how to use transaction isolation

In the java code below, I set the transaction isolation to serialization.

public class IsolationTest {

private static String select = "select * from tmp where url = 'aaa'";

public static void main(String[] args) throws Exception{
//ConncetionFactory is a factory class for managing connection
Connection con = ConnectionFactory.getConnection();
con.setTransactionIsolation(Connection.TRANSACTION_SERIALIZABLE);
con.setAutoCommit(false);

Statement smt = con.createStatement();
ResultSet r1 = smt.executeQuery(select);
System.out.println(r1.next()); //(1)
con.commit(); //(2)

ResultSet r2 = smt.executeQuery(select);
System.out.println(r2.next()); //(3)
con.commit();
smt.close();

ConnectionFactory.closeConnection();
}
}

I set a break point at (2), then I run this code in debug mode. When it suspended at (2), line(1) print "false". Then, I execute an insert statement in pgadmin: insert into tmp values('aaa'), after that I continued to run the code, and line(3) print "true". I have set the transaction isolation to serialization, didn't the two select statements print the same result?

btw: postgresql version is 8.2, jdbc version is postgresql-8.2-506.jdbc3.jar, and jdk version is 1.5