Core APIs & Data Manipulation Flashcards

What does `String.format("%05d", 42)` produce?

Answer: 00042

The `%05d` format specifier pads the integer with leading zeros to a width of 5.

Which method returns an `OptionalInt` containing the sum of an `IntStream`?

Answer: reduce(Integer::sum)

`IntStream.reduce(Integer::sum)` returns an `OptionalInt` because the stream might be empty; `sum()` returns a plain `int`.

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

Answer: ll

`substring(beginIndex, endIndex)` extracts characters from index 2 (inclusive) to 4 (exclusive), yielding "ll".

Which `LocalDate` method adds a specified number of months and adjusts the day if the result month is shorter?

Answer: plusMonths()

`plusMonths()` adds months and normalizes the day-of-month if the target month has fewer days.

Which interface does `HashMap` NOT implement?

Answer: SortedMap

`HashMap` implements `Map`, `Cloneable`, and `Serializable`, but NOT `SortedMap` — that is implemented by `TreeMap`.

What does `Arrays.asList(1, 2, 3).remove(0)` do at runtime?

Answer: Throws UnsupportedOperationException

The list returned by `Arrays.asList` is fixed-size; structural modifications like `remove` throw `UnsupportedOperationException`.

What is the output of `Math.round(2.5)`?

Answer: 3

`Math.round(double)` uses half-up rounding, so 2.5 rounds to 3L (returns a `long`).