如何在java中将用户输入与我的数据库进行比较

Question

As shown on the title, I have tried so hard to compare the user input with my database and it works with a true input. However, when the input is not exist on my database I don't know what is wrong in my below code, please help.

private class da implements ActionListener{
 public void actionPerformed(ActionEvent e){
           Connection con = null;

      String url = "jdbc:mysql://localhost:3306/المكتبة";
      String unicode = "?useUnicode=yes&characterEncoding=UTF-8";
  try{
con = DriverManager.getConnection(url+unicode,"root","");

 PreparedStatement upd = con.prepareStatement("SELECT * FROM library WHERE author =?'");
 upd.setString(1,name.getText());
 ResultSet rs = upd.executeQuery();


while(rs.next()){
    String authorname = rs.getString("author");
    String bookname = rs.getString("bookname");
    String categort = rs.getString("category");
    int isbn = Integer.parseInt(rs.getString("ISBN"));
    String data = "اسم المؤلف: "+authorname+"\n"+"اسم الكتاب: "+bookname+"\n"+"التصنيف: "+categort+"\n"+"ISBN: "+isbn;


 if(name.getText().equals(authorname))
     txt.setText(data);
 else
     txt.setText("no matches");

Answer 1

The problem is related to this section:

while(rs.next()){

 // ... other code 

 if(name.getText().equals(authorname))
     txt.setText(data);
 else
     txt.setText("no matches");

If the value of name.getText() is not found in your database, rs.next() will never return true. Since you've enclosed your if block inside of the while loop , it will never be executed if no match is found in your database. You could solve it a number of ways, one way is to do something like this instead:

boolean has_results = rs.next();

if(has_results){

    do {
        // ... your loop code
    }while(rs.next());

}else {
    text.setText("No Matches");
}

In this code, if the first call to rs.next() returns false , we know nothing we returned from the database. Otherwise, loop as normal. In this instance, I changed the loop to a post-checking do...while loop instead, since we want to check rs.next() at the end of the loop now.

Note: This answer demonstrates another way of checking whether the result set contains rows.

Answer 2

You should close the while() loop before the if/else.

It is not the prettiest of code though.