public ArrayPtg(RecordInputStream in) 
{
	   
		field_1_reserved = new byte[RESERVED_FIELD_LEN];
		// TODO - add readFully method to RecordInputStream
		for(int i=0; i<  RESERVED_FIELD_LEN; i++) {
			field_1_reserved[i] = in.readByte();
		}
}

 public Object clone() 
{
	  ArrayPtg ptg = (ArrayPtg) super.clone();
	  ptg.field_1_reserved = (byte[]) field_1_reserved.clone();
	  ptg.token_3_arrayValues = (Object[]) token_3_arrayValues.clone();
	  return ptg;
}

 public short getColumnCount() 
{
		return token_1_columns;
}

 public byte getDefaultOperandClass() 
{
		return Ptg.CLASS_ARRAY;
}

 public short getRowCount() 
{
		return token_2_rows;
}

 public int getSize() 
{
		int size = 1+7+1+2;
		size += ConstantValueParser.getEncodedSize(token_3_arrayValues);
		return size;
}

This size includes the size of the array Ptg plus the Array Ptg Token value size

 public Object[] getTokenArrayValues() 
{
		return (Object[]) token_3_arrayValues.clone();
}

 int getValueIndex(int colIx,
    int rowIx) 
{
		if(colIx <  0 || colIx  >= token_1_columns) {
			throw new IllegalArgumentException("Specified colIx (" + colIx 
					+ ") is outside the allowed range (0.." + (token_1_columns-1) + ")");
		}
		if(rowIx <  0 || rowIx  >= token_2_rows) {
			throw new IllegalArgumentException("Specified rowIx (" + rowIx 
					+ ") is outside the allowed range (0.." + (token_2_rows-1) + ")");
		}
		return rowIx + token_2_rows * colIx;
}

Note - (2D) array elements are stored column by column

 public boolean isBaseToken() 
{
		return false;
}

 public  void readTokenValues(RecordInputStream in) 
{
		short nColumns = in.readUByte();
		short nRows = in.readShort();
		//The token_1_columns and token_2_rows do not follow the documentation.
		//The number of physical rows and columns is actually +1 of these values.
		//Which is not explicitly documented.
		nColumns++;
		nRows++;
		token_1_columns = nColumns;
		token_2_rows = nRows;
		int totalCount = nRows * nColumns;
		token_3_arrayValues = ConstantValueParser.parse(in, totalCount);
}

Read in the actual token (array) values. This occurs AFTER the last Ptg in the expression. See page 304-305 of Excel97-2007BinaryFileFormat(xls)Specification.pdf

 public String toFormulaString(HSSFWorkbook book) 
{
		StringBuffer b = new StringBuffer();
		b.append("{");
		for (int x=0;x< getColumnCount();x++) {
		  	if (x  > 0) {
				b.append(";");
			}
		  	for (int y=0;y< getRowCount();y++) {
				if (y  > 0) {
					b.append(",");
				}
		  		Object o = token_3_arrayValues[getValueIndex(x, y)];
		  		b.append(getConstantText(o));
		  	}
		  }
		b.append("}");
		return b.toString();
}

 public String toString() 
{
		StringBuffer buffer = new StringBuffer("[ArrayPtg]\n");
		buffer.append("columns = ").append(getColumnCount()).append("\n");
		buffer.append("rows = ").append(getRowCount()).append("\n");
		for (int x=0;x< getColumnCount();x++) {
			for (int y=0;y< getRowCount();y++) {
				Object o = token_3_arrayValues[getValueIndex(x, y)];
	   			buffer.append("[").append(x).append("][").append(y).append("] = ").append(o).append("\n"); 
			}
		}
		return buffer.toString();
}

 public  void writeBytes(byte[] data,
    int offset) 
{
		
		LittleEndian.putByte(data, offset + 0, sid + getPtgClass());
		System.arraycopy(field_1_reserved, 0, data, offset+1, RESERVED_FIELD_LEN);
}

 public int writeTokenValueBytes(byte[] data,
    int offset) 
{
		LittleEndian.putByte(data,  offset + 0, token_1_columns-1);
		LittleEndian.putShort(data, offset + 1, (short)(token_2_rows-1));
		ConstantValueParser.encode(data, offset + 3, token_3_arrayValues);
		return 3 + ConstantValueParser.getEncodedSize(token_3_arrayValues);
}