Initialize HashMap in Java: Capacity, Load Factor, newHashMap
Learn to initialize HashMap in Java with Map.of, newHashMap and streams, and see how initial capacity and load factor decide when the map resizes.
In Java, HashMap is a widely used data structure that belongs to the Java Collections Framework. It is part of the java.util package and provides an efficient way to store and retrieve key-value pairs.
A HashMap allows you to associate keys with values and quickly retrieve values based on their keys.
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Tutorials
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.
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.
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 Collections & Arrays Tutorial
Learn the Java Collections Framework with a Java 21 hierarchy diagram, how to choose List, Set, Map and Queue classes, and links to every tutorial.
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 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.
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.
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.