer: If the source is unordered (e.g., HashSet), operations like forEach may visit elements in any order unless you enforce ordering.
Correct Answer: They avoid boxing overhead and provide numeric-specific operations like sum and average.
Correct Answer: The second terminal operation throws IllegalStateException: stream has already been operated upon or closed.
Correct Answer: Implementing Runnable decouples task from thread and is generally preferred.
Correct Answer: Execute synchronously on the current thread.
Correct Answer: Atomicity and visibility for the guarded critical section, with mutual exclusion per monitor.
Correct Answer: Publishing a stop flag that multiple threads read frequently.
Correct Answer: Ability to attempt timed/non-blocking lock acquisition and to interrupt lock waits.
Correct Answer: It manages thread creation, reuse, and shutdown, decoupling task submission from execution.
Correct Answer: Blocks until the task completes or throws due to interruption/cancellation/failure.
Correct Answer: thenApply transforms value synchronously; thenCompose flattens a future-of-future pipeline.
Correct Answer: When you need lock-free, thread-safe, single-variable updates with CAS semantics.
Correct Answer: put and take block automatically when full or empty, coordinating producers and consumers without custom wait/notify.
Correct Answer: It uses work-stealing to balance fine-grained, CPU-bound tasks across worker threads.
Correct Answer: InputStream/OutputStream → byte data; Reader/Writer → character data
Correct Answer: To reduce costly kernel calls by reading or writing larger chunks
Correct Answer: Use InputStreamReader with an explicit Charset, such as UTF_8
Correct Answer: Files.walk returns a lazily traversed stream that must be closed
Correct Answer: Use Files.readAttributes(path, BasicFileAttributes.class)
Correct Answer: Directory change notifications with events like CREATE, DELETE, MODIFY
Correct Answer: They support non-blocking I/O and efficient bulk transfers, mapping well to OS primitives
Correct Answer: When random access to very large files is needed and OS page cache can help
Correct Answer: Implementing java.io.Serializable
Correct Answer: To control compatibility across versions and avoid InvalidClassException when the compiler’s computed value would differ
Correct Answer: Use transient to exclude a field; customize with writeObject/readObject for special handling
Correct Answer: Files.move can perform an atomic move when supported by the filesystem