Four Maps and Four Keys: Which Combinations Work

Four map types and four things people put in them. Decide which combinations work, which throw and which compile and misbehave. All before the reveal.

The four combinations to judge
	The combination
1	A `TreeMap` with a key class that implements nothing
2	A `HashMap` with one null key
3	A `HashMap` with a key class that overrides `equals` only
4	A `LinkedHashMap` read in a loop, expecting insertion order

What each of the four does

Each combination, what happens, and why
	What happens	Why
1	Throws on the second put	A sorted map must compare keys, and the class is not `Comparable`
2	Works	A hash map special-cases null and stores it in the first bucket
3	Compiles, and entries go missing	Equal keys get different hashes, so `get` searches the wrong bucket
4	Works	Insertion order is exactly what this map promises

The first throws on the second put rather than the first, which is worth noticing. One key needs no comparison, so a test with a single entry passes.

The third is the only silent one, and it is the most common. It is why equals and hashCode are written together or not at all.