I/O Operations Flashcards

Which class is used for high-performance binary file I/O using memory-mapped files in Java NIO?

Answer: MappedByteBuffer

`MappedByteBuffer` maps a region of a file directly into memory, enabling fast random access without traditional read/write calls.

What is the purpose of `FileChannel.transferTo(position, count, target)`?

Answer: Efficiently transfers bytes from one channel to another, potentially using OS-level zero-copy

`transferTo` can leverage OS zero-copy mechanisms to move data between channels without buffering in the JVM heap.

Which interface must be implemented to receive notifications from a `WatchService`?

Answer: No interface — you poll `WatchKey` events from the service

`WatchService` is poll-based: you call `take()` or `poll()` on the service to retrieve `WatchKey` objects containing `WatchEvent` lists.

What does `Serializable` marker interface indicate for Java I/O?

Answer: The class can be serialized by `ObjectOutputStream` and deserialized by `ObjectInputStream`

Implementing `Serializable` signals that `ObjectOutputStream`/`ObjectInputStream` may serialize and deserialize instances of the class.

What is the role of `serialVersionUID` in serialization?

Answer: It is used to verify that the sender and receiver of a serialized object have loaded classes that are compatible

During deserialization, the JVM compares `serialVersionUID` between the class definition and the stream; a mismatch throws `InvalidClassException`.

Which keyword prevents a field from being serialized by `ObjectOutputStream`?

Answer: transient

Fields marked `transient` are skipped by the default serialization mechanism and restored to their default values on deserialization.

What does `Console.readPassword()` return that makes it preferable to `readLine()` for passwords?

Answer: A `char[]` that can be zeroed out after use, unlike `String` which is immutable

Returning `char[]` allows the caller to overwrite the array in memory after use, while `String` values persist in the string pool.