Why parsing a String into Date in Java is slow? Can we accelerate it?

Question

I am reading a text file containing dates, and I want to parse the Strings representing the dates into Date objects in java. What I notice is the operation is slow. Why? is there any way to accelerate it? My file looks like:

2012-05-02 12:08:06:950, secondColumn, thirdColumn
2012-05-02 12:08:07:530, secondColumn, thirdColumn
2012-05-02 12:08:08:610, secondColumn, thirdColumn

I am reading the file line by line, then I am getting the date String from each line, then I am parsing it into a Date object using a SimpleDateFormat as follow:

DataInputStream in = new DataInputStream(myFileInputStream);
BufferedReader  br = new BufferedReader(new InputStreamReader(in));
String strLine;

SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
while ((strLine = br.readLine()) != null)
{
    ....Do things....
    Date myDateTime = (Date)formatter.parse(myDateString);
    ...Do things....
}

Answer 1

The converting of dates and timezone is expensive. If you can assume your date/times are similar to each other, you can convert the date and hours/minutes (or only dates if you use GMT) whenever minutes change and generate the seconds yourself.

This will call parse once per minute. Depending on your assumptions you could make it once per hours or once per day.

String pattern = "yyyy-MM-dd HH:mm";
SimpleDateFormat formatter = new SimpleDateFormat(pattern);
String lastTime = "";
long lastDate = 0;
while ((strLine = br.readLine()) != null) {
    String myDateString = strLine.split(", ")[0];
    if (!myDateString.startsWith(lastTime)) {
        lastTime = myDateString.substring(0, pattern.length());
        lastDate = formatter.parse(lastTime).getTime();
    }
    Date date = new Date(lastDate + Integer.parseInt(myDateString.substring(pattern.length() + 1).replace(":", "")));
}