Collections

Learn

Existing chapters for this concept.

• Collections

  Core Java

  Average lookup and update are O(1) when hashes spread well. Heavy collisions degrade toward O(n) — interview material covers capacity and load factor in depth.

  Open lesson →

See it in code

Existing Java examples.

• core 19 Collections Demo

  Core Java · pkg1core/core19CollectionsDemo.java · pkg1core · core19CollectionsDemo.java

  - List: ordered, indexed, allows duplicates (ArrayList / LinkedList). - Set: no duplicates (HashSet=unordered, LinkedHashSet=insertion, TreeSet=sorted). -…

  Open example →
• jvm 3 Garbage Collection Demo

  JVM · pkg6jvm/jvm3GarbageCollectionDemo.java · pkg6jvm · jvm3GarbageCollectionDemo.java

  Open example →
• core29HashMapDemo

  Core Java · pkg1core/core29HashMapDemo.java · pkg1core · core29HashMapDemo.java

  - Keys are unique; put with an existing key replaces the value. - Lookup uses hashCode (bucket) then equals (match within the bucket). - Mutating a key after…

  Open example →
• core28ComparatorDemo

  Core Java · pkg1core/core28ComparatorDemo.java · pkg1core · core28ComparatorDemo.java

  - Comparable: built into the class (`compareTo`). One natural ordering. - Comparator: external, multiple orderings, lambdas/method refs. - Use…

  Open example →
• datastructures6HashTableImpl

  Data structures · pkg3datastructures/datastructures6HashTableImpl.java · pkg3datastructures · datastructures6HashTableImpl.java

  Open example →
• blind75_LC1TwoSum

  Blind 75 · Array · pkg5leetcode/blind75/blind75_LC1TwoSum.java · pkg5leetcode · blind75_LC1TwoSum.java

  One-pass hash map stores value->index; check complement each step. Time O(n), Space O(n)

  Open example →
• blind75_LC49GroupAnagrams

  Blind 75 · String · pkg5leetcode/blind75/blind75_LC49GroupAnagrams.java · pkg5leetcode · blind75_LC49GroupAnagrams.java

  HashMap keyed by sorted char signature. Time O(n k log k), Space O(nk)

  Open example →

Prepare

Existing interview questions.

• Autobox in collections?

  Core Java — Interview Questions (200+)

  Clear naming, small methods, immutability, proper exceptions, tests, and idiomatic APIs.

  Open question →
• Collections Framework — Interview Questions (120+)

  Collections Framework — Interview Questions (120+)

  By access pattern: order, uniqueness, sorting, concurrency.

  Open question →
• Overview of the Collections hierarchy?

  Collections Framework — Interview Questions (120+)

  → → List/Set/Queue; is separate.

  Open question →
• How to make a collection thread-safe?

  Collections Framework — Interview Questions (120+)

  Use concurrent collections or .

  Open question →
• Comparable vs Comparator (collections context)?

  Collections Framework — Interview Questions (120+)

  Natural ordering vs custom/multiple orderings.

  Open question →
• when to use concurrent collections?

  Concurrency & Multithreading — Interview Questions (150+)

  Prefer immutability and high-level concurrency utilities over low-level locks.

  Open question →
• How does garbage collection decide what to collect?

  JVM Internals & GC — Interview Questions (120+)

  Reachability from GC roots.

  Open question →
• What is a Stream and how does it differ from a Collection?

  Functional Programming & Streams — Interview Questions (100+)

  A pipeline for processing data; not storage.

  Open question →
• fail-fast collections?

  Exceptions — Interview Questions (77+)

  @throws tags.

  Open question →
• Collections.sort signature?

  Generics — Interview Questions (60+)

  Use bounded wildcards on input params (PECS), exact types on outputs.

  Open question →
• Java 9 — collection factories?

  Java 9–21 Features — Interview Questions (95+)

  , , — immutable, null-hostile.

  Open question →
• Java 21 — sequenced collections?

  Java 9–21 Features — Interview Questions (95+)

  , , — uniform first/last/reversed.

  Open question →
• copyOf collections?

  Java 9–21 Features — Interview Questions (95+)

  infers raw-ish; specify generic.

  Open question →
• Serializable collections?

  I/O, NIO & Serialization — Interview Questions (82+)

  Use virtual threads executor Java 21.

  Open question →
• Return empty collections, not null?

  Effective Java & Best Practices — Interview Questions (82+)

  or — callers skip null checks.

  Open question →
• EnumSet for enum collections?

  Effective Java & Best Practices — Interview Questions (82+)

  Named constants.

  Open question →
• Collections Framework — Interview Questions (120+)

  Collections Framework — Interview Questions (120+)

  Array of buckets; index from hash; chaining; treeify on heavy collisions.

  Open question →
• Collections Framework — Interview Questions (120+)

  Collections Framework — Interview Questions (120+)

  HashMap (not synced, allows null), Hashtable (legacy, fully synced), ConcurrentHashMap (scalable concurrency).

  Open question →
• Collections Framework — Interview Questions (120+)

  Collections Framework — Interview Questions (120+)

  Same trade-offs as the Set variants.

  Open question →
• Collections Framework — Interview Questions (120+)

  Collections Framework — Interview Questions (120+)

  Capacity = bucket count; load factor = fill threshold (0.75).

  Open question →

Java versions

• Sequenced collections (JEP 431)

  Java 21

  Java 21. getFirst/getLast/reversed on ordered collections.

  Open version note →
• Convenience Factory Methods for Collections

  JEP 269 · Final

  Convenience Factory Methods for Collections. List.of, Set.of, and Map.of build immutable collections and reject null elements.

  Open version note →
• Abortable Mixed Collections for G1

  JEP 344 · Final

  Abortable Mixed Collections for G1. G1 can abort a mixed collection that is taking too long, so a pause is more likely to meet its goal.

  Open version note →
• Sequenced Collections

  JEP 431 · Final

  Sequenced Collections. SequencedCollection, SequencedSet, and SequencedMap add getFirst, getLast, and reversed to ordered collections.

  Open version note →