Implemented the file identifier functionality for Java.

Also fixed flatc not outputting these identifiers for files
compiled on the command-line.

Bug: 16983987
Change-Id: I8b714cfea3a8e144fa52133f62b2f7eda6eb044a
Tested: on Linux

java/flatbuffers/Constants.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright 2014 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package flatbuffers;
+
+// Class that holds shared constants.
+
+public class Constants {
+    // Java doesn't seem to have these.
+    static final int SIZEOF_SHORT = 2;
+    static final int SIZEOF_INT = 4;
+    static final int FILE_IDENTIFIER_LENGTH = 4;
+}
+

java/flatbuffers/FlatBufferBuilder.java

@@ -25,7 +25,7 @@ import java.nio.charset.Charset;
 // Class that helps you build a FlatBuffer.
 // See the section "Use in Java" in the main FlatBuffers documentation.
 
-public class FlatBufferBuilder {
+public class FlatBufferBuilder extends Constants {
     ByteBuffer bb;       // Where we construct the FlatBuffer.
     int space;           // Remaining space in the ByteBuffer.
     static final Charset utf8charset = Charset.forName("UTF-8");
@@ -36,10 +36,6 @@ public class FlatBufferBuilder {
     int num_vtables = 0;          // Number of entries in `vtables` in use.
     int vector_num_elems = 0;     // For the current vector being built.
 
-    // Java doesn't seem to have these.
-    final int SIZEOF_SHORT = 2;
-    final int SIZEOF_INT = 4;
-
     // Start with a buffer of size `initial_size`, then grow as required.
     public FlatBufferBuilder(int initial_size) {
         if (initial_size <= 0) initial_size = 1;
@@ -251,6 +247,17 @@ public class FlatBufferBuilder {
         addOffset(root_table);
     }
 
+    public void finish(int root_table, String file_identifier) {
+        prep(minalign, SIZEOF_INT + FILE_IDENTIFIER_LENGTH);
+        if (file_identifier.length() != FILE_IDENTIFIER_LENGTH)
+            throw new AssertionError("FlatBuffers: file identifier must be length " +
+                                     FILE_IDENTIFIER_LENGTH);
+        for (int i = FILE_IDENTIFIER_LENGTH - 1; i >= 0; i--) {
+            addByte((byte)file_identifier.charAt(i));
+        }
+        addOffset(root_table);
+    }
+
     public ByteBuffer dataBuffer() { return bb; }
 
     // The FlatBuffer data doesn't start at offset 0 in the ByteBuffer:

java/flatbuffers/Table.java

@@ -21,12 +21,10 @@ import java.nio.ByteBuffer;
 import java.nio.charset.Charset;
 
 // All tables in the generated code derive from this class, and add their own accessors.
-public class Table {
+public class Table extends Constants {
   protected int bb_pos;
   protected ByteBuffer bb;
 
-  final int SIZEOF_INT = 4;
-
   // Look up a field in the vtable, return an offset into the object, or 0 if the field is not
   // present.
   protected int __offset(int vtable_offset) {
@@ -75,4 +73,14 @@ public class Table {
     t.bb = bb;
     return t;
   }
+
+  protected static boolean __has_identifier(ByteBuffer bb, int offset, String ident) {
+    if (ident.length() != FILE_IDENTIFIER_LENGTH)
+        throw new AssertionError("FlatBuffers: file identifier must be length " +
+                                 FILE_IDENTIFIER_LENGTH);
+    for (int i = 0; i < FILE_IDENTIFIER_LENGTH; i++) {
+      if (ident.charAt(i) != (char)bb.get(offset + SIZEOF_INT + i)) return false;
+    }
+    return true;
+  }
 }