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466

Chapter 18 JDBC and Database Connection Pooling

while(resultSet.next()) {

  System.out.println(results.getString(1) + " " +

                     results.getString(2) + " " +

                     results.getString(3));

}

In addition to the 

get

Xxx and 

next

 methods, other useful methods in the

ResultSet

 class include 

findColumn

 (get the index of the named column),

wasNull

 (was the last 

get

Xxx result SQL 

NULL

? Alternatively, for strings you

can simply compare the return value to 

null

), and 

getMetaData

 (retrieve

information about the 

ResultSet

 in a 

ResultSetMetaData

 object). 

The 

getMetaData

 method is particularly useful. Given only a 

ResultSet

,

you have to know about the name, number, and type of the columns to be

able to process the table properly. For most fixed format queries, this is a rea 

sonable expectation. For ad hoc queries, however, it is useful to be able to

dynamically discover high level information about the result. That is the role

of the 

ResultSetMetaData

 class: it lets you determine the number, names,

and types of the columns in the 

ResultSet

. Useful 

ResultSetMetaData

methods include 

getColumnCount

 (the number of columns), 

getColumn 

Name(colNumber)

 (a column name, indexed starting at 1), 

getColumnType

(an 

int

 to compare against entries in 

java.sql.Types

), 

isReadOnly

 (is

entry a read only value?), 

isSearchable

 (can it be used in a 

WHERE

 clause?),

isNullable

 (is a null value permitted?), and several others that give details

on the type and precision of the column. 

ResultSetMetaData

 does not

include the number of rows, however; the only way to determine that is to

repeatedly call 

next

 on the 

ResultSet

 until it returns 

false

.

Close the Connection

To close the connection explicitly, you would do:

connection.close();

You should postpone this step if you expect to perform additional database

operations, since the overhead of opening a connection is usually large. In

fact, reusing existing connections is such an important optimization that Sec 

tion 18.7 develops a library just for that purpose and Section 18.8 shows some

typical timing results.

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