Caches are widely used in software applications to improve performance by storing frequently accessed data. However, since the cache has a limited size, it becomes necessary to implement an eviction policy to remove least-recently-used or least-frequently-used items when the cache is full. In this blog post, we will explore how to implement a cache eviction policy using a custom comparator with the TreeMap and HashMap classes in Java.

Table of Contents

1. Introduction
2. Implementing the Cache
3. Defining a Custom Comparator
4. Eviction Policy
5. Conclusion

Introduction

The Java collections framework provides various data structures to efficiently store and manage data. We can leverage the TreeMap and HashMap classes to implement a cache with an eviction policy. TreeMap is a sorted map implementation that maintains the keys in sorted order, while HashMap is an unordered map.

Implementing the Cache

To start, let’s create a class called Cache that encapsulates the TreeMap and HashMap to store and manage the cache entries. The keys of the TreeMap will be used to maintain the desired order for eviction.

import java.util.TreeMap;
import java.util.HashMap;

public class Cache<K, V> {
    private final TreeMap<K, V> orderedMap;
    private final HashMap<K, V> hashMap;
    private final int maxSize;

    public Cache(int maxSize) {
        this.orderedMap = new TreeMap<>();
        this.hashMap = new HashMap<>();
        this.maxSize = maxSize;
    }

    // Other methods for cache operations
}

Defining a Custom Comparator

To implement a custom eviction policy, we need to define a custom comparator that determines the order of the keys in the TreeMap. For example, we can sort the keys based on their access time or frequency, allowing us to easily evict the least-recently-used or least-frequently-used items when the cache is full.

Let’s assume we want to sort the cache entries based on their access time, with the least recently used item at the start. We can define a custom comparator for our cache class:

import java.util.Comparator;

public class AccessTimeComparator<K> implements Comparator<K> {
    private final HashMap<K, Long> accessTimes;

    public AccessTimeComparator(HashMap<K, Long> accessTimes) {
        this.accessTimes = accessTimes;
    }

    @Override
    public int compare(K key1, K key2) {
        Long accessTime1 = accessTimes.getOrDefault(key1, 0L);
        Long accessTime2 = accessTimes.getOrDefault(key2, 0L);
        return accessTime1.compareTo(accessTime2);
    }
}

Eviction Policy

Now that we have our cache implementation and custom comparator, let’s add the eviction logic. We’ll modify the Cache class to evict entries when the cache is full, using the custom comparator to determine the order of eviction.

public class Cache<K, V> {
    // ...

    private void evictEntriesIfNeeded() {
        while (orderedMap.size() > maxSize) {
            K keyToRemove = orderedMap.firstKey();
            orderedMap.remove(keyToRemove);
            hashMap.remove(keyToRemove);
        }
    }

    public void put(K key, V value) {
        // Add the key-value pair to the map
        orderedMap.put(key, value);
        hashMap.put(key, value);

        // Evict entries if needed
        evictEntriesIfNeeded();
    }

    // ...
}

Conclusion

By combining the TreeMap and HashMap data structures with a custom comparator, we can implement a cache eviction policy in Java. The TreeMap ensures that the keys are sorted according to our desired ordering, while the HashMap allows for efficient retrieval of cached items.

Eviction policies are crucial in managing cache efficiency, ensuring that the cache contains the most relevant and frequently used data. The custom comparator enables us to define the eviction policy based on specific criteria, such as access time, frequency, or any other metric.

Overall, leveraging TreeMap and HashMap with a custom comparator provides an effective approach to implementing a cache eviction policy in Java.

References

1. Java TreeMap documentation
2. Java HashMap documentation