Java Map computeIfAbsent(): Multimaps, Caches and null Rules
Learn how Java Map computeIfAbsent() works, with multimap and cache examples, null and exception rules, thread safety and the putIfAbsent() difference.
Java ConcurrentSkipListMap: Sorted Thread-Safe Map Guide
Java ConcurrentSkipListMap keeps keys sorted for many threads. Learn skip lists, floor and ceiling keys, live range views, polling and TreeMap differences.
Max Value in a Java Map: Find the Largest, Smallest and Key
Find the max value in a Java Map and its key with streams, Collections.max() or one loop, and handle min values, ties, empty maps and null values safely.
Convert a Map to a List, Array or Set in Java
Convert a map to a list, array or set in Java using keySet(), values() and entrySet(), with streams, toArray(), ordering rules and lists of records.
Convert List to Map in Java: toMap(), Duplicate Keys and Order
Convert List to Map in Java with Collectors.toMap() and groupingBy(), fix the duplicate key IllegalStateException with a merge function, keep order.
Invert a Map in Java: Swap Keys and Values, Even With Duplicates
Invert a map in Java by swapping keys and values with a loop or Collectors.toMap(), handle duplicate values with groupingBy(), and use Guava BiMap.
Java HashMap: Complete Guide With Examples
Java HashMap guide with examples for put, get, merge, compute, iteration, keys, internals and thread safety, plus links to every HashMap how-to.
Java Map: HashMap vs LinkedHashMap vs TreeMap and More
A Java Map stores unique keys with one value each. Quick put/get/merge/computeIfAbsent examples, a comparison of HashMap, LinkedHashMap, TreeMap, ConcurrentHashMap, EnumMap, Hashtable and Map.of(), and all our Map tutorials grouped by topic.
Java Case-Insensitive Map: TreeMap, Commons and Spring
Create a Java case insensitive map with TreeMap and CASE_INSENSITIVE_ORDER, Commons CaseInsensitiveMap or Spring LinkedCaseInsensitiveMap, side by side.
Java EnumMap with Examples: Usage, Streams and vs HashMap
Java EnumMap stores enum keys in an array by ordinal. Learn its constructors, null rules, key order, stream collectors and how it differs from HashMap.
Java Nested Map: Create, Update and Iterate a Map of Maps
Create a Java nested map with computeIfAbsent and streams, add, remove and iterate entries, read values safely, deep copy, flatten and choose alternatives.
TreeMap vs HashMap in Java: Differences and When to Use Each
TreeMap vs HashMap in Java compared on ordering, speed, null keys, memory and key equality, with Java 25 examples and a table for picking the right map.
Java IdentityHashMap: Reference Equality vs HashMap
Java IdentityHashMap compares keys with == instead of equals(). Learn how it differs from HashMap, the String and Integer traps and the Java 20 change.
Java WeakHashMap: Weak Keys, GC Behavior and Use Cases
Learn how Java WeakHashMap removes entries after GC clears weak keys, the String and value-reference traps, WeakHashMap vs HashMap and real use cases.
Java TreeMap: Sorted Keys, floorKey(), subMap() and headMap()
Java TreeMap keeps keys sorted in a red-black tree. Learn comparators, floorKey(), ceilingKey(), subMap(), headMap(), tailMap() views and null key rules.
Java LinkedHashMap: Insertion Order, Access Order and LRU Cache
Java LinkedHashMap keeps keys in insertion or access order. See SequencedMap methods from Java 21 and build an LRU cache with removeEldestEntry().
Deep Copy a HashMap in Java and When a Shallow Copy Is Enough
Learn to deep copy a HashMap in Java with copy constructors, streams and serialization, and compare shallow copies with new HashMap, clone() and copyOf().
ConcurrentHashMap vs synchronizedMap: Synchronize a HashMap
Compare ConcurrentHashMap vs synchronizedMap in Java by locking, iteration, null handling and atomic updates, and pick the right map for shared data.
Merge Two Maps in Java: putAll(), merge() and Streams
Merge two maps in Java with putAll(), putIfAbsent(), merge() or Collectors.toMap(), and control duplicate keys, sums, list values and unchanged inputs.
Compare Two Maps in Java: equals(), Keys, Values and Diff
Compare two maps in Java with equals(), keySet() and value counts, list added, removed and changed entries, and build a full diff with Guava.
Jackson – Marshal and Unmarshal Java Maps
Learn to convert JSON to Map and convert a Map to JSON string using Jackson 2. Given example uses Jackson ObjectMapper for conversions.
ConcurrentHashMap in Java: Internals, Atomic Methods, Pitfalls
Learn how ConcurrentHashMap works since Java 8, with atomic merge and compute methods, caching, bulk operations, newKeySet, sizing and common mistakes.
Design a Good Key for HashMap in Java: Rules and Examples
A good HashMap key overrides equals() and hashCode() together, uses fields that never change and spreads its hash codes well. Records, a mutable key that gets lost, collisions, Comparable keys, EnumMap and Lombok, with tested Java 25 code.
Iterate HashMap in Java: Fastest Way With JMH Results
Learn to iterate HashMap in Java with entrySet, forEach, keySet and streams, remove entries safely, and compare each way with JMH results on JDK 25.
How HashMap Works Internally in Java: Buckets, Trees, Resize
Learn how HashMap works internally in Java with hashing, bucket index, collisions, Java 8 tree bins, resizing and why equal keys need equal hash codes.
