write some bytes to stdout. the bytes are wrong

examples/src/main/java/org/capnproto/examples/AddressbookMain.java

@@ -1,17 +1,22 @@
 package org.capnproto.examples;
 
+import java.io.FileOutputStream;
+import java.io.FileDescriptor;
+
 import org.capnproto.MessageBuilder;
 import org.capnproto.MessageReader;
-import org.capnproto.StructList;
 import org.capnproto.InputStreamMessageReader;
+import org.capnproto.Serialize;
+import org.capnproto.StructList;
 import org.capnproto.Text;
 
 import org.capnproto.examples.Addressbook.*;
 
 public class AddressbookMain {
 
-    public static void writeAddressBook() {
+    public static void writeAddressBook() throws java.io.IOException {
-        System.out.println("WARNING: writing is not yet fully implemented");
+        System.err.println("WARNING: writing is not yet fully implemented");
+
         MessageBuilder message = new MessageBuilder();
         AddressBook.Builder addressbook = message.initRoot(AddressBook.Builder.factory);
         StructList.Builder<Person.Builder> people = addressbook.initPeople(2);
@@ -36,6 +41,9 @@ public class AddressbookMain {
         bobPhones.get(1).setNumber(new Text.Reader("555-7654"));
         bobPhones.get(1).setType(Person.PhoneNumber.Type.WORK);
         bob.getEmployment().setUnemployed();
+
+        Serialize.writeMessage((new FileOutputStream(FileDescriptor.out)).getChannel(),
+                               message);
     }
 
     public static void printAddressBook() throws java.io.IOException {

runtime/src/main/java/org/capnproto/BuilderArena.java

@@ -5,6 +5,8 @@ import java.nio.ByteOrder;
 import java.util.Vector;
 
 public final class BuilderArena implements Arena {
+
+    // Maybe this should be ArrayList?
     public final Vector<SegmentBuilder> segments;
 
     public BuilderArena() {
@@ -34,7 +36,15 @@ public final class BuilderArena implements Arena {
         throw new Error("unimplemented");
     }
 
-    public final Vector<ByteBuffer> getSegmentsForOutput() {
+    public final ByteBuffer[] getSegmentsForOutput() {
-        throw new Error();
+        ByteBuffer[] result = new ByteBuffer[this.segments.size()];
+        for (int ii = 0; ii < this.segments.size(); ++ii) {
+            SegmentBuilder segment = segments.get(ii);
+            segment.buffer.reset();
+            ByteBuffer slice = segment.buffer.slice();
+            slice.limit(segment.currentSize() * 8);
+            result[ii] = slice;
+        }
+        return result;
     }
 }

runtime/src/main/java/org/capnproto/MessageBuilder.java

@@ -23,5 +23,7 @@ public final class MessageBuilder {
         return ptr.initAsStruct(factory);
     }
 
-    //public final getSegmentsForOutput()
+    public final java.nio.ByteBuffer[] getSegmentsForOutput() {
+        return this.arena.getSegmentsForOutput();
+    }
 }

runtime/src/main/java/org/capnproto/SegmentBuilder.java

@@ -18,7 +18,7 @@ public final class SegmentBuilder extends SegmentReader {
     }
 
     // return how many words have already been allocated
-    private final int currentSize() {
+    public final int currentSize() {
         return this.pos;
     }
 

runtime/src/main/java/org/capnproto/SegmentReader.java

@@ -3,6 +3,8 @@ package org.capnproto;
 import java.nio.ByteBuffer;
 
 public class SegmentReader {
+
+    // invariant: buffer's mark is at its beginning.
     final ByteBuffer buffer;
 
     public SegmentReader(ByteBuffer buffer) {