Java: Added access object for vector of struct and vector of tables. (#5233)

* Java: Added access object for vector of struct and vector of tables.

* Java: Workarounds removed when accessing the union vector.

java/com/google/flatbuffers/BaseVector.java

@@ -0,0 +1,96 @@
+/*
+ * Copyright 2019 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 com.google.flatbuffers;
+
+import java.nio.ByteBuffer;
+
+/// @cond FLATBUFFERS_INTERNAL
+
+/**
+ * All vector access objects derive from this class, and add their own accessors.
+ */
+public class BaseVector {
+  /** Used to hold the vector data position. */
+  private int vector;
+  /** Used to hold the vector size. */
+  private int length;
+  /** Used to hold the vector element size in table. */
+  private int element_size;
+  /** The underlying ByteBuffer to hold the data of the vector. */
+  protected ByteBuffer bb;
+
+  /**
+   * Get the start data of a vector.
+   *
+   * @return Returns the start of the vector data.
+   */
+  protected int __vector() {
+    return vector;
+  }
+
+  /**
+   * Gets the element position in vector's ByteBuffer.
+   *
+   * @param j An `int` index of element into a vector.
+   * @return Returns the position of the vector element in a ByteBuffer.
+   */
+  protected int __element(int j) {
+    return vector + j * element_size;
+  }
+
+  /**
+   * Re-init the internal state with an external buffer {@code ByteBuffer}, an offset within and
+   * element size.
+   *
+   * This method exists primarily to allow recycling vector instances without risking memory leaks
+   * due to {@code ByteBuffer} references.
+   */
+  protected void __reset(int _vector, int _element_size, ByteBuffer _bb) { 
+    bb = _bb;
+    if (bb != null) {
+      vector = _vector;
+      length = bb.getInt(_vector - Constants.SIZEOF_INT);
+      element_size = _element_size;
+    } else {
+      vector = 0;
+      length = 0;
+      element_size = 0;
+    }
+  }
+
+  /**
+   * Resets the internal state with a null {@code ByteBuffer} and a zero position.
+   *
+   * This method exists primarily to allow recycling vector instances without risking memory leaks
+   * due to {@code ByteBuffer} references. The instance will be unusable until it is assigned
+   * again to a {@code ByteBuffer}.
+   */
+  public void reset() {
+    __reset(0, 0, null);
+  }
+
+  /**
+   * Get the length of a vector.
+   *
+   * @return Returns the length of the vector.
+   */
+  public int length() {
+    return length;
+  }
+}
+
+/// @endcond