セットとマップ

Beyond the ArrayList class (which represents an ordered list of elements), the Java Collections Framework offers two other fundamental interfaces: Set (for unique elements) and Map (for key-value associations).

Set: Collections of Unique Elements

A Set is a collection that cannot contain duplicate elements. Its most common implementation is HashSet, based internally on a hash table, which guarantees excellent performance for insertion, removal, and presence verification operations.

import java.util.HashSet;
import java.util.Set;

Set<String> uniqueNames = new HashSet<>();
uniqueNames.add("Alice");
uniqueNames.add("Bob");
uniqueNames.add("Alice"); // This insertion will be ignored since "Alice" is already present

System.out.println(uniqueNames.size()); // Prints 2

Map: Key-Value Associations

A Map maps unique keys to values. It cannot contain duplicate keys: each key is associated with at most one value. Its standard implementation is HashMap.

import java.util.HashMap;
import java.util.Map;

Map<String, Integer> ageMap = new HashMap<>();
// Insert or update key-value pairs
ageMap.put("Alice", 25);
ageMap.put("Bob", 30);

// Retrieve value by key
int aliceAge = ageMap.get("Alice"); // Returns 25

// Verify if a key or value exists
boolean hasBob = ageMap.containsKey("Bob"); // Returns true

Iterating over a Map

To iterate over key-value pairs, we can use a for-each loop on the set of entries (entrySet()) provided by the map.

for (Map.Entry<String, Integer> entry : ageMap.entrySet()) {
    System.out.println(entry.getKey() + " is " + entry.getValue() + " years old.");
}

Try it yourself