What is the simplest way to implement caching?

In-memory caching using Map.

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

public class SimpleCache<K, V> {

    private final Map<K, V> cache = new HashMap<>();

    public V get(K key) {
        return cache.get(key); // Get value by key
    }

    public void put(K key, V value) {
        cache.put(key, value); // Add or update value in cache
    }

    public void remove(K key) {
        cache.remove(key); // Remove key from cache
    }

    public void clear() {
        cache.clear(); // Clear entire cache
    }
}

This approach is the simplest but has limitations in memory volume and does not support eviction of old elements (TTL/LRU). For more advanced scenarios, libraries like Guava Cache, Ehcache, or Caffeine are used.