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Java · Question Set
Java I/O, NIO.2 & Serialization interview questions for placements and exams.
Questions
14
Included in this set
Subject
Java
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Difficulty
Mixed
Level of this set
Go through each question and its explanation. Use this set as a focused practice pack for Java.
Stream classes move raw bytes; Reader/Writer classes decode and encode characters using a charset. They’re not interchangeable, but bridges like InputStreamReader and OutputStreamWriter adapt bytes to characters and vice-versa.
Buffering batches I/O into fewer, larger operations, which is faster than many tiny reads and writes. It doesn’t add compression, thread safety, or change encodings by itself.
Platform defaults vary and can break portability. Always specify the charset explicitly when converting bytes to characters to avoid mojibake and subtle data loss.
Files.walk produces a Stream that must be closed (try-with-resources). Path can represent non-existing locations; Paths.get can be relative. readAllLines loads all content into memory.
NIO.2 exposes metadata via attribute views like BasicFileAttributes and PosixFileAttributes without reading file payloads, which is lightweight and efficient.
WatchService lets you register a directory for change events. It’s best effort, may coalesce events, and is not a content diff tool or replication system.
Memory mapping lets the OS page data on demand and can be very fast for large random access workloads. It uses native resources and requires careful unmapping and bounds handling.
Default Java object serialization activates when a class implements Serializable. You can customize via writeObject/readObject, but the marker interface is the trigger.
Mark sensitive or derived fields transient to omit them. For tailored behavior, implement private writeObject/readObject hooks to transform, validate, or reconstruct state.
Deserialization can trigger gadget chains leading to remote code execution when untrusted data is accepted; the object graph is opaque, brittle across versions, and slow. Safer options: JSON or CBOR with vetted mappers, ProtoBuf/Avro/FlatBuffers for schema-driven, version-tolerant formats, or dedicated DTOs with explicit parsing. If you must deserialize, enable object filters (JEP 290), restrict classes, and do input validation.
Use CopyOption flags to control behavior. For example, REPLACE_EXISTING to overwrite, COPY_ATTRIBUTES to preserve metadata, and ATOMIC_MOVE for all-or-nothing moves when supported.
It guarantees timely, exception-safe closing of Closeable and AutoCloseable resources. Benefits: 1) Avoids leaks by closing even when exceptions occur. 2) Keeps code concise and readable; suppressed exceptions are preserved, aiding debugging.
Channels integrate with selectors and ByteBuffers for non-blocking or zero-copy patterns, allowing efficient, scalable I/O. Throughput still depends on workload and hardware.
serialVersionUID is a version stamp. Declaring it stabilizes compatibility decisions across builds and code changes. It has nothing to do with compression or encryption.
For complete preparation, combine this set with full subject-wise practice for Java. You can also explore other subjects and sets from the links below.