Find Bugs in an LRU Cache

Problem Statement

A colleague submitted a pull request implementing a fixed-capacity Least Recently Used (LRU) cache in Java using java.util.LinkedHashMap. The code compiles cleanly, but testing reveals that entries are evicted incorrectly under common access patterns.

Review the implementation below, identify all behavioral defects, supply minimal failing test cases for each defect, and write the corrected implementation.

import java.util.LinkedHashMap;
import java.util.Map;

public class SimpleLRUCache {
    private final int capacity;
    private final Map cache;

    public SimpleLRUCache(int capacity) {
        if (capacity <= 0) {
            throw new IllegalArgumentException("Capacity must be positive");
        }
        this.capacity = capacity;
        // Intended: LRU eviction with initial capacity and default load factor
        this.cache = new LinkedHashMap(capacity, 0.75f) {
            @Override
            protected boolean removeEldestEntry(Map.Entry eldest) {
                return size() >= capacity;
            }
        };
    }

    public V get(K key) {
        return cache.get(key);
    }

    public void put(K key, V value) {
        cache.put(key, value);
    }

    public int size() {
        return cache.size();
    }
}

Tasks

1. Identify the Bugs: Find the specific defects in the implementation that violate standard LRU cache semantics.
2. Minimal Failing Tests: For each bug, describe a short sequence of put and get operations that passes on a correct LRU cache but fails on this code.
3. Correction: Provide the corrected Java implementation.
4. Semantics Explanation: Briefly explain why the Java compiler cannot catch these errors and why behavioral tests are required to validate cache invariants.