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Java provides a rich set of standard libraries and third-party libraries that make working with data structures easier and more efficient. Here are some of the most popular libraries and frameworks for data structures in Java:

1. Java Collections Framework (JCF)

The Java Collections Framework is a part of the Java Standard Library (java.util package) and provides a set of interfaces and classes for working with data structures such as lists, sets, maps, and queues. It includes:

• List Interface: Implemented by classes like ArrayList, LinkedList, Vector.
• Set Interface: Implemented by classes like HashSet, LinkedHashSet, TreeSet.
• Map Interface: Implemented by classes like HashMap, LinkedHashMap, TreeMap, Hashtable.
• Queue Interface: Implemented by classes like PriorityQueue, LinkedList.

Example Usage:

import java.util.*;

List<String> list = new ArrayList<>();
list.add("Apple");
list.add("Banana");

Set<Integer> set = new HashSet<>();
set.add(1);
set.add(2);

Map<String, Integer> map = new HashMap<>();
map.put("Key1", 100);
map.put("Key2", 200);

2. Apache Commons Collections

Apache Commons Collections extends or augments the Java Collections Framework. It provides additional types of collections, such as bag, buffer, and bidirectional maps, as well as utilities for manipulating and creating collection objects.

• Bag: A collection that allows duplicate elements and keeps track of the number of occurrences.
• BidiMap: A map that allows you to look up keys from values as well as values from keys.
• MultiMap: A map that can hold multiple values against each key.

Example Usage:

import org.apache.commons.collections4.Bag;
import org.apache.commons.collections4.bag.HashBag;

Bag<String> bag = new HashBag<>();
bag.add("Apple", 3);
bag.add("Banana", 2);

3. Google Guava

Google Guava is an open-source set of common libraries for Java, which includes collections, caching, primitives support, and more. It enhances the capabilities of the Java Collections Framework with additional data structures and utilities.

• Multimap: A collection that maps keys to multiple values.
• Immutable Collections: Immutable versions of collection classes like ImmutableList, ImmutableSet.
• BiMap: A map that maintains a one-to-one correspondence between keys and values.
• Table: A collection that represents a 2D map, similar to a table with rows and columns.

Example Usage:

import com.google.common.collect.*;

Multimap<String, String> multimap = ArrayListMultimap.create();
multimap.put("Fruit", "Apple");
multimap.put("Fruit", "Banana");

ImmutableList<String> immutableList = ImmutableList.of("a", "b", "c");

BiMap<Integer, String> biMap = HashBiMap.create();
biMap.put(1, "One");
biMap.put(2, "Two");

4. Trove

Trove provides high-performance implementations of primitive collections. It offers similar interfaces to the Java Collections Framework but is optimized for performance with primitive types, reducing the memory and performance overhead of autoboxing.

• TIntArrayList: An array list for int primitives.
• TIntHashSet: A hash set for int primitives.
• TIntIntHashMap: A hash map for int keys and int values.

Example Usage:

import gnu.trove.list.array.TIntArrayList;

TIntArrayList intList = new TIntArrayList();
intList.add(10);
intList.add(20);
intList.add(30);

5. Eclipse Collections

Eclipse Collections, formerly known as GS Collections, is a comprehensive collections library. It includes a wide range of data structures, including additional list types, sets, bags, maps, and multimaps, with features like primitive collections, immutable collections, and more.

• MutableList: An interface extending List with additional methods for better usability.
• MutableMap: An interface extending Map with more methods.
• Bag: An unordered collection that allows duplicates.
• Multimap: A map that allows multiple values for a single key.

Example Usage:

import org.eclipse.collections.api.list.MutableList;
import org.eclipse.collections.impl.factory.Lists;

MutableList<String> list = Lists.mutable.of("Apple", "Banana", "Cherry");
list.add("Date");

MutableBag<String> bag = Bags.mutable.with("Apple", "Apple", "Banana");

6. JGraphT

JGraphT is a free Java graph library that provides mathematical graph-theory objects and algorithms. It supports a variety of graph types, such as directed, undirected, weighted, unweighted, and more.

• Graph: The core interface representing a graph.
• DirectedGraph: A graph where edges have direction.
• UndirectedGraph: A graph where edges do not have direction.

Example Usage:

import org.jgrapht.graph.DefaultEdge;
import org.jgrapht.graph.SimpleGraph;

SimpleGraph<String, DefaultEdge> graph = new SimpleGraph<>(DefaultEdge.class);
graph.addVertex("A");
graph.addVertex("B");
graph.addEdge("A", "B");

7. Koloboke

Koloboke is a high-performance collections library that provides optimized collections for primitive data types, and for reference types, it offers efficient memory usage.

• ObjObjMap: A map from an object to an object.
• IntIntMap: A map from int to int.

Example Usage:

import com.koloboke.collect.map.IntIntMap;
import com.koloboke.collect.map.hash.HashIntIntMaps;

IntIntMap map = HashIntIntMaps.newMutableMap();
map.put(1, 100);
map.put(2, 200);

8. FastUtil

FastUtil provides fast and memory-efficient implementations of standard Java data structures for both primitive and object types. It includes sets, lists, maps, and other utilities.

• IntArrayList: A list for int primitives.
• Int2IntOpenHashMap: A hash map for int to int mappings.

Example Usage:

import it.unimi.dsi.fastutil.ints.IntArrayList;
import it.unimi.dsi.fastutil.ints.Int2IntOpenHashMap;

IntArrayList list = new IntArrayList();
list.add(10);
list.add(20);

Int2IntOpenHashMap map = new Int2IntOpenHashMap();
map.put(1, 100);
map.put(2, 200);

Conclusion

Java developers have access to a wide range of libraries for working with data structures. The Java Collections Framework provides a solid foundation with its standard data structures, while libraries like Apache Commons Collections, Google Guava, and Eclipse Collections offer additional functionality and specialized data structures. For specific use cases, such as working with primitive types or graph structures, libraries like Trove, JGraphT, Koloboke, and FastUtil provide optimized solutions.

Choosing the right library and data structure depends on the specific requirements of your application, including performance needs, memory usage, and the type of data you are working with. By leveraging these libraries, Java developers can build efficient, robust, and scalable applications.