Why SelfEncapsulation?

AppendAsGZip.java

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.nio.charset.StandardCharsets;
import java.nio.file.Path;
import java.util.logging.Level;
import java.util.logging.Logger;
import java.util.stream.Collectors;
import java.util.zip.GZIPInputStream;
import java.util.zip.GZIPOutputStream;

/**
 * Appends the entry to the file as GZipped.
 */
final class AppendEntryAsGZip<P> extends WriteToFileTask<P> {
    /**
     * A log.
     */
    private static final Logger LOG = Logger.getLogger(AppendEntryAsGZip.class.getName());

    /**
     * constructs the task.
     *
     * @param entry  should not be null
     * @param toFile should not be null
     */
    AppendEntryAsGZip(final P entry, final Path toFile) {
        super(entry, toFile);
    }

    @Override
    protected void writeEntryToFile(final Path toFile, final P entry) throws IOException {
        final StringBuilder sb = new StringBuilder();
        sb.append(slurpGZippedFile(toFile));
        sb.append(entry.toString());
        writeOutGZippedFile(toFile, sb.toString());
    }

    /**
     * reads in a GZipped file.
     * @param toFile the file to be read
     * @return the lines of the file
     * @throws IOException if something goes wrong
     */
    private String slurpGZippedFile(final Path toFile) throws IOException {
        if (toFile.toFile().length() == 0) {
            return "";

        }
        try (final FileInputStream fis = new FileInputStream(toFile.toFile());
            final GZIPInputStream gis = new GZIPInputStream(fis);
            final BufferedReader br = new BufferedReader(new InputStreamReader(gis, "UTF-8"))) {
            return br.lines().collect(Collectors.joining());
        } catch (final IOException e) {
            LOG.log(Level.SEVERE, "Could not open file " + toFile.toString(), e);
            throw e;
        }
    }

    /**
     * writes out a string to a file, gzipped.
     * @param toFile the file to be written
     * @param log the string to write, as gzipped
     */
    private void writeOutGZippedFile(final Path toFile, final String log) {
        try (final FileOutputStream fos = new FileOutputStream(toFile.toFile());
            final GZIPOutputStream gos = new GZIPOutputStream(fos)) {
            gos.write(log.getBytes(StandardCharsets.UTF_8));
        } catch (final IOException e) {
            LOG.log(Level.SEVERE, "Could not write file " + toFile.toString(), e);
        }
    }
}

AppendAsGZipTest.java

import org.junit.Before;
import org.junit.Test;
import org.mockito.Mock;
import org.mockito.MockitoAnnotations;

import java.io.File;
import java.io.IOException;
import java.nio.file.Path;

import static org.mockito.BDDMockito.*;

public class AppendEntryAsGZipTest {
    @Mock Path toFile;
    ServiceQueryLogEntry entry;
    AppendEntryAsGZip toTest;

    @Before
    public void setup() {
        MockitoAnnotations.initMocks(this);
        entry = new ServiceQueryLogEntry.Builder()
                .build();
        toTest = new AppendEntryAsGZip(entry, toFile);
    }

    @Test(expected = NullPointerException.class)
    public void rejectsNullEntry() {
        new AppendEntryAsGZip(null, toFile);
    }

    @Test(expected = NullPointerException.class)
    public void rejectsNullPath() {
        new AppendEntryAsGZip(entry, null);
    }

    @Test
    public void canWriteAnEntryToFile() throws IOException {
        final File f = mock(File.class);
        given(f.getPath()).willReturn("./service_log.2018-04-19");
        given(toFile.toFile()).willReturn(f);
        // surprising NullPointerException:
        // at java.io.File.isInvalid(File.java:187)
        //	at java.io.FileOutputStream.<init>(FileOutputStream.java:205)
        //	at java.io.FileOutputStream.<init>(FileOutputStream.java:162)
        //	at AppendEntryAsGZip.writeOutGZippedFile(AppendEntryAsGZip.java:70)
        //	at AppendEntryAsGZip.writeEntryToFile(AppendEntryAsGZip.java:40)
        //	at AppendEntryAsGZipTest.canWriteAnEntryToFile(AppendEntryAsGZipTest.java:42)
        toTest.writeEntryToFile(toFile, entry);
    }
}