Core Java API Flashcards

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

Answer: NullPointerException

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

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

Answer: reverse()

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

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

Answer: Throws UnsupportedOperationException

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

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

Answer: -3.0

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

Which interface does `HashMap` directly implement?

Answer: Map

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

What is the result of `"hello".substring(2, 4)`?

Answer: "ll"

`substring(beginIndex, endIndex)` returns characters from index 2 (inclusive) to 4 (exclusive): 'l' and 'l'.

Which `Optional` method returns the value if present, otherwise throws `NoSuchElementException`?

Answer: get()

`Optional.get()` returns the value if present, but throws `NoSuchElementException` if the Optional is empty.