Collections Framework interview questions

Implementation internals, resizing behaviour, ordering guarantees, and choosing the right collection.

3 questions in Java & Spring.

Java and the Spring ecosystemA 22-minute read on how the topic fits together, before the questions.Java fundamentals75 short answers on one page, for revising rather than studying.

medium3

mediumConceptCoding

Sorting a list throws IllegalArgumentException saying the comparison method violates its general contract. What did the comparator get wrong?

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.

5 minmid, senior, staff

mediumConcept

How does HashMap work internally, and what happens when it resizes?

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.

3 minentry, mid, senior

mediumConceptCoding

How do you remove elements from a collection while iterating it, and what makes an iterator fail-fast?

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.

5 minentry, mid, senior