Support to auto-generate Java files from flatbuffer (#3749)

* auto gen flatbuffers for Java

* Add auto_gen_tool.py

* Refine

* Add a comment

* address comments.

* Address comments.

* Addressed

* Refine

* Address comments

* Fix typo

* Add exception

* Address comments.

* Refine

* Fix lint

* Fix

* Fix lint and address comment.

* Fix lint error

java/test/src/main/java/org/ray/api/test/UniqueIdTest.java

@@ -2,6 +2,7 @@ package org.ray.api.test;
 
 import java.nio.ByteBuffer;
 import java.util.Arrays;
+import java.util.List;
 import javax.xml.bind.DatatypeConverter;
 import org.junit.Assert;
 import org.junit.Test;
@@ -79,4 +80,20 @@ public class UniqueIdTest {
     Assert.assertEquals("FCFCFDFE123456789ABCDEF123456789ABCDEF00".toLowerCase(), putId.toString());
   }
 
+  @Test
+  public void testUniqueIdsAndByteBufferInterConversion() {
+    final int len = 5;
+    UniqueId[] ids = new UniqueId[len];
+    for (int i = 0; i < len; ++i) {
+      ids[i] = UniqueId.randomId();
+    }
+
+    ByteBuffer temp = UniqueIdUtil.concatUniqueIds(ids);
+    UniqueId[] res = UniqueIdUtil.getUniqueIdsFromByteBuffer(temp);
+
+    for (int i = 0; i < len; ++i) {
+      Assert.assertEquals(ids[i], res[i]);
+    }
+  }
+
 }