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().
Java collections framework is a unified architecture for representing and manipulating collections. All collections frameworks contain the following:
Interfaces: These are abstract data types that represent collections. Interfaces allow collections to be manipulated independently of the details of their representation. In object-oriented languages, interfaces generally form a hierarchy.
Implementations, i.e., Classes: These are the concrete implementations of the collection interfaces. In essence, they are reusable data structures.
Algorithms: These are the methods that perform useful computations, such as searching and sorting, on objects that implement collection interfaces. The algorithms are said to be polymorphic: that is, the same method can be used on many different implementations of the appropriate collection interface.
In addition to collections, the framework defines several map interfaces and classes. Maps store key/value pairs. Although maps are not collections in the proper use of the term, but they are fully integrated with collections.
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Tutorials
Remove Duplicates from ArrayList in Java and Keep the Order
Learn to remove duplicates from ArrayList in Java with distinct(), LinkedHashSet and removeIf(), plus duplicate objects by a field and ignoring case.
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.
ArrayList removeIf() in Java: Remove Elements by Condition
ArrayList removeIf() removes every element that matches a Predicate. See return values, exceptions, Map views, lists that refuse it and stream filters.
ArrayList forEach() in Java: Lambdas, Limits and Examples
ArrayList forEach() runs a Consumer on each element. Learn method references, andThen(), break and index workarounds, CME rules and Map.forEach().
Java ArrayList clone(): Shallow Copy vs Deep Copy
Java ArrayList clone() creates a shallow copy. Learn how it differs from a deep copy and how to deep copy a list with copy constructors and streams.
LinkedList vs ArrayList in Java: Differences and When to Use
LinkedList vs ArrayList in Java compared on memory, time complexity and real use cases, plus ArrayDeque advice and how to convert between them.
ArrayList vs Vector in Java: Differences and Modern Alternatives
ArrayList vs Vector in Java compared on synchronization, capacity growth and iteration, with modern replacements for Vector, Stack and Enumeration.
Convert Set to List in Java (HashSet to ArrayList and Back)
Learn to convert Set to List in Java with the ArrayList constructor, List.copyOf() and streams, keep or sort the order, and convert a List to a Set.
Convert Array to ArrayList in Java (Mutable and Unmodifiable)
Convert array to ArrayList in Java with new ArrayList<>(Arrays.asList()), List.of() or streams. Covers fixed-size views, int[] arrays and the way back.
Check if Element Exists in ArrayList: contains() and indexOf()
Learn how to check if element exists in ArrayList in Java with contains(), indexOf(), lastIndexOf() and anyMatch(), incl. custom objects and fast lookups.
Replace Element in ArrayList in Java: set() and replaceAll()
Learn to replace element in ArrayList in Java by index or value with set(), indexOf(), Collections.replaceAll() and List.replaceAll(), with safe examples.
Check if ArrayList Is Empty in Java: isEmpty() vs size()
Check if ArrayList is empty in Java with isEmpty() or size(), handle null lists with Commons and Spring, and empty a list with clear() vs removeAll().
Merge Two ArrayLists in Java, With or Without Duplicates
Merge two ArrayLists in Java with addAll() or Stream.concat(), drop duplicates with LinkedHashSet, merge sorted lists and merge objects by key.
Serialize and Deserialize an ArrayList in Java (Examples)
Serialize an ArrayList to a file or a byte array with ObjectOutputStream, read it back with ObjectInputStream, handle the unchecked cast, and see when JSON with Jackson is the better choice.
Swap Two Elements in an ArrayList With Collections.swap()
Swap two elements in ArrayList with Collections.swap(), by value or with set(), avoid IndexOutOfBoundsException and move items with rotate().
Synchronized ArrayList in Java: Thread-Safe List Options
Create a synchronized ArrayList with Collections.synchronizedList() or CopyOnWriteArrayList, lock iteration correctly and pick the right option.
Compare Two Lists in Java: Equality, Differences, Common Items
Compare two lists in Java with equals() or ignoring order, count duplicates correctly, and find additional, missing and common items with removeAll().
Add Multiple Elements to ArrayList in Java: addAll() and More
Learn how to add multiple elements to ArrayList in Java with addAll(), Collections.addAll(), add(index), addFirst() and streams, plus common exceptions.
Remove Element from ArrayList in Java: remove() and removeIf()
Learn to remove element from ArrayList in Java by index, value or condition with remove(), removeIf(), removeAll() and retainAll(), plus common traps.
Iterate ArrayList in Java: for, forEach(), Iterator and Streams
Learn to iterate ArrayList in Java with the for-each loop, index loop, forEach(), Iterator, reversed() and streams, and remove elements without errors.
Create a Java List with a Single Element (List.of, singletonList)
Create a Java list with a single element using List.of(), Collections.singletonList(), Arrays.asList() or ArrayList. Mutability and null rules compared.
Immutable Collections Factory Methods: List.of, Set.of, Map.of
Immutable collections factory methods List.of(), Set.of() and Map.of() in Java 25, with null and duplicate rules, copyOf(), toList() and Arrays.asList().
Iterate a Collection in Java: Loops for List, Set and Map
How to iterate a collection in Java with for-each, forEach(), Iterator, index loops and streams, remove elements safely and compare loop performance.
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.