Limiting the max size of a HashMap in Java

Simple solution is usually the best, so use unmodifiable or Immutable hashmap.

If you can not change amount of elements, then the size will be fixed - problem solved.

Sometimes simpler is better.

public class InstrumentedHashMap<K, V> implements Map<K, V> {

    private Map<K, V> map;

    public InstrumentedHashMap() {
        map = new HashMap<K, V>();
    }

    public boolean put(K key, V value) {
        if (map.size() >= MAX && !map.containsKey(key)) {
             return false;
        } else {
             map.put(key, value);
             return true;
        }
    }

    ...
}

You could create a new class like this to limit the size of a HashMap:

public class MaxSizeHashMap<K, V> extends LinkedHashMap<K, V> {
    private final int maxSize;

    public MaxSizeHashMap(int maxSize) {
        this.maxSize = maxSize;
    }

    @Override
    protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
        return size() > maxSize;
    }
}