Implementation internals, resizing behaviour, ordering guarantees, and choosing the right collection.
4 questions in Java & Spring.
A declaration shows whether you chose a structure for its access pattern: Stack or LinkedList where ArrayDeque belongs, a List used for membership tests, a concrete type on a signature. Programming to interfaces helps only if the code stays efficient for every implementation it accepts.
The sort detected that your comparator contradicted itself partway through a merge. The usual causes are a subtraction that overflows, an ordering that is not transitive, and a sort key that changes while the sort is running. Use this collections answer to show the decision, trade-off, and evidence rather than a memorised definition. It also connects sorting to the point an interviewer is testing.
HashMap stores entries in an array of buckets indexed by a hash of the key; collisions chain into a linked list that converts to a red-black tree past a threshold, and exceeding the load factor triggers a resize that reallocates and redistributes every entry. It also connects collections to the point an interviewer is testing.
Structural modification through the collection during a for-each invalidates the iterator, which detects it by comparing a modification counter and throws ConcurrentModificationException. Remove through the iterator, or use removeIf, or pick a collection whose iterator is weakly consistent instead.