1Z0-811 Core Java API 2

Question 1

What does `String.valueOf(null)` return when the argument is a `char[]` variable set to null?

Accepted Answer

NullPointerException

Answer

Calling `String.valueOf((char[]) null)` throws a NullPointerException because the char[] overload dereferences the array.

Question 2

Which method of `StringBuilder` reverses the character sequence in place?

Accepted Answer

reverse()

Answer

`StringBuilder.reverse()` reverses the sequence of characters and returns the same StringBuilder.

Question 3

Given `List<String> list = List.of("a","b","c");`, what happens when you call `list.set(0, "z")`?

Accepted Answer

Throws UnsupportedOperationException

Answer

Lists created with `List.of()` are immutable, so any structural or element-change operation throws UnsupportedOperationException.

Question 4

What is the output of `System.out.println(Math.floor(-2.5))`?

Accepted Answer

-3.0

Answer

`Math.floor()` returns the largest double less than or equal to the argument, so -2.5 floors to -3.0.

Question 5

Which interface does `HashMap` directly implement?

Accepted Answer

Map

Answer

`HashMap` directly implements the `Map` interface and extends `AbstractMap`.

(1Z0-811) Oracle Java SE11 Programmer Practice Test