step1）先建立一个缓冲大小确定（capacity）的二维数组指针data；

step2）通过循环读取数据，并填充二维数组；

step3）判断二维数组大小是否等于原定的缓冲大小（capacity）；

step3.1）若等于：则增大缓冲区大小 为 newcapacity，并建立容量为 newcapacity 的二维数组指针datacopy，将原来的data数组的内容copy或填充到datacopy，并使得data指针的指向等于datacopy的指向；

step3.2）若不等于：继续下一步循环；

step4）循环完毕后，建立容量为 newcapacity 的二维数组指针datacopy，将原来的data数组的内容copy或填充到datacopy，并使得data指针的指向等于datacopy的指向；（bingo）

A1）本文的capacity设定为10，你可以设定其他值，视具体情况而定；

A2）本文代码 在某些地方吧 data设置为null， 是为了便于 jvm 回收其内存；

// the core code begins.int capacity=10; // buffer size, of course you can specify other values of capacity. step1		double[][] data = new double[capacity][];int lineNum = 0;while((str=reader.readLine())!=null) { // step2String[] array = str.split(",");double[] temp = new double[array.length-1];for (int i = 0; i < array.length-1; i++) {temp[i] = Double.valueOf(array[i]);}if(lineNum%capacity==0) {  // step3double[][] datacopy = new double[lineNum+capacity][]; // step3.1for (int i = 0; i < data.length; i++) {datacopy[i] = data[i];}data = null;data = datacopy;				} data[lineNum++] = temp;	// step3.2	}double[][] datacopy = new double[lineNum][]; // step4.for (int i = 0; i < datacopy.length; i++) {datacopy[i] = data[i];}data = null;data = datacopy;// the core code ends.<span style="font-family: SimSun; background-color: rgb(255, 255, 255);"> </span>

public class XlsxDataModel
{private Workbook workbook;private Sheet sheet;static XlsxDataModel model;private XlsxDataModel(String sheetName){this.workbook = new XSSFWorkbook();;this.sheet = workbook.createSheet(sheetName);}public static XlsxDataModel getInstance(String sheetName){if(model == null){model = new XlsxDataModel(sheetName);}return model;}public Workbook getWorkbook() {return workbook;}public Sheet getSheet() {return sheet;}
}

XlsxDataModel model = XlsxDataModel.getInstance(sheetName);
Row row = model.getSheet().createRow(0);
Cell cell = row.createCell(0);
cell.setCellValue("");

// 获取数据行 迭代器final Iterator<Row> readIterator() {Iterator<Row> itr = null;try {File excel = new File(this.dataPath);FileInputStream fis = new FileInputStream(excel);// 创建工作簿XSSFWorkbook book = new XSSFWorkbook(fis);// 创建工作簿下的第一页纸张XSSFSheet sheet = book.getSheetAt(0);// 纸张的迭代器，用于遍历行itr = sheet.iterator();// Iterating over Excel file in Java} catch (Exception e) {// TODO: handle exceptione.printStackTrace();}return itr;}

step1）从文本读取数据；

step2）将存储文本数据的二维数组写入到xlsx；

public class DataProcess { // source code for step1 begins.public static void main(String[] args) throws IOException {String basedir = System.getProperty("user.dir") + File.separator;BufferedReader reader = new BufferedReader(new InputStreamReader(new FileInputStream(basedir+"iris_data.txt")));String str;// the core code begins.int capacity=10; // buffer size, of course you can specify other values of capacity.		double[][] data = new double[capacity][];int lineNum = 0;while((str=reader.readLine())!=null) {String[] array = str.split(",");double[] temp = new double[array.length-1];for (int i = 0; i < array.length-1; i++) {temp[i] = Double.valueOf(array[i]);}if(lineNum%capacity==0) { double[][] datacopy = new double[lineNum+capacity][];for (int i = 0; i < data.length; i++) {datacopy[i] = data[i];}data = null;data = datacopy;				} data[lineNum++] = temp;			}double[][] datacopy = new double[lineNum][];for (int i = 0; i < datacopy.length; i++) {datacopy[i] = data[i];}data = null;data = datacopy;// source code for step1 ends.DataWrite writer = new DataWrite(basedir+"gmeans_irise1.xlsx");// source code for step2 begins.writer.writeArray(data, "item", data[0].length, "iris"); // source code for step2 ends. highlight line.}
}

