How can I compress/uncompress a long string or a file (of any type)?

Check out the java.util.zip.GZIPInputStream and java.util.zip.GZIPOutputStream classes. The following demonstrates compressing a file:

import java.io.*;
import java.net.*;
import java.util.zip.*;

public class CompressIt {
  public static void main(String[] args) {
    if (args.length !=1) {
      displayUsage();
      System.exit(-1);
    } else {
      String filename = args[0];
      compressIt(filename);
    }
  }
  private static void displayUsage() {
    System.err.println(
      "Usage: java CompressIt filename");
  }
  private static void compressIt(String filename) {
    try {
      File file = new File(filename);
      // small files only
      int length = (int) file.length();
      FileInputStream fis = new FileInputStream(file);
      BufferedInputStream bis = 
        new BufferedInputStream(fis);
      ByteArrayOutputStream baos = 
        new ByteArrayOutputStream(length);
      GZIPOutputStream gos = 
        new GZIPOutputStream(baos);
      byte[] buffer = new byte[1024];
      int bytesRead;
      while ((bytesRead = bis.read(buffer)) != -1) {
        gos.write(buffer, 0, bytesRead);
      }
      bis.close();
      gos.close();
      System.out.println("Input Length: " + length);
      System.out.println("Output Length: " + 
        baos.size());
    } catch (FileNotFoundException e) {
      System.err.println("Invalid Filename");
    } catch (IOException e) {
      System.err.println("I/O Exception in transit");
    }
  }
}

The following table demonstrates the compression results. Depending upon the type of input, the results can vary considerably.