simple LoadingCache implementation as seen with Guava

LoadingCache.java

public class LoadingCache<K, V> extends HashMap<K, V>
{
    
    private final long timeout;
    
    private final Function<K, V> function;
    
    private Map<K, Long> timeouts = new HashMap<K, Long>();
    
    public LoadingCache(long timeout, Function<K, V> function)
    {
        this.timeout = timeout;
        this.function = function;
    }

    private boolean validate(K key)
    {
        final long now = System.currentTimeMillis();
        Long l = this.timeouts.get(key);
        if(l == null || l < now)
        {
            return false;
        }
        return true;
    }
    
    @Override
    public V get(K key)
    {
        V val;
        boolean invalid = !validate();
        if(invalid || !super.containsKey(key))
        {
            // Reset the lifespan
            this.timeouts.put(System.currentTimeMillis() + this.timeout);
            val = this.function.compute(key);
            super.put(key, val);
        }
        else
            val = super.get(key);
        return val;
    }
    
    @Override
    public V put(K key, V val)
    {
        throw new UnsupportedOperationException("Use get(K key)");
    }
    
    @Override
    public void putAll(Map<K, V> map)
    {
        throw new UnsupportedOperationException("Use get(K key)");
    }
}

public interface Function<K, V>
{
    public V compute(K key);
}