KataEight.java

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.Collection;
import java.util.HashSet;

public class WordOfTwo {

    private Collection<String> dict = new HashSet<String>();

    public Collection<String> find() {
        Collection<String> ret = new HashSet<String>();
        for (String word : dict) {
            if (word.length() != 6)
                continue;
            for (int i = 2; i <= 4; i++) {
                if (dict.contains(word.substring(0, i)) && dict.contains(word.substring(i))) {
                    ret.add(word);
                    break;
                }
            }
        }
        return ret;
    }

    public void loadFile(String path) throws IOException {
        BufferedReader reader = new BufferedReader(new FileReader(path));
        String aux;
        while ((aux = reader.readLine()) != null) {
            dict.add(aux);
        }
    }

    public static void main(String[] args) {
        for (int i = 0; i < 2; i++) {
    		test();
    	}
    }

    public static void test() {
        WordOfTwo dictionary = new WordOfTwo();

        long startTime = System.nanoTime();
        try {
            dictionary.loadFile("res/wordsEn.txt");
        } catch (IOException e) {
        	throw new RuntimeException("Failed to open dictionary.", e);
        }

        long searchTime = System.nanoTime();
        Collection<String> match = dictionary.find();
        long endTime = System.nanoTime();
        
        //System.out.println(match);

        System.out.println("Load Time: " + (searchTime - startTime) / 1000000 + "ms");
        System.out.println("Search Time: " + (endTime - searchTime) / 1000000 + "ms");
        System.out.println("Total Time: " + (endTime - startTime) / 1000000 + "ms");
        
        int expected = 3715;
        int count = match.size();
        System.out.println("Found: " + count + " words");
        
        if (count != expected)
        	throw new RuntimeException("Expected " + expected + " matches, got " + count + " matches.");
    }
}

Read's faster once the file is cached:

Load Time: 0.068796
Search Time: 0.030156
Total Time: 0.098952
Load Time: 0.027102
Search Time: 0.025036
Total Time: 0.052138
Load Time: 0.034772
Search Time: 0.00759
Total Time: 0.042362
Load Time: 0.030268
Search Time: 0.00723
Total Time: 0.037498
Load Time: 0.033377
Search Time: 0.006703
Total Time: 0.04008
Load Time: 0.040017
Search Time: 0.005629
Total Time: 0.045646
Load Time: 0.025513
Search Time: 0.007767
Total Time: 0.03328
Load Time: 0.027963
Search Time: 0.007702
Total Time: 0.035665
Load Time: 0.026415
Search Time: 0.007613
Total Time: 0.034028
Load Time: 0.026203
Search Time: 0.007872
Total Time: 0.034075
Load Time: 0.034507
Search Time: 0.007146
Total Time: 0.041653
Load Time: 0.06376
Search Time: 0.007154
Total Time: 0.070914
Load Time: 0.026322
Search Time: 0.033975
Total Time: 0.060297
Load Time: 0.026814
Search Time: 0.009131
Total Time: 0.035945
Load Time: 0.026638
Search Time: 0.008183
Total Time: 0.034821
Load Time: 0.024653
Search Time: 0.011622
Total Time: 0.036275
Load Time: 0.029263
Search Time: 0.007243
Total Time: 0.036506
Load Time: 0.035769
Search Time: 0.007342
Total Time: 0.043111
Load Time: 0.044243
Search Time: 0.005369
Total Time: 0.049612
Load Time: 0.026526
Search Time: 0.015027
Total Time: 0.041553