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Implementing an LRU cache

You're developing a web application that frequently retrieves user profile information as strings. To improve performance, you want to implement a simple cache that stores these profile strings and can identify which entries were least recently used.

Cet exercice fait partie du cours

Optimizing Code in Java

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Instructions

  • In the get() method, retrieve the cache entry for the specified key.
  • After retrieving the key, update its access time.
  • After putting an entry in the cache, if the capacity has been exceeded, remove the least recently used entry.

Exercice interactif pratique

Essayez cet exercice en complétant cet exemple de code.

import java.util.*;

public class StringCache {
    private final int capacity = 100;
    private final Map cache = new HashMap<>();
    
    public String get(String key) {
        // Get the entry for the specified key
        CacheEntry entry = ____;
        if (entry == null) return null;
        // Update its access time
        entry.____;
        return entry.value;
    }
    
    public void put(String key, String value) {
        cache.put(key, new CacheEntry(value));
        if (cache.size() > capacity) {
            // If cache exceeds capacity, remove least recently used
            ____;
        }
    }

    void removeLeastRecentlyUsed() {
        String lruKey = null;
        long oldest = Long.MAX_VALUE;
        for (Map.Entry e : cache.entrySet()) {
            if (e.getValue().lastAccessed < oldest) {
                oldest = e.getValue().lastAccessed;
                lruKey = e.getKey();
            }
        }
        if (lruKey != null) { cache.remove(lruKey); }
    }

    static class CacheEntry {
        String value;
        long lastAccessed;
        CacheEntry(String value) {
            this.value = value;
            updateAccessTime();
        }
        void updateAccessTime() {
            lastAccessed = System.currentTimeMillis();
        }
    }

    public static void main(String[] args) {}
}
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