/**
* Returns a string representation of the receiver, containing
* the String representation of each key-value pair, sorted ascending by value, according to natural ordering.
*/
public String toStringByValue() {
LongArrayList theKeys = new LongArrayList();
keysSortedByValue(theKeys);
StringBuffer buf = new StringBuffer();
buf.append("[");
int maxIndex = theKeys.size() - 1;
for (int i = 0; i <= maxIndex; i++) {
long key = theKeys.get(i);
buf.append(String.valueOf(key));
buf.append("->");
buf.append(String.valueOf(get(key)));
if (i < maxIndex) buf.append(", ");
}