How to convert a collection to String in Java

private Map<String, Integer> collection = new HashMap<String, Integer>();

collection.put("row #1", 1);
collection.put("row #2", 2);

for ( String key : collection.keySet() ) {
    System.out.println( key );
}

Source: http://stackoverflow.com/questions/10462819/get-keys-from-hashmap-in-java