abc bx      cde

e 
      fz  g


qwerty

qwerty
g
fz
e
cde
bx
abc

import java.io.*;
import java.util.*;

public class FileWordsReverser2 {
    public static void main(String[] args) {
        ArrayList<String> aWords = new ArrayList<String>();
        String xline;
        String[] words;
        BufferedReader br;

        System.out.println("start:  "+System.currentTimeMillis());
        try {
            br = new BufferedReader(new FileReader("C:\\hugetext.txt"));
            try {
                while ((xline = br.readLine()) != null) {

                    words = xline.split("\\s+");
                    aWords.addAll(Arrays.asList(words));
                }
                br.close();
                //no effect in memory consumption:
                //aWords.trimToSize();

                File file = new File("C:\\hugetxt2.rev");

                if (!file.exists()) {
                    file.createNewFile();
                }

                FileWriter fw = new FileWriter(file.getAbsoluteFile());
                BufferedWriter bw = new BufferedWriter(fw);

                for (int i = aWords.size()-1; i >= 0; i--) {
                    bw.write( aWords.get(i) +"\n");
                }
                bw.close();
                System.out.println("Finish: "+System.currentTimeMillis());

            } catch (IOException ex) {
                ex.printStackTrace();
            }
        } catch (FileNotFoundException ex) {
            ex.printStackTrace();
        }
        //---------------------------------------------------------------------

    }
}

start:  1364152678296
Finish: 1364152682812

import java.io.*;
import java.util.*;
public class FileWordsReverser3 {
    public static void main(String[] args) {
        BufferedReader istream;
        List<String> tokens = new ArrayList<String>();

        System.out.println("Start:  "+System.currentTimeMillis());

        try {
        istream = new BufferedReader(new FileReader("c:\\hugetext.txt"));
            try {

                while (istream.ready()) {
                    for (String token: istream.readLine().split("\\s+")) {
                        if (token.trim().length() > 0) {
                            tokens.add(token);
                        }
                    }
                }
                istream.close();

                BufferedWriter ostream = new BufferedWriter(new FileWriter("c:\\hugetxt3.rev"));
                for (int i = tokens.size(); --i >= 0;) {
                    ostream.write(tokens.get(i));
                    ostream.write("\n");
                    ostream.flush();
                }
                ostream.close();

            } catch (IOException ex) {
                ex.printStackTrace();
            }
       } catch (FileNotFoundException ex) {ex.printStackTrace();}

       System.out.println("Finish: "+System.currentTimeMillis());

    }

}

Start:  1364152777984
Finish: 1364152791375

D:\JAVA\1Probe>java -Xmx:64m FileWordsReverser2
Invalid maximum heap size: -Xmx:64m
Could not create the Java virtual machine.

D:\JAVA\1Probe>java -Xmx:96m FileWordsReverser2
Invalid maximum heap size: -Xmx:96m
Could not create the Java virtual machine.

D:\JAVA\1Probe>java -Xmx:128m FileWordsReverser2
Invalid maximum heap size: -Xmx:128m
Could not create the Java virtual machine.

D:\JAVA\1Probe>java -Xmx:256m FileWordsReverser2
Invalid maximum heap size: -Xmx:256m
Could not create the Java virtual machine.

-Xmx

This option sets the maximum Java heap size. The Java heap (the “heap”) is the part of the memory where blocks of memory are allocated to objects and freed during garbage collection. Depending upon the kind of operating system you are running, the maximum value you can set for the Java heap can vary.
Note: 	 -Xmx does not limit  the total amount of memory that the JVM can use.

...
for (long pos = istream2.length() - 1; pos >= 0;) {
    StringBuilder word = new StringBuilder();
    for (byte ch = 0; ch != '\n' && --pos >= 0;) {
        istream2.seek(pos);
        ch = istream2.readByte();
        word.append((char)ch);
    }