Web / Apache Commons Collections Interview questions

What is a ReferenceMap in Apache Commons Collections?

ReferenceMap<K,V> is a Map whose keys and/or values are held through java.lang.ref soft or weak references rather than ordinary strong references.

ReferenceMap<String, byte[]> cache =
    new ReferenceMap<>(ReferenceStrength.HARD, ReferenceStrength.SOFT);

cache.put("payload", largeByteArray);
// entry may be reclaimed automatically if the JVM needs memory

Because the garbage collector can reclaim soft references under memory pressure (or weak references as soon as nothing else holds them), entries can disappear from a ReferenceMap on their own - which is a deliberate trade-off, not a bug.

This makes it well suited to caches or lookup tables where you'd rather silently lose stale entries than risk an OutOfMemoryError from a plain HashMap that never shrinks.

Take quiz

Entries in a ReferenceMap can disappear on their own because: ReferenceMap has a bug that randomly drops entries

✗ Try again if not.

the JVM can reclaim soft/weak referenced entries under memory pressure

✓ Correct! Well done.

entries expire after a fixed time-to-live

✗ Try again if not.

ReferenceMap is most useful for: storing configuration values that must never be lost

✗ Try again if not.

caches that should shrink under memory pressure instead of risking OutOfMemoryError

✓ Correct! Well done.

guaranteeing sorted iteration order

✗ Try again if not.

More Related questions...

What is Apache Commons Collections? What are the main packages in Apache Commons Collections 4? What is a Bag in Apache Commons Collections? What are the types of Bag implementations in Commons Collections? What is a BidiMap in Apache Commons Collections? What is a MultiValuedMap in Apache Commons Collections? What is the purpose of CollectionUtils in Apache Commons Collections? What is the purpose of MapUtils in Apache Commons Collections? What is the purpose of ListUtils in Apache Commons Collections? What are Predicates in Apache Commons Collections? What are Transformers in Apache Commons Collections? What are Closures in Apache Commons Collections? What is a Factory in Apache Commons Collections? Define CircularFifoQueue in Apache Commons Collections? What is an LRUMap in Apache Commons Collections? What is a ReferenceMap in Apache Commons Collections? What is a MultiKeyMap in Apache Commons Collections? What is the purpose of IteratorUtils in Apache Commons Collections? Describe the LoopingIterator class in Apache Commons Collections? What is an OrderedMap in Apache Commons Collections? What is a SortedBidiMap in Apache Commons Collections? What is the purpose of ComparatorUtils in Apache Commons Collections? Define FixedOrderComparator in Apache Commons Collections? What is a PredicatedCollection in Apache Commons Collections? What is a TransformedCollection in Apache Commons Collections? What is the difference between a Map and a MultiValuedMap? What is the difference between a BidiMap and a regular Map? Why is CollectionUtils.isEmpty() preferred over calling isEmpty() directly? How does a TreeBag maintain element ordering internally? How does LRUMap decide which entry to evict? What is the difference between HashBag and TreeBag? Why do we use predicate chaining with allPredicate and anyPredicate? How is UnmodifiableMap in Commons Collections different from java.util's Collections.unmodifiableMap? What happens when you add a duplicate value to a BidiMap? How does PredicatedList enforce validation on add operations? Explain the internal working of CircularFifoQueue? How can you optimize repeated multi-field lookups using MultiKeyMap? What is the difference between Apache Commons Collections 3.x and 4.x? Why should you use TransformedMap instead of manual validation in setters? When should you choose Apache Commons Collections over Guava collections? Explain the execution flow of CollectionUtils.collect()? How is FactoryUtils used to lazily create objects? Why is Apache Commons Collections associated with a well-known deserialization vulnerability? What is the difference between the legacy MultiMap (3.x) and MultiValuedMap (4.x)? How does ReferenceMap help prevent memory leaks in long-running caches? Why doesn't a Bag simply behave like a Set? How do you troubleshoot a ConcurrentModificationException when using CollectionUtils.filter()? Explain the lifecycle of a ClosureUtils.chainedClosure() execution? What is the difference between SetUtils.union() and manually merging two sets? How does Commons Collections' Trie support prefix-based lookups?

Show more question and Answers...

Comments & Discussions

Angular Interview questions 124 ReactJS Interview questions 64 Typescript interview questions 22 NodeJS Interview questions 14 Nativescript interview questions 5 React native Interview questions 51 ES6 Interview questions 7 ES8 Interview questions 3 RxJs Interview questions 17 HTML Interview questions 9 JSP interview questions 6 TanStack Interview Questions 38 Caddy Server Interview questions 50 NGINX Interview questions 50 Traefik Interview questions 50 Apache Lucene Interview questions 50 Apache Solr Interview questions 50 Apache OFBiz Interview questions 50 Apache Axiom Interview questions 50 Apache Commons Collections Interview questions 50

Recently added questions

What are Transformers in Apache Commons Collections? What is a MultiKeyMap in Apache Commons Collections? What is a MultiValuedMap in Apache Commons Collections? What are Predicates in Apache Commons Collections? Define CircularFifoQueue in Apache Commons Collections? What is a ReferenceMap in Apache Commons Collections? What are the main packages in Apache Commons Collections 4? What is a BidiMap in Apache Commons Collections? What is the purpose of MapUtils in Apache Commons Collections? What is the purpose of ListUtils in Apache Commons Collections? What are Closures in Apache Commons Collections? What is a Factory in Apache Commons Collections? What is the difference between SetUtils.union() and manually merging two sets? What is an LRUMap in Apache Commons Collections? How does Commons Collections' Trie support prefix-based lookups? What is Apache Commons Collections? What is a Bag in Apache Commons Collections? What are the types of Bag implementations in Commons Collections? What is the purpose of CollectionUtils in Apache Commons Collections? What is the purpose of IteratorUtils in Apache Commons Collections?

	Interviews Questions Java Spring Hibernate DevOps Testing API BigData Web Erlang AI Database Integration Cloud Python Scala Tools Golang SAP	About Javapedia.net Javapedia.net is for Java and J2EE developers, technologist and college students who prepare of interview. Also this site includes many practical examples. This site is developed using J2EE technologies by Steve Antony, a senior Developer/lead at one of the logistics based company. Follow Javapedia updates
	contact: javatutorials2016[at]gmail[dot]com
Kindly consider donating for maintaining this website. Thanks.
	Copyright © 2026, javapedia.net, all rights reserved. Privacy Policy · Terms of Service.