Question

I have an ArrayList of String[].

Each String[] contains two values: value and date.

If I print it, it will look something like this:

 value | date
-------|----------
 357.0 | 2011/05/30
-234.0 | 2011/05/31
-123.0 | 2011/05/30

i want to make a new ArrayList, where all values in same date are summed. Like this:

 value | date
-------|----------
 234.0 | 2011/05/30
-234.0 | 2011/05/31

Can anyone help?

Answer 1

You too, are suffering from object denial.

String is not a good type for handling numbers or dates. For a number I’d use int (if you only have integer values), double or BigDecimal (if it’s a monetary ammount or some other exact decimal number).

For the date I’d use either a Date/Calendar or LocalDate from Joda Time (if you can use an external library).

Obviously this means that you can no longer store your data in a String[], but that’s not really an appropriate type for any kind of structured data, anyway.

You’d want to write a class that reflects what your values are about:

public class DatedValue {
  private final Date date;
  private final BigDecimal value;

  public DatedValue(final Date date, final BigDecimal value) {
    this.date = date;
    this.value = value;
  }

  public Date getDate() {
    return date;
  }

  public BigDecimal getValue() {
    return value;
  }
}

Then you can simply create the new values by iterating over the original ArrayList<DatedValue>:

Map<Date,DatedValue> result = new LinkedHashMap<Date,DatedValue>();
for(DatedValue dv : datedValues) {
  DatedValue newDV = result.get(dv.getDate());
  if (newDV == null) {
    newDV = dv;
  } else {
    newDV = new DatedValue(dv.getDate(), dv.getValue().add(newDV.getValue()));
  }
  result.put(dv.getDate(), newDV);
}
List<DatedValue> sums = new ArrayList<DatedValue>(result.values());