Core Java API Flashcards

What does `Collections.unmodifiableList(list)` guarantee?

Answer: Mutations through the wrapper throw UnsupportedOperationException

The unmodifiable wrapper throws UnsupportedOperationException on mutating calls, but mutations to the backing list are still visible through the wrapper.

What is printed by `System.out.println(Integer.toBinaryString(-1))`?

Answer: "11111111111111111111111111111111"

`Integer.toBinaryString(-1)` returns the unsigned 32-bit binary representation of -1, which is 32 ones.

Which `Map` method returns the value associated with a key, or a default if the key is absent?

Answer: getOrDefault()

`Map.getOrDefault(key, defaultValue)` returns the mapped value or the specified default without modifying the map.

What is the result of `String.format("%05d", 42)`?

Answer: "00042"

The format specifier `%05d` pads the integer with leading zeros to a width of 5, producing "00042".

Which statement about `ArrayList` vs `LinkedList` is accurate?

Answer: ArrayList has O(1) random get by index

ArrayList backed by an array supports O(1) index-based access, while LinkedList requires O(n) traversal to reach an element.

What does `Math.round(2.5)` return in Java?

Answer: 3

`Math.round(2.5)` returns 3 (a long); it uses half-up rounding, so .5 always rounds toward positive infinity.

What exception is thrown when you access an index beyond the end of an array?

Answer: ArrayIndexOutOfBoundsException

Accessing an invalid array index throws `ArrayIndexOutOfBoundsException`, a subclass of IndexOutOfBoundsException.