How To Cache Data Reading From Collection Of Text Files In A Directory Using TreeMap

How can I Cache the data I'm reading from a collection of text files in a directory using a TreeMap? Currently my program reads the data from several text files in a directory and the saves that information in a text file called output.txt. I would like to cache this data in order to use it later. How can I do this using the TreeMap Class? These are the keys,values: TreeMap The data I'd like to Cache is (date from the file, time of the file, current time).
 
import java.io.*; 
public class CacheData {
  public static void main(String[] args) throws IOException {
  String target_dir = "C:Files";
  String output = "C:Filesoutput.txt";
  File dir = new File(target_dir);
  File[] files = dir.listFiles();
 
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