Java: Calculation of vtable and vtable size moved to the __init method. (#5210)

vtable and vtable size depends only on `Table#bb_pos` but calculated in
`Table#_offset` method on each field lookup.
Doing this with every call of `Table#__offset` is redundant.

These values can be read once with change of `Table#bb_pos` and reused
for any field lookup.

java/com/google/flatbuffers/Table.java

@@ -37,6 +37,10 @@ public class Table {
   protected int bb_pos;
   /** The underlying ByteBuffer to hold the data of the Table. */
   protected ByteBuffer bb;
+  /** Used to hold the vtable position. */
+  protected int vtable_start;
+  /** Used to hold the vtable size. */
+  protected int vtable_size;
   Utf8 utf8 = Utf8.getDefault();
 
   /**
@@ -53,8 +57,7 @@ public class Table {
    * @return Returns an offset into the object, or `0` if the field is not present.
    */
   protected int __offset(int vtable_offset) {
-    int vtable = bb_pos - bb.getInt(bb_pos);
-    return vtable_offset < bb.getShort(vtable) ? bb.getShort(vtable + vtable_offset) : 0;
+    return vtable_offset < vtable_size ? bb.getShort(vtable_start + vtable_offset) : 0;
   }
 
   protected static int __offset(int vtable_offset, int offset, ByteBuffer bb) {
@@ -169,6 +172,8 @@ public class Table {
     offset += bb_pos;
     t.bb_pos = offset + bb.getInt(offset);
     t.bb = bb;
+    t.vtable_start = t.bb_pos - bb.getInt(t.bb_pos);
+    t.vtable_size = bb.getShort(t.vtable_start);
     return t;
   }
 
@@ -268,6 +273,8 @@ public class Table {
   public void __reset() {
     bb = null;
     bb_pos = 0;
+    vtable_start = 0;
+    vtable_size = 0;
   }
 }