1Z0-819 I/O Operations 5

Question 1

What is the output of `System.out.printf("%05.2f", 3.7)` ?

Accepted Answer

"03.70"

Answer

`%05.2f` formats to total width 5, 2 decimal places, zero-padded, producing `03.70`.

Question 2

Which `PrintWriter` constructor automatically flushes the buffer after every `println`, `printf`, or `format` call?

Accepted Answer

Both A and C

Answer

Passing `true` as the auto-flush argument to either the `Writer` or `OutputStream` constructor enables auto-flush on line-terminating methods.

Question 3

What does `DataInputStream.readUTF()` expect about the byte stream format?

Accepted Answer

A 2-byte length prefix followed by modified UTF-8 encoded bytes

Answer

`readUTF` reads a 2-byte big-endian length then the modified UTF-8 bytes, matching the format written by `DataOutputStream.writeUTF`.

Question 4

Which NIO.2 attribute view provides access to POSIX file permissions?

Accepted Answer

PosixFileAttributeView

Answer

`PosixFileAttributeView` exposes owner, group, and permission bits on POSIX-compliant filesystems.

Question 5

What is the default behavior of `Files.copy(InputStream in, Path target)` if the target file already exists?

Accepted Answer

It throws `FileAlreadyExistsException`

Answer

Without `REPLACE_EXISTING`, `Files.copy` fails with `FileAlreadyExistsException` to avoid silent data loss.

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