Added example how to write structures

examples/zip-file-format/src/test/java/examples/zip/ZipFileWritingTest.java

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+package examples.zip;
+
+import org.junit.Test;
+import static org.junit.Assert.assertEquals;
+
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+import java.util.zip.CRC32;
+import java.util.zip.Checksum;
+import java.util.zip.ZipEntry;
+import java.util.zip.ZipInputStream;
+
+public class ZipFileWritingTest {
+
+    @Test
+    public void testWritingZipFile() throws IOException {
+        ByteBuffer buffer = ByteBuffer.allocate(4096);
+        buffer.order(ByteOrder.LITTLE_ENDIAN);
+
+        // our file content to store
+        byte[] fileData = "example file content".getBytes("UTF-8");
+
+        // compute the CRC32 checksum of the file content
+        Checksum checksum = new CRC32();
+        checksum.update(fileData, 0, fileData.length);
+        long crc32 = checksum.getValue();
+
+        // first we create and write the local file header
+        long positionOfLocalFileHeader = buffer.position();
+        LocalFileHeader fhFile = new LocalFileHeader();
+        fhFile.setSignature(ByteBuffer.wrap(new byte[]{ 0x50, 0x4b, 0x03, 0x04 }));
+        fhFile.setFileName("example.txt");
+        fhFile.setCompressedSizeInBytes(fileData.length);
+        fhFile.setUncompressedSizeInBytes(fileData.length);
+        fhFile.setCrc32OfUncompressedData(crc32);
+        fhFile.write(buffer);
+        // right after the local file header, the actual file content needs to be stored
+        buffer.put(fileData);
+
+        // now we need to create the central directory header
+        long positionOfFirstCentralDirectoryHeader = buffer.position();
+        CentralDirectoryFileHeader cdfh = new CentralDirectoryFileHeader();
+        cdfh.setSignature(ByteBuffer.wrap(new byte[]{ 0x50, 0x4b, 0x01, 0x02 }));
+        cdfh.setFileName("example.txt");
+        cdfh.setCrc32OfUncompressedData(crc32);
+        cdfh.setCompressedSizeInBytes(fileData.length);
+        cdfh.setUncompressedSizeInBytes(fileData.length);
+        cdfh.setRelativeOffsetOfLocalFileHeader(positionOfLocalFileHeader);
+        cdfh.write(buffer);
+
+        // finally, we need to write the closing end of central directory header
+        EndOfCentralDirectoryRecord eocdr = new EndOfCentralDirectoryRecord();
+        eocdr.setSignature(ByteBuffer.wrap(new byte[]{ 0x50, 0x4b, 0x05, 0x06 }));
+        eocdr.setOffsetOfStartOfCentralDirectory(positionOfFirstCentralDirectoryHeader);
+        eocdr.setNumberOfCentralDirectoryRecordsOnThisDisk(1);
+        eocdr.setTotalNumberOfCentralDirectoryRecords(1);
+        eocdr.write(buffer);
+
+        // now we read the ZIP file using Java on-board classes and verify
+        ZipInputStream zis = new ZipInputStream(new ByteArrayInputStream(buffer.array()));
+        ZipEntry fileEntry = zis.getNextEntry();
+        assertEquals("example.txt", fileEntry.getName());
+        byte[] fileContentRead = zis.readAllBytes();
+        assertEquals("example file content", new String(fileContentRead, "UTF-8"));
+    }
+}

examples/zip-file-format/src/test/resources/README.md

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+The `example.zip` file was created without compression using
+```
+zip -0 -r example.zip zip-content/
+```