/*** @author Rong Tang* @version 1.0* @since 20150911*/
public class DataWrite {private String filepath;private FileOutputStream out;public DataWrite(String filepath) {this.filepath = filepath;}/*** @param data is a double array storing data written into xlsx.* @param colPrefix is a row tag.* @param headColNum is column number.* @param sheetName is the name of sheet.* @throws IOException*/public void writeArray(double[][] data, String colPrefix, int headColNum, String sheetName)throws IOException {XlsxDataModel model = XlsxDataModel.getInstance(sheetName);Row row = model.getSheet().createRow(0);Cell cell = row.createCell(0);cell.setCellValue("");for (int i = 1; i <= headColNum; i++) {cell = row.createCell(i);cell.setCellValue(i);} // build the head line overfor (int i = 0; i < data.length; i++) {row = model.getSheet().createRow(i + 1);cell = row.createCell(0);cell.setCellValue(colPrefix + (i + 1));for (int j = 0; j < data[i].length; j++) {cell = row.createCell(j + 1);cell.setCellValue(data[i][j]);}}// write the cluster result(centroid vector) into xlsx overout = new FileOutputStream(filepath);model.getWorkbook().write(out);out.flush();out.close();System.out.println("write " + filepath + " over");}/*** @param data is a int array storing data written into xlsx.* @param colPrefix is a row tag.* @param headColNum is column number.* @param sheetName is the name of sheet.* @throws IOException*/public void writeArray(int[][] data, String colPrefix, int headColNum, String sheetName)throws IOException {XlsxDataModel model = XlsxDataModel.getInstance(sheetName);Row row = model.getSheet().createRow(0);Cell cell = row.createCell(0);cell.setCellValue("");for (int i = 1; i <= headColNum; i++) {cell = row.createCell(i);cell.setCellValue(i);} // build the head line overfor (int i = 0; i < data.length; i++) {row = model.getSheet().createRow(i + 1);cell = row.createCell(0);cell.setCellValue(colPrefix + (i + 1));for (int j = 0; j < data[i].length; j++) {cell = row.createCell(j + 1);cell.setCellValue(data[i][j]);}}// write the cluster result(centroid vector) into xlsx overout = new FileOutputStream(filepath);model.getWorkbook().write(out);out.flush();out.close();System.out.println("write " + filepath + " over");}
}

/*** @author Rong Tang* @version 1.0* @since 20150911*/
public class DataRead {private String dataPath;public DataRead(String dataPath) {this.dataPath = dataPath;}/*** * @param row_start is a startup row startup index for reading. * @param col_start is a startup column index for reading.* @param array is a double array storing the data read from some xlsx. */public final void readDataToArray(int row_start, int col_start, double[][] array) {Iterator<Row> itr = readIterator(); // 获得遍历行 的迭代器Row row = null; // 行对象int row_index = 0;// 行索引int col_index = 0;// 列索引int row_length = array.length; // 数据行数int col_length = array[0].length; // 数据列数// the first row is ommited for it stores column indexif (itr.hasNext()) {itr.next();}// 定位行指针到 row_startwhile(itr.hasNext()){row_index++;if(row_index == row_start) {row_index = 0;break;}itr.next();}// 定位 over// other rows stores time series datawhile (itr.hasNext() && (row_index<row_length)) {col_index = 0;row = itr.next();Iterator<Cell> cellIterator = row.cellIterator(); // 遍历每行单元格的迭代器Cell cell = null; // the first column is ommited for it stores row indexif(cellIterator.hasNext()) { cellIterator.next();}// 定位列指针到 col_startwhile(cellIterator.hasNext()) {col_index++;if(col_index == col_start) {col_index = 0;break;}cellIterator.next();}// 定位 overwhile (cellIterator.hasNext() && (col_index<col_length)) {cell = cellIterator.next();array[row_index][col_index++] = cell.getNumericCellValue();}// 一行数据读取完毕row_index++;			} // 数据行读取完毕}// read data from xlsx to arraypublic final void readDataToArray(int row_start, int row_end) {Iterator<Row> itr = readIterator(); // 获得遍历行 的迭代器Row row = null; // 行对象int index = 0;// 行索引int row_length = row_end-row_start+1; // 数据行数// the first row is ommited for it stores column indexif (itr.hasNext()) {row = itr.next();}// 定位行指针到 row_startwhile(itr.hasNext()){index++;if(index == row_start) {break;}row = itr.next();}index -= row_start;// other rows stores time series datawhile (itr.hasNext() && (index!=row_length)) {int j = 0;row = itr.next();Iterator<Cell> cellIterator = row.cellIterator(); // 遍历每行单元格的迭代器Cell cell = null; // the first column is ommited for it stores row indexif(cellIterator.hasNext())cell = cellIterator.next();while (cellIterator.hasNext()) {cell = cellIterator.next();ClusterData.items[index][j++] = cell.getNumericCellValue();  }// 一行数据读取完毕index++;			} // 数据行读取完毕}// 获取数据行 迭代器final Iterator<Row> readIterator() {Iterator<Row> itr = null;try {File excel = new File(this.dataPath);FileInputStream fis = new FileInputStream(excel);// 创建工作簿XSSFWorkbook book = new XSSFWorkbook(fis);// 创建工作簿下的第一页纸张XSSFSheet sheet = book.getSheetAt(0);// 纸张的迭代器，用于遍历行itr = sheet.iterator();// Iterating over Excel file in Java} catch (Exception e) {// TODO: handle exceptione.printStackTrace();}return itr;}
}