Java Collections Framework Flashcards
6 cards from real SCJP practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Java Collections Framework flashcards as text
Which method must be overridden in a class for its objects to work correctly as HashMap keys?
Answer: equals() and hashCode()
HashMap uses `hashCode()` to find the bucket and `equals()` to confirm key equality, so both must be correctly overridden.
Which List implementation provides O(1) add/remove at the ends but O(n) for random access?
Answer: LinkedList
`LinkedList` uses a doubly-linked list, giving O(1) insertions at head/tail but O(n) for index-based access.
What is the difference between `Iterator` and `ListIterator` in Java?
Answer: ListIterator can traverse in both directions; Iterator is forward-only
`ListIterator` extends `Iterator` with the ability to traverse a list in both forward and backward directions and also supports `add()` and `set()`.
Which collection class is synchronized and considered the thread-safe version of ArrayList?
Answer: Vector
`Vector` is a synchronized, thread-safe dynamic array, essentially the legacy thread-safe counterpart to `ArrayList`.
What does the `Collections.sort()` method require from the objects it sorts?
Answer: Objects must implement Comparable
`Collections.sort()` requires list elements to implement `Comparable` so their natural ordering can be used.
Which Map implementation maintains keys in their insertion order?
Answer: LinkedHashMap
`LinkedHashMap` extends `HashMap` and maintains a linked list of entries in insertion order.
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