数据类型转换：
包括Double,Float,Integer,Long,String等类型转换成BigDecimal

public static void testDecimal(){
        //new BigDecima()
        System.out.println(new BigDecimal("0.03"));//0.03
        System.out.println(new BigDecimal(3));//3
        System.out.println(new BigDecimal(3L));//3
        System.out.println(new BigDecimal(0.03));//0.0299999999999999988897769753748434595763683319091796875
        System.out.println(new BigDecimal(0.03f));//0.02999999932944774627685546875
        
        //BigDecimal.valueOf(double val)
        //BigDecimal.valueOf(long val)
        System.out.println(BigDecimal.valueOf(3L));//3
        System.out.println(BigDecimal.valueOf(0.03));//0.03
        System.out.println(BigDecimal.valueOf(0.03f));//0.029999999329447746
        System.out.println(BigDecimal.valueOf(1000L));//100

        // 使用setScale()精确小数位
        System.out.println(new BigDecimal(0.03).setScale(4,BigDecimal.ROUND_HALF_UP));//0.300
        System.out.println(new BigDecimal(0.99f).setScale(4,BigDecimal.ROUND_HALF_UP));//0.0300
        System.out.println(BigDecimal.valueOf(0.03f).setScale(4,BigDecimal.ROUND_HALF_UP));//0.0300
    }

Rounding Mode精确小数位数(7种模式)：

public static void testRoundingMode(){
        // 1.BigDecimal.ROUND_UP 直接进位
        System.out.println(BigDecimal.valueOf(3.51).setScale(1,BigDecimal.ROUND_UP));// 3.6
        // 2.BigDecimal.ROUND_DOWN 直接退位
        System.out.println(BigDecimal.valueOf(3.57).setScale(1,BigDecimal.ROUND_DOWN));// 3.5
        // 3.BigDecimal.ROUND_CEILING
        // 正数时同ROUND_UP，负数时同ROUND_DOWN（绝对值进位）
        System.out.println(BigDecimal.valueOf(3.51).setScale(1,BigDecimal.ROUND_CEILING));// 3.6
        System.out.println(BigDecimal.valueOf(-3.57).setScale(1,BigDecimal.ROUND_CEILING));// -3.5
        // 4.ROUND_HALF_UP 四舍五入
        System.out.println(BigDecimal.valueOf(3.55).setScale(1,BigDecimal.ROUND_HALF_UP));// 3.6
        // 5.ROUND_HALF_DOWN 五舍六入
        System.out.println(BigDecimal.valueOf(3.55).setScale(1,BigDecimal.ROUND_HALF_DOWN));// 3.5
        System.out.println(BigDecimal.valueOf(3.56).setScale(1,BigDecimal.ROUND_HALF_DOWN));// 3.6
        // 6.ROUND_HALF_EVEN
        // 前一位是奇数，则同ROUND_HALF_UP
        System.out.println(BigDecimal.valueOf(3.55).setScale(1,BigDecimal.ROUND_HALF_EVEN));//3.6
        System.out.println(BigDecimal.valueOf(3.55).setScale(1,BigDecimal.ROUND_HALF_UP));//3.6
        System.out.println(BigDecimal.valueOf(3.55).setScale(1,BigDecimal.ROUND_HALF_DOWN));//3.5
        // 前一位是偶数，则同ROUND_HALF_DOWN
        System.out.println(BigDecimal.valueOf(4.665).setScale(2,BigDecimal.ROUND_HALF_EVEN));//4.66
        System.out.println(BigDecimal.valueOf(4.665).setScale(2,BigDecimal.ROUND_HALF_UP));//4.67
        System.out.println(BigDecimal.valueOf(4.665).setScale(2,BigDecimal.ROUND_HALF_DOWN));//4.66
}