use netty buffer since java's ByteBuffer can't grow on its own

pom.xml

@@ -40,6 +40,11 @@
       <artifactId>junit-jupiter-params</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+    	<groupId>io.netty</groupId>
+    	<artifactId>netty-buffer</artifactId>
+    	<version>4.1.114.Final</version>
+    </dependency>
   </dependencies>
 
   <build>

src/main/java/com/mykola2312/retracker/bencode/BTree.java

@@ -1,6 +1,6 @@
 package com.mykola2312.retracker.bencode;
 
-import java.nio.ByteBuffer;
+import static io.netty.buffer.Unpooled.buffer;
 import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 
@@ -10,6 +10,8 @@ import com.mykola2312.retracker.bencode.error.BDecodeParseError;
 import com.mykola2312.retracker.bencode.error.BErrorNoRoot;
 import com.mykola2312.retracker.bencode.error.BErrorValueCast;
 
+import io.netty.buffer.ByteBuf;
+
 public class BTree {
 	private BValue root = null;
 	
@@ -17,6 +19,12 @@ public class BTree {
 		return root;
 	}
 	
+	@SuppressWarnings("unchecked")
+	public <T extends BValue> T setRoot(T root) {
+		this.root = root;
+		return (T)this.root;
+	}
+	
 	private static final byte BE_INTEGER = (byte)'i';
 	private static final byte BE_LIST = (byte)'l';
 	private static final byte BE_DICT = (byte)'d';
@@ -143,7 +151,7 @@ public class BTree {
 	}
 	
 	class BEncoder {
-		private ByteBuffer buffer = ByteBuffer.allocate(0);
+		private ByteBuf buffer = buffer();
 		
 		private byte[] encodeInt(int value) {
 			return Integer.toString(value).getBytes(StandardCharsets.UTF_8);
@@ -156,39 +164,39 @@ public class BTree {
 		public void encode(BValue node) {
 			switch (node.getType()) {
 			case INTEGER:
-				buffer.put(BE_INTEGER);
-				buffer.put(encodeLong(((BInteger)node).get()));
-				buffer.put(BE_END);
+				buffer.writeByte(BE_INTEGER);
+				buffer.writeBytes(encodeLong(((BInteger)node).get()));
+				buffer.writeByte(BE_END);
 				break;
 			case LIST:
-				buffer.put(BE_LIST);
+				buffer.writeByte(BE_LIST);
 				for (BValue item : node) {
 					encode(item);
 				}
-				buffer.put(BE_END);
+				buffer.writeByte(BE_END);
 				break;
 			case DICT:
-				buffer.put(BE_DICT);
+				buffer.writeByte(BE_DICT);
 				for (BValue key : node) {
 					encode(key);
 					encode(key.getChild());
 				}
-				buffer.put(BE_END);
+				buffer.writeByte(BE_END);
 				break;
 			case STRING:
 				byte[] bytes = ((BString)node).get();
-				buffer.put(encodeInt(bytes.length));
-				buffer.put(BE_STRING_SEP);
-				buffer.put(bytes);
+				buffer.writeBytes(encodeInt(bytes.length));
+				buffer.writeByte(BE_STRING_SEP);
+				buffer.writeBytes(bytes);
 				break;
 			}
 		}
 		
 		public byte[] get() {
-			int length = buffer.position();
+			int length = buffer.writerIndex();
 			byte[] out = new byte[length];
 			
-			buffer.get(out, 0, length);
+			buffer.getBytes(0, out);
 			return out;
 		}
 	}

src/test/java/com/mykola2312/retracker/bencode/BTreeTest.java

@@ -1,5 +1,6 @@
 package com.mykola2312.retracker.bencode;
 
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
@@ -140,6 +141,16 @@ public class BTreeTest {
 		});
 	}
 	
+	@Test
+	public void testEncode() {
+		assertDoesNotThrow(() -> {
+			BTree tree = new BTree();
+			
+			tree.setRoot(new BInteger(1));
+			assertArrayEquals("i1e".getBytes(), tree.encode());
+		});
+	}
+	
 	@Test
 	public void testTorrentFile() throws BError, IOException {
 		final byte[] data = Files.readAllBytes(Path.of("test", "test.torrent"));