In Java 8+, when does a HashMap bucket turn into a red-black tree?
TREEIFY_THRESHOLD is 8 and MIN_TREEIFY_CAPACITY is 64; with a smaller table, HashMap resizes instead.
QuizzesModern Java
Collections & Streams
HashMap internals, choosing collections, iterators, lambdas, streams and collectors.
16 questions
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Question 1 of 16Score 0
A HashMap has the default capacity and load factor. At what size does it first resize?
Threshold = capacity × load factor = 16 × 0.75 = 12.
What does this throw?
List<String> list = new ArrayList<>(List.of("a", "b", "c"));
for (String s : list) {
if (s.equals("a")) list.remove(s);
}Structural modification during for-each iteration trips the fail-fast iterator. Use removeIf or Iterator.remove().
What happens here?
List<Integer> nums = List.of(1, 2, 3);
nums.add(4);List.of returns an immutable list; mutators throw at runtime.
Which collection keeps keys sorted and supports floorKey / ceilingKey?
TreeMap is a red-black tree implementing NavigableMap, with O(log n) operations.
What is the best general-purpose choice for a FIFO queue or a stack?
ArrayDeque is faster than LinkedList and Stack (which is synchronized and legacy).
Which LinkedHashMap constructor argument enables LRU-style ordering?
With access order, get moves an entry to the end; override removeEldestEntry to evict.
What does this print?
Stream.of("a", "bb", "ccc")
.peek(s -> System.out.print(s + " "))
.filter(s -> s.length() > 1)
.findFirst();Streams are lazy and process elements one at a time. findFirst short-circuits after "bb" passes the filter, so "ccc" is never peeked.
What does this print?
Stream.of("a", "b").map(String::toUpperCase);
System.out.println("done");Without a terminal operation, intermediate operations never execute.
Which operation turns Stream<List<String>> into Stream<String>?
map would give Stream<Stream<String>>; flatMap flattens.
What happens when Collectors.toMap(Person::city, Person::name) meets two people in the same city?
Without a merge function toMap throws on duplicate keys. Pass one, e.g. (a, b) -> a + "," + b.
Which collector builds Map<Boolean, List<T>> that always contains both keys?
partitioningBy always has true and false entries; groupingBy only creates keys that occur.
Why does this fail to compile?
int count = 0;
list.forEach(x -> count++);Captured locals must not be reassigned. Use list.size(), a stream count(), or an AtomicInteger.
What is the difference between orElse(x) and orElseGet(() -> x) on an Optional with a value?
Arguments are evaluated before a method is called, so an expensive default passed to orElse always runs.
Which is a poor use of a parallel stream?
Blocking I/O on the shared common ForkJoinPool starves the whole JVM. Use virtual threads or a dedicated executor for I/O fan-out.
String::compareToIgnoreCase is used as a Comparator<String>. What kind of method reference is it?
The first lambda parameter becomes the receiver: (a, b) -> a.compareToIgnoreCase(b).
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