Add KiwiMaps methods to get a non-null value or throw an exception

[sleberknight]

Map has get which returns "the value to which the specified key is mapped, or null if this map contains no mapping for the key", and getOrDefault which returns "the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key".

But many times I want to get a non-null value from a map for a specific key. In other words, the map should have a mapping (i.e., there is an entry in the map with that key) which contains a non-null value. And if it doesn't , I want to treat it an exceptional condition and throw an exception.

So, we can add a method to KiwiMaps that has the following signature:

public static <K, V> V getOrThrow(Map<K, V> map, K key)

It should throw a NoSuchElementException when the map doesn't contain the specified key, or if it has the key but the associated value is null.

Since it might also be desirable to throw some other kind of exception, e.g., a ValidationException or IllegalStateException, there should be the more generic method:

public static <K, V> V getOrThrow(Map<K, V> map, K key, Supplier<RuntimeException> exceptionSupplier)

Note this only permits runtime exceptions, so that we don't force a try/catch (or a Lombok @SneakyThrows).

To permit throwing checked exceptions, we can also add:

public static <K, V, E extends Exception> V getOrThrowChecked(Map<K, V> map,
                                                              K key,
                                                              Supplier<E> exceptionSupplier) throws E {

This last method would be the exception, not the rule, i.e., we expect the normal case to be that a RuntimeException is preferred.

[sleberknight]

These methods should throw IlegalArgumentException if any of the arguments are null