In Java, the java.util package provides various hash-based data structures, such as HashMap, HashSet, and HashTable. These structures use hashing to store and retrieve elements efficiently. Below is an overview of the most commonly used hash-based data structures and how to use them:

HashMap

A HashMap stores key-value pairs. It allows for fast retrieval of values based on their keys.

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import java.util.HashMap;

HashMap<Integer, String> map = new HashMap<>();
map.put(1, "Apple");
map.put(2, "Banana");

// Retrieve a value
String value = map.get(1); // "Apple"

// Check if a key exists
boolean exists = map.containsKey(2); // true

// Iterate over the map
for (Map.Entry<Integer, String> entry : map.entrySet()) {
    System.out.println(entry.getKey() + ": " + entry.getValue());
}

HashSet

A HashSet stores a collection of unique elements.

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import java.util.HashSet;

HashSet<String> set = new HashSet<>();
set.add("Apple");
set.add("Banana");

// Check if an element exists
boolean exists = set.contains("Apple"); // true

// Iterate over the set
for (String element : set) {
    System.out.println(element);
}

Hashtable

Similar to HashMap, but it is synchronized and does not allow null keys or values.

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import java.util.Hashtable;

Hashtable<Integer, String> table = new Hashtable<>();
table.put(1, "Apple");
table.put(2, "Banana");

// Retrieve a value
String value = table.get(1); // "Apple"

Notes:

• Choosing Between Structures: The choice of which hash-based structure to use depends on your specific requirements, like order maintenance (LinkedHashMap), sorting (TreeMap), or just a set of unique elements (HashSet).
• Null Handling: HashMap and HashSet allow one null key and multiple null values, but Hashtable does not allow any null key or value.
• Concurrency: If you’re working in a multithreaded environment and require thread-safe operations, consider using ConcurrentHashMap instead of HashMap or Hashtable.