1Z0-819 I/O Operations 4

Question 1

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

Accepted Answer

MappedByteBuffer

Answer

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

Question 2

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

Accepted Answer

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

Answer

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

Question 3

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

Accepted Answer

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

Answer

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

Question 4

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

Accepted Answer

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

Answer

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

Question 5

What is the role of `serialVersionUID` in serialization?

Accepted Answer

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

Answer

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

(1Z0-819) Oracle Java SE 11 Developer Practice Test