Add FLATBUFFERS_COMPATIBILITY string (#5381)

* Add FLATBUFFERS_COMPATIBILITY string

- Add a new __reset method NET/JAVA which hides internal state

* Resolve PR notes

* Use operator new() to __init of Struct and Table

* Restrict visibility of C# Table/Struct to internal level

java/com/google/flatbuffers/Struct.java

@@ -29,6 +29,21 @@ public class Struct {
   /** The underlying ByteBuffer to hold the data of the Struct. */
   protected ByteBuffer bb;
 
+  /**
+   * Re-init the internal state with an external buffer {@code ByteBuffer} and an offset within.
+   *
+   * This method exists primarily to allow recycling Table instances without risking memory leaks
+   * due to {@code ByteBuffer} references.
+   */
+  protected void __reset(int _i, ByteBuffer _bb) { 
+    bb = _bb;
+    if (bb != null) {
+      bb_pos = _i;
+    } else {
+      bb_pos = 0;
+    }
+  }
+
   /**
    * Resets internal state with a null {@code ByteBuffer} and a zero position.
    *
@@ -39,8 +54,7 @@ public class Struct {
    * @param struct the instance to reset to initial state
    */
   public void __reset() {
-    bb = null;
-    bb_pos = 0;
+    __reset(0, null);
   }
 }
 

java/com/google/flatbuffers/Table.java

@@ -38,9 +38,9 @@ public class Table {
   /** The underlying ByteBuffer to hold the data of the Table. */
   protected ByteBuffer bb;
   /** Used to hold the vtable position. */
-  protected int vtable_start;
+  private int vtable_start;
   /** Used to hold the vtable size. */
-  protected int vtable_size;
+  private int vtable_size;
   Utf8 utf8 = Utf8.getDefault();
 
   /**
@@ -263,6 +263,25 @@ public class Table {
     return len_1 - len_2;
   }
 
+  /**
+   * Re-init the internal state with an external buffer {@code ByteBuffer} and an offset within.
+   *
+   * This method exists primarily to allow recycling Table instances without risking memory leaks
+   * due to {@code ByteBuffer} references.
+   */
+  protected void __reset(int _i, ByteBuffer _bb) { 
+    bb = _bb;
+    if (bb != null) {
+      bb_pos = _i;
+      vtable_start = bb_pos - bb.getInt(bb_pos);
+      vtable_size = bb.getShort(vtable_start);
+    } else {
+      bb_pos = 0;
+      vtable_start = 0;
+      vtable_size = 0;
+    }
+  }
+
   /**
    * Resets the internal state with a null {@code ByteBuffer} and a zero position.
    *
@@ -271,10 +290,7 @@ public class Table {
    * again to a {@code ByteBuffer}.
    */
   public void __reset() {
-    bb = null;
-    bb_pos = 0;
-    vtable_start = 0;
-    vtable_size = 0;
+    __reset(0, null);
   }
 }