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  • Solve null pointer exception

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    Answer it

    Here is my Code:

    import java.sql.*;
    
    public class dbcon {
    
     Connection con;
     Statement st;
     ResultSet rs;
     dbcon() {
      try {
       Class.forName("net.ucanaccess.jdbc.UcanaccessDriver");
       Connection con = DriverManager.getConnection("jdbc:ucanaccess://E:\\code\\db5.mdb");
    
      } catch (Exception e) {
       System.out.println("hihihhaha 1");
       System.out.println(e);
      }
     }
     public String check(String u, String p) {
      String s = null;
    
    
      try {
       System.out.println("hihihhaha");
       st = con.createStatement();
       rs = st.executeQuery("select * from login where user='" + u + "' and pass='" + p + "' ");
    
       if (rs.next())
    
       {
        s = "ok";
       } else {
        s = "notok";
       }
      } catch (Exception e) {
       System.out.println(e);
      }
    
      return s;
     }
    
     public String check_key(String u, String key) {
      String s = null;
    
    
      try {
    
       st = con.createStatement();
       rs = st.executeQuery("select * from login where user='" + u + "' and s_code='" + key + "' ");
    
       if (rs.next())
    
       {
        s = "ok";
       } else {
        s = "notok";
       }
      } catch (Exception e) {
       System.out.println(e);
      }
    
      return s;
     }
    
     public String checkip(String ip) {
      String s = null;
    
    
      try {
    
    
       st = con.createStatement();
       rs = st.executeQuery("select * from ADDRESS where ipaddress='" + ip + "' ");
    
       if (rs.next())
    
       {
        s = "ok";
       } else {
        s = "notok";
       }
      } catch (Exception e) {
       System.out.println(e);
      }
    
      return s;
     }
    
     public String checkfileoption(String option1) {
      String s = null;
    
    
      try {
    
    
       st = con.createStatement();
       rs = st.executeQuery("select * from RESOURCES where files='" + option1 + "' ");
    
       if (rs.next())
    
       {
        s = rs.getString(3);
       }
      } catch (Exception e) {
       System.out.println(e);
      }
    
      return s;
     }
    
     public String listfile(String clientIp) {
      String s = "";
    
    
      try {
    
       //select distinct username from userinfo
       st = con.createStatement();
       rs = st.executeQuery("select distinct files from RESOURCES where allowedip='" + clientIp + "' ");
    
       while (rs.next())
    
       {
        s = s + rs.getString(1) + ";";
       }
    
      } catch (Exception e) {
       System.out.println(e);
      }
    
      return s;
     }
     public String listfile() {
      String s = "";
    
    
      try {
    
    
       st = con.createStatement();
       rs = st.executeQuery("SELECT distinct files FROM  RESOURCES");
    
       while (rs.next())
    
       {
        s = s + rs.getString(1) + ";";
       }
    
      } catch (Exception e) {
       System.out.println(e);
      }
    
      return s;
     }
     public String listip() {
      String s = "";
    
    
      try {
    
    
       st = con.createStatement();
       rs = st.executeQuery("select * from ADDRESS ");
    
       while (rs.next())
    
       {
        s = s + rs.getString(1) + ";";
       }
    
      } catch (Exception e) {
       System.out.println(e);
      }
    
      return s;
     }
    
     public String getfake(String fil) {
      String s = null;
    
    
      try {
    
    
       st = con.createStatement();
       rs = st.executeQuery("select * from RESOURCES where files='" + fil + "'");
    
       if (rs.next())
    
       {
        s = rs.getString(2);
       }
    
      } catch (Exception e) {
       System.out.println(e);
      }
      System.out.println(s);
      return s;
     }
    
     public void removede(String data, String table1, String wh) {
    
    
      try {
    
    
       st = con.createStatement();
    
       String sql = "delete  from " + table1 + " where " + wh + "='" + data + "'";
       System.out.println(sql);
    
       st.executeUpdate(sql);
       st.executeUpdate(sql);
      } catch (Exception e) {
       System.out.println(e);
      }
      System.out.println("deleted");
    
     }
    
     public void add_ip(String s) {
    
      try {
    
    
       st = con.createStatement();
    
       String sql = "insert into ADDRESS values('" + s + "')";
       System.out.println(sql);
    
       st.executeUpdate(sql);
       //st.executeUpdate(sql);
      } catch (Exception e) {
       System.out.println(e);
      }
      System.out.println("added");
    
     }
    
     public void add_res(String f, String ff, String op, String allowedip) {
    
      try {
       st = con.createStatement();
    
       String sql = "insert into RESOURCES values('" + f + "','" + ff + "','" + op + "','" + allowedip + "')";
       System.out.println(sql);
    
       st.executeUpdate(sql);
       //st.executeUpdate(sql);
      } catch (Exception e) {
       System.out.println(e);
      }
      System.out.println("added");
    
     }
    
     public void add_user(String user, String pass, String code, int age, String gender, String phone, String address, String mobile) {
    
      try {
    
    
       st = con.createStatement();
    
       String sql = "insert into LOGIN values('" + user + "','" + pass + "','" + code + "'," + age + ",'" + gender + "','" + phone + "','" + address + "','" + mobile + "')";
       System.out.println(sql);
    
       st.executeUpdate(sql);
       //st.executeUpdate(sql);
      } catch (Exception e) {
       System.out.println(e);
      }
      System.out.println("added");
    
     }
    
    
     public static void main(String arg[]) {
      dbcon ob = new dbcon();
      /*System.out.println(ob.check("a","a"));
      System.out.println(ob.checkip("127.0.0.1"));
      System.out.println(ob.listfile());
      System.out.println(ob.getfake("Login.java"));
      ob.removede("dsds","ADDRESS","IPADDRESS");
      */
      //ob.add_ip("aasd");
      //ob.add_res("aasd","sss","ddd");
      //ob.add_user("a","a","a","w","s","d");
      System.out.println(ob.checkfileoption("Login.java"));
     }
    
    
    }

     

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