oracle数据库连接 出错

出错信息
Hit uncaught exception java.lang.NullPointerException
error creating connection:
Exception in thread "main" java.lang.NullPointerException
at oracledatabase.DatabaseJdbc.getResultsByColumnName
at oracledatabase.DatabaseJdbc.main
代码如下:
package oracledatabase;import java.sql.ResultSet;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Connection;
import java.sql.Statement;
import java.sql.ResultSetMetaData;public class DatabaseJdbc
{
    public static void main(String[] args) {
        DatabaseJdbc databasejdbc = new DatabaseJdbc();
        databasejdbc.getResultsByColumnName();
        databasejdbc.getResultsByColumnPosition();
        databasejdbc.getAllColumn();
        databasejdbc.closeConnection();
    }
    public DatabaseJdbc(){
        try {
            Class.forName("oracle.jdbc.driver.OracleDriver");
            connection = DriverManager.getConnection(sourceURL);
            statement = connection.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_UPDATABLE);
        } catch (SQLException sqle) {
            System.err.println("error creating connection:");
        } catch (ClassNotFoundException ex) {
            /** @todo Handle this exception */
            System.err.println(ex.toString());
        }
    }
    void getResultsByColumnName() {
        try {
            ResultSet resultSet = statement.executeQuery(allName);
            int row = 0;
            while(resultSet.next()) {
                System.out.println("Row " + (++row) + ") " +
                        resultSet.getString("name") + " " +
                        resultSet.getString("id"));
            }
            resultSet.close();
        } catch (SQLException sqle) {
            System.out.println("\nSQLException: \n--- ");
            System.out.println("SQL state: " + sqle.getSQLState());
//            System.out.println("Exception deal: " + sqle.getNextException());
            System.out.println("Message: " + sqle.getMessage());        }
    }
    void getResultsByColumnPosition() {
        try {
             ResultSet resultSet = statement.executeQuery(allName);
            int row = 0;
            while(resultSet.next()) {
                System.out.println("\nRow " + (++row) + ")");
                for(int i=0; i<=2;i++){
                    System.out.println((i>1?", ":" ") + resultSet.getString(i));
           }
        }
        resultSet.close();
        } catch (SQLException sqle) {
            System.out.println("SQLException: \n--- ");
            System.out.println("SQL state: " + sqle.getSQLState());
//            System.out.println("Exception deal: " + sqle.getNextException());
            System.out.println("Message: " + sqle.getMessage());
        }
    }
    void getAllColumn() {
        try {
            ResultSet resultSet = statement.executeQuery(allName);
            ResultSetMetaData metadata = resultSet.getMetaData();
//            Column count
            int column = metadata.getColumnCount();
            int row = 0;
            while(resultSet.next()) {
                System.out.println("\nRow " + (++row) + ")");
                for(int i=0; i<=2;i++){
                    System.out.println((i>1?", ":" ") + resultSet.getString(i));
           }
        }
        resultSet.close();
        } catch (Exception ex) {        }
    }
    void closeConnection() {
        if (connection != null) {
            try {
                connection.close();
                connection = null;
            } catch (SQLException ex) {
                System.out.println("SQLException: \n--- ");
                System.out.println("SQL state: " + ex.getSQLState());
//                System.out.println("Exception deal: " + ex.getNextException());
                System.out.println("Message: " + ex.getMessage());
            }
        }
    }
    Connection connection;
    Statement statement;
    String sourceURL = "jdbc:oracle:thin:@localhost:1521:system";
    String userNameAndID = "select name,id from SYS_WORKER";
    String allName = "select * from SYS_WORKER";
}