Hallo Programmierer,
habe eine kleine einfache Frage über neue Java Datenbank HSQLDB, wäre echt sehr dankbar wenn ich da weiterkommen könnte...
nun zur meiner Frage, meiste von ihnen kennen eh schon die Testdb.java file, ganz unten habe ich eine Methode
erstellt und möchte nur wissen, wie ich diese Methode von einer anderen Klasse aufrufen kann, ohne NullPointerException zu bekommen...
Danke im Voraus
Methodenaufruf von anderen Klasse:
Viele Grüsse
Danke nochmals
habe eine kleine einfache Frage über neue Java Datenbank HSQLDB, wäre echt sehr dankbar wenn ich da weiterkommen könnte...
nun zur meiner Frage, meiste von ihnen kennen eh schon die Testdb.java file, ganz unten habe ich eine Methode
Code:
public static void addCategroy()Danke im Voraus
Code:
package database;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;
/**
* Title: Testdb
* Description: simple hello world db example of a
* standalone persistent db application
*
* every time it runs it adds four more rows to sample_table
* it does a query and prints the results to standard out
*
* Author: Karl Meissner [email]karl@meissnersd.com[/email]
*/
public class Testdb {
Connection conn; //our connnection to the db - presist for life of program
static Testdb db = null;
// we dont want this garbage collected until we are done
public Testdb() throws Exception { // note more general exception
// Load the HSQL Database Engine JDBC driver
// hsqldb.jar should be in the class path or made part of the current jar
Class.forName("org.hsqldb.jdbcDriver");
// connect to the database. This will load the db files and start the
// database if it is not alread running.
// db_file_name_prefix is used to open or create files that hold the state
// of the db.
// It can contain directory names relative to the
// current working directory
conn = DriverManager.getConnection("jdbc:hsqldb:C:/Programme/ContactManager/db",
"sa", // username
""); // password
}
public void shutdown() throws SQLException {
Statement st = conn.createStatement();
// db writes out to files and performs clean shuts down
// otherwise there will be an unclean shutdown
// when program ends
st.execute("SHUTDOWN");
conn.close(); // if there are no other open connection
}
//use for SQL command SELECT
public synchronized void query(String expression) throws SQLException {
Statement st = null;
ResultSet rs = null;
st = conn.createStatement(); // statement objects can be reused with
// repeated calls to execute but we
// choose to make a new one each time
rs = st.executeQuery(expression); // run the query
// do something with the result set.
dump(rs);
st.close(); // NOTE!! if you close a statement the associated ResultSet is
// closed too
// so you should copy the contents to some other object.
// the result set is invalidated also if you recycle an Statement
// and try to execute some other query before the result set has been
// completely examined.
}
//use for SQL commands CREATE, DROP, INSERT and UPDATE
public synchronized void update(String expression) throws SQLException {
Statement st = null;
st = conn.createStatement(); // statements
int i = st.executeUpdate(expression); // run the query
if (i == -1) {
System.out.println("db error : " + expression);
}
st.close();
} // void update()
public static void dump(ResultSet rs) throws SQLException {
// the order of the rows in a cursor
// are implementation dependent unless you use the SQL ORDER statement
ResultSetMetaData meta = rs.getMetaData();
int colmax = meta.getColumnCount();
int i;
Object o = null;
// the result set is a cursor into the data. You can only
// point to one row at a time
// assume we are pointing to BEFORE the first row
// rs.next() points to next row and returns true
// or false if there is no next row, which breaks the loop
for (; rs.next(); ) {
for (i = 0; i < colmax; ++i) {
o = rs.getObject(i + 1); // Is SQL the first column is indexed
// with 1 not 0
System.out.print(o.toString() + " ");
}
System.out.println(" ");
}
} //void dump( ResultSet rs )
public static void main(String[] args) {
Testdb db = null;
try {
db = new Testdb();
} catch (Exception ex1) {
ex1.printStackTrace(); // could not start db
return; // bye bye
}
try {
//make an empty table
//
// by declaring the id column IDENTITY, the db will automatically
// generate unique values for new rows- useful for row keys
db.update(
"CREATE TABLE sample_table ( id INTEGER IDENTITY, str_col VARCHAR(256), num_col INTEGER)");
} catch (SQLException ex2) {
//ignore
//ex2.printStackTrace(); // second time we run program
// should throw execption since table
// already there
//
// this will have no effect on the db
}
try {
// add some rows - will create duplicates if run more then once
// the id column is automatically generated
db.update(
"INSERT INTO sample_table(str_col,num_col) VALUES('Ford', 100)");
db.update(
"INSERT INTO sample_table(str_col,num_col) VALUES('Toyota', 200)");
db.update(
"INSERT INTO sample_table(str_col,num_col) VALUES('Honda', 300)");
db.update(
"INSERT INTO sample_table(str_col,num_col) VALUES('GM', 400)");
// do a query
db.query("SELECT * FROM sample_table WHERE num_col < 250");
addCategroy();
// at end of program
db.shutdown();
} catch (SQLException ex3) {
ex3.printStackTrace();
}
} // main()
public static void addCategroy() {
try {
db = new Testdb();
} catch (Exception ex1) {
ex1.printStackTrace(); // could not start db
return; // bye bye
}
try {
db.update(
"INSERT INTO category(cat_id,cat_name,cat_descr) VALUES(21, 'Kunde','Kontaktverwaltung')");
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
} // class TestdbMethodenaufruf von anderen Klasse:
Code:
private void bt_addCategoryDialogActionPerformed(ActionEvent e) {
Testdb.addCategroy();
}Viele Grüsse
Danke nochmals
