java常用工具方法

1、两个数相除，保留两位小数

public static void main(String[] args) {
        double divide = divide(1, 3, 2);
        System.out.println(divide);//0.33
        System.out.println(divide * 100 + "%");//33.0%
    }
    public static double divide(double a, double b, int scale) {
        BigDecimal bd1 = new BigDecimal(Double.toString(a));
        BigDecimal bd2 = new BigDecimal(Double.toString(b));
        return bd1.divide(bd2, scale, BigDecimal.ROUND_HALF_UP).doubleValue();
    }
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2、文件转字节数组，字节数组转文件

应用场景: 需要像第三方系统传数据，并且以json格式传。记得中间要用base64把字节转成一个编码串。

    public static void main(String[] args) {
        byte[] bytesByFile = getBytesByFile("D:\\aa.txt");
        //对字节数组进行base64编码
        String encoded = Base64.getEncoder().encodeToString(bytesByFile);
        System.out.println(encoded);

        //把base64编码转成字节
        byte[] decoded = Base64.getDecoder().decode(encoded);
        getFileByBytes(decoded,"D:\\","bb.txt");
    }

    /**
     * 将文件转换成Byte数组
     * @param pathStr
     * @return
     */
    public static byte[] getBytesByFile(String pathStr) {
        File file = new File(pathStr);
        try {
            FileInputStream fis = new FileInputStream(file);
            ByteArrayOutputStream bos = new ByteArrayOutputStream(1000);
            byte[] b = new byte[1000];
            int n;
            while ((n = fis.read(b)) != -1) {
                bos.write(b, 0, n);
            }
            fis.close();
            byte[] data = bos.toByteArray();
            bos.close();
            return data;
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    /**
     * 将Byte数组转换成文件
     * @param bytes
     * @param filePath
     * @param fileName
     */
    public static void getFileByBytes(byte[] bytes, String filePath, String fileName) {
        BufferedOutputStream bos = null;
        FileOutputStream fos = null;
        File file = null;
        try {
            File dir = new File(filePath);
            if (!dir.exists() && dir.isDirectory()) {// 判断文件目录是否存在
                dir.mkdirs();
            }
            file = new File(filePath + "\\" + fileName);
            fos = new FileOutputStream(file);
            bos = new BufferedOutputStream(fos);
            bos.write(bytes);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (bos != null) {
                try {
                    bos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (fos != null) {
                try {
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
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