1Z0-819 Core APIs & Data Manipulation 4

Question 1

What does `String.valueOf(null)` return?

Accepted Answer

"null"

Answer

`String.valueOf((Object) null)` returns the string literal `"null"` rather than throwing an exception.

Question 2

Which `List` factory method (Java 9+) produces an immutable list?

Accepted Answer

List.of()

Answer

`List.of()` introduced in Java 9 returns a truly immutable list; `Arrays.asList` is only structurally fixed but not immutable.

Question 3

What is printed by: `var sb = new StringBuilder("abc"); sb.reverse(); System.out.println(sb);`?

Accepted Answer

cba

Answer

`StringBuilder.reverse()` reverses the character sequence in-place and returns the same `StringBuilder`, so printing it shows "cba".

Question 4

Which `Map` method atomically replaces the value for a key using a remapping function if the key exists?

Accepted Answer

computeIfPresent()

Answer

`computeIfPresent(key, biFunction)` calls the function with the key and current value only when the key is already mapped to a non-null value.

Question 5

What does `Stream.of("a","b","c").skip(1).limit(1).findFirst()` return?

Accepted Answer

Optional[b]

Answer

`skip(1)` discards "a", leaving "b" and "c"; `limit(1)` keeps only "b"; `findFirst()` returns `Optional[b]`.

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