/**
 * Creates a pseudo-unique filename for the specified cache key.
 *
 * @param key The key to generate a file name for.
 * @return A pseudo-unique filename.
 */
private String getFilenameForKey(String key) {
    int firstHalfLength = key.length() / 2;
    String localFilename = String.valueOf(key.substring(0, firstHalfLength).hashCode());
    localFilename += String.valueOf(key.substring(firstHalfLength).hashCode());
    return localFilename;
}

Este código do Google VolleyDiskBasedCache.
Por que emendar sem acesso direto.
por exemplo:

return String.valueOf(key.hashCode());