[Java][FlexBuffers] Make FlexBuffersBuilder reusable by adding clear() (#5889) (#5890)

java/com/google/flatbuffers/ArrayReadWriteBuf.java

@@ -34,6 +34,11 @@ public class ArrayReadWriteBuf implements ReadWriteBuf {
     this.writePos = startPos;
   }
 
+  @Override
+  public void clear() {
+    this.writePos = 0;
+  }
+
   @Override
   public boolean getBoolean(int index) {
     return buffer[index] != 0;

java/com/google/flatbuffers/ByteBufferReadWriteBuf.java

@@ -12,6 +12,11 @@ public class ByteBufferReadWriteBuf implements ReadWriteBuf {
     this.buffer.order(ByteOrder.LITTLE_ENDIAN);
   }
 
+  @Override
+  public void clear() {
+    buffer.clear();
+  }
+
   @Override
   public boolean getBoolean(int index) {
     return get(index) != 0;

java/com/google/flatbuffers/FlexBuffersBuilder.java

@@ -151,6 +151,17 @@ public class FlexBuffersBuilder {
         this(bb, BUILDER_FLAG_SHARE_KEYS);
     }
 
+    /**
+     * Reset the FlexBuffersBuilder by purging all data that it holds.
+     */
+    public void clear(){
+        bb.clear();
+        stack.clear();
+        keyPool.clear();
+        stringPool.clear();
+        finished = false;
+    }
+
     /**
      * Return `ByteBuffer` containing FlexBuffer message. {@code #finish()} must be called before calling this
      * function otherwise an assert will trigger.

java/com/google/flatbuffers/ReadWriteBuf.java

@@ -5,6 +5,13 @@ package com.google.flatbuffers;
  * FlexBuffers message.
  */
 interface ReadWriteBuf extends ReadBuf {
+
+    /**
+     * Clears (resets) the buffer so that it can be reused. Write position will be set to the
+     * start.
+     */
+    void clear();
+
     /**
      * Put a boolean into the buffer at {@code writePosition()} . Booleans as stored as single
      * byte. Write position will be incremented.