private void btnAdd1ActionPerformed(java.awt.event.ActionEvent evt) {
    // TODO add your handling code here:
     Connection conn = null;
            Statement st = null;
            ResultSet rs = null;

        try{

  conn = DriverManager.getConnection("jdbc:mysql://localhost/check","root","");
    st = conn.createStatement();

    String sql4 =("SELECT AVG(unit4) as num FROM lo where unit4 IS NOT NULL");
         PreparedStatement pstmt3 = conn.prepareStatement(sql4);
         ResultSet rs4 = pstmt3.executeQuery();
     rs4.next();
     double a3 = rs4.getDouble("num");
     double b3 = Math.round(a3*10);
     double res5 = b3/10;
     rs4.next();
     avg1.setText(String.valueOf(res5));
     String a1 =avg1.getText();
     String ans ;

    if(a1.equals("0.0")){
        ans = null;
    }else{
        ans = a1;
    }

    String query = "INSERT INTO chk(id) VALUES ('"+ans+"')";
    executeSQlQuery(query, "Inserted");

    }

String query = "INSERT INTO chk(id) VALUES ('"+ans+"')";

String query = "INSERT INTO chk(id) VALUES ("+ ans +")";

if(ans != null)
    ans = "'" + ans + "'";

PreparedStatement ps = connection.prepareStatement("INSERT INTO chk(id) VALUES (?)");
ps.setNull(1, java.sql.Types.VARCHAR);
ps.executeUpdate();

if (rs4.next()) {//<<<--------------------
    double a3 = rs4.getDouble("num");
    //-----------^^-----------------
}