[Java] mysql treiber kann nicht aufgelöst werden

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;



public class handler {
 
 private String user="",pw="",database="";
 Connection conn = null;
 
 
 public void setDatabase(String database) {
 this.database = database;
 }



 public String getDatabase() {
 return database;
 }


 public void setPw(String pw) {
 this.pw = pw;
 }


 public String getPw() {
 return pw;
 }


 public void setUser(String user) {
 this.user = user;
 }


 public String getUser() {
 return user;
 }

 public boolean close() {
 try {
 if(!conn.isClosed()) {
 conn.close();
 }
 System.out.println("Connection closed!");
 return true;
 } catch (Exception ex) {
 return false;
 }
 }
 
 public boolean changeDB(String database) {
 try {
 System.out.println("Query: 'use " + database + ";'");
 Statement stat = conn.createStatement();
 stat.executeQuery("use " + database + ";");
 return true;
 } catch (Exception e) {
 return false;
 }
 }

 public boolean connect() {
 if(user.length()>0 && pw.length()>0 && database.length()>0) {
 return connect(user,pw,database);
 }
 return false;
 }
 
 public boolean connect(String userName, String userPassword,
 String databaseUrl) {
 try {
 System.out.println("Connecting to:" + databaseUrl);
 Class.forName("com.mysql.jdbc.Driver").newInstance();
 conn = DriverManager.getConnection(databaseUrl,
 userName, userPassword);
 System.out.println("Connected!");
 return true;
 } catch (SQLException e) {
 System.out.println("Connection refused!");
 return false;
 } catch (InstantiationException e) {
 System.out.println("Connection refused!");
 return false;
 } catch (IllegalAccessException e) {
 System.out.println("Connection refused!");
 return false;
 } catch (ClassNotFoundException e) {
 System.out.println("Connection refused!");
 return false;
 }
 }
 
 public ResultSet executeQuery(String query) {
 Statement stat;
 try {
 stat = conn.createStatement();
 return stat.executeQuery(query);
 } catch (SQLException e) {
 return null;
 }

 }