http://qiita.com/aono-masashi/items/0ef2a0ddc8d901ff27dd

Divider.java

package divide;

public class Divider {

    private final String inputFilePath;

    private final String outputFileDirectory;

    private final Map<String, FileWriter> outputWriters = new HashMap<>();

    public Divider(String inputFilePath, String outputFileDirectory) {
        this.inputfilePath = inputFilePath;
        this.outputFileDirectory = outputFileDirectory;
    }

    public void divide() throws IOException {
        try {
            try(BufferedReader reader = new BufferedReader(new File(inputFilePath)) {
                reader.lines().forEach(line -> {
                    String[] elements = line.split("\t");
                    if (elements.length != 2) {
                        return;
                    }
                    try {
                        FileWriter writer = outputWriters.get(elements[0]);
                        if (writer == null) {
                            writer = new FileWriter(new File(outputFileDirectory, elements[0] + ".tsv"));
                            outputWriters.put(elements[0], writer);
                        }
                        writer.write(line + "\n");
                    } catch (IOException e) {
                        throw new UncheckedIOException(e);
                    }
                });
            }
        } catch (UncheckedIOException e) {
            throw e.getCause();
        } finally {
            try {
                outputWriters.entrySet().forEach(o -> {
                    try {
                        o.close();
                    } catch (IOException e) {
                        throw new UncheckedIOException(e);
                    }
                });
            } catch (UncheckedIOException e) {
                throw e.getCause();
            }
        }
    }

}