Use Object#toString().
String string = map.toString();
That's after all also what System.out.println(object) does under the hoods. The format for maps is described in AbstractMap#toString().
Returns a string representation of this map. The string representation consists of a list of key-value mappings in the order returned by the map's
entrySetview's iterator, enclosed in braces ("{}"). Adjacent mappings are separated by the characters ", " (comma and space). Each key-value mapping is rendered as the key followed by an equals sign ("=") followed by the associated value. Keys and values are converted to strings as byString.valueOf(Object).