How to use BigDecimal in Java to keep a two-bit valid number for a double type

One, scene: read from the data table decimal type of data is directly plugged into a double type of object, there is no exception.

If you want to calculate on this basis again, an exception will occur.

For example: read out the data is 0.0092, multiply it by 100, then become 0.919999999999999 ...

Second, Reason:

The Java MySQL data types are compared as follows:

Type name	Display length	Database type	Java type	JDBC Type index (int)	Describe
VARCHAR	L+n	VARCHAR	Java.lang.String	12
CHAR	N	CHAR	Java.lang.String	1
Blob	L+n	Blob	Java.lang.byte[]	-4
TEXT	65535	VARCHAR	Java.lang.String	-1
INTEGER	4	INTEGER UNSIGNED	Java.lang.Long	4
TINYINT	3	TINYINT UNSIGNED	Java.lang.Integer	-6
SMALLINT	5	SMALLINT UNSIGNED	Java.lang.Integer	5
Mediumint	8	Mediumint UNSIGNED	Java.lang.Integer	4
BIT	1	BIT	Java.lang.Boolean	-7
BIGINT	20	BIGINT UNSIGNED	Java.math.BigInteger	-5
FLOAT	4+8	FLOAT	Java.lang.Float	7
DOUBLE	22	DOUBLE	Java.lang.Double	8
DECIMAL	11	DECIMAL	Java.math.BigDecimal	3
BOOLEAN	1	With tinyint
Id	11	PK (INTEGER UNSIGNED)	Java.lang.Long	4
DATE	10	DATE	Java.sql.Date	91
Time	8	Time	Java.sql.Time	92
Datetime	19	Datetime	Java.sql.Timestamp	93
TIMESTAMP	19	TIMESTAMP	Java.sql.Timestamp	93
Year	4	Year	Java.sql.Date	91

Three, the solution:

1, the double type re-bigdecimal type, the final result also to obtain a significant number of digits

Double d = 111231.5585;

BigDecimal B = new BigDecimal (f);

Double df = B.setscale (2, bigdecimal.round_half_up). Doublevalue ();

2, because the type of the data table is a decimal, so the corresponding member variables in the Java object type is changed to BigDecimal.

Iv. introduction of BigDecimal

In MySQL, for high-precision data storage, such as money, the decimal type is required, not the float or double type, because the latter has a large data error.

The declaration syntax for the decimal column is decimal (m,d).

In MySQL 5.1, the value range of the parameter:

1, M is the maximum number (precision) of the number. The range is 1~65 (in the older MySQL version, the allowable range is 1~254).

2, D is the number of digits to the right of the decimal point (scale). Its scope is 0~30, but must not exceed M.
Description: Float accounted for 4 bytes, double was 8 bytes, decimail (m,d) accounted for m+2 bytes.
The maximum value for decimal (5, 2) is 9 9 9 9. 9 9, because there are 7 bytes available.

Note:
The effect of M and D on the range of decimal (M, d) values

Type description value range (MySQL < 3.23) range (MySQL >= 3.23)
DECIMAL (4, 1)-9.9 to 99.9-999.9 to 9999.9
DECIMAL (5, 1)-99.9 to 999.9-9999.9 to 99999.9
DECIMAL (6, 1)-999.9 to 9999.9-99999.9 to 999999.9
DECIMAL (6, 2)-99.99 to 999.99-9999.99 to 99999.99
DECIMAL (6, 3)-9.999 to 99.999-999.999 to 9999.999

# in MySQL 3.23 and later versions, the value range of decimal (M, d) equals the range of decimal (M + 2, D) in earlier versions.

Another type of data:
Longblob, this type of data can be directly stored in the data table of the image file!

In the study of MySQL's decimal data type, data experiment results are now published:
Database version: Server version:5.0.45 Source Distribution

1. CREATE TABLE structure

CREATE Table Ta (a float,b decimal (10,5));

2. Inserting data

INSERT into TA (A, B) values (1,12345.123423);

The column B data actually inserted is: 12345.12342

INSERT into TA (A, B) values (1,123456.1234);

The column B data actually inserted is: 99999.99999

Conclusion: The decimal data type,
1. When the value of the inserted integer part exceeds its representation, the value of the fractional part is ignored and populated with the maximum value.
2, when the integer part of the legal, fractional part of the extra digits, directly truncated.

How to use BigDecimal in Java to keep a two-bit valid number for a double type