Database Programming with JDBC & Java, Second Edition

SQL Datatypes and Java Datatypes

Support for different datatypes in SQL2 is poor. Since Java is an object-oriented language, however, datatype support is extremely rich. Therefore a huge disconnect exists between what sits in the database and the way you want it represented in your Java application. The SQL concept of a variable width, single-byte character array, for example, is the VARCHAR datatype. Java actually has no concept of a variable width, single-byte character array; Java doesn’t even have a single-byte character type.^[6] The closest thing is the String class.

To make matters worse, many database engines internally support their own datatypes and loosely translate them to a SQL2 type. All Oracle numeric types, for example, map to the SQL NUMERIC type. JDBC, fortunately, lets you retrieve data in their Java forms defined by a JDBC-specified datatype mapping. You do not need to worry that a SQL LONG has a different representation in Sybase than it does in Oracle. You just call the ResultSet getLong() method to retrieve numbers you wish to treat as Java longs.

You do need to be somewhat concerned when designing the database, however. If you pull a 64-bit number into a Java application via getInt(), you risk getting bad data. Similarly, if you save a Java float into a numeric field with a scale of 0, you will lose data. The important rule of thumb for Java programming, however, is think and work in Java and use the database to support the Java application. Do not let ...