Java：检查自动生成的 ID

Question

My Java program adds items into database. I have a code for generating random string that will be used as an item ID. I want to make IDchecker(id) that will check whether the ID already exists into database.

If I have codeIDgenerator() and IDchecker(id) methods, how do I make a loop that will generate new code if the ID already exists, or will exit the loop if the ID is unique and doesn't come up in the database?

Also I'm having trouble with my IDchecker(id) method, I'm using ResultSet to bring back the data from SQL, but I can't find a way to determine how many rows does ResultSet has (if at all). There is no isEmpty() for resultSet?

Here's the code:

public void AddItem() {
    boolean checkCode = false;
    while (checkCode == false) {
        Random r = new Random();
        int numbers = 100000 + (int) (r.nextFloat() * 899900);
        String ID= Integer.toString(numbers);
        try {
            if (DatabaseConnection.checkID(ID) == false) {
                checkCode = true;
                System.out.println("ID is unique");
            } else if (DatabaseConnection.checkID(ID) == true) {
                System.out.println("ID is NOT unique");
            }
        } catch (SQLException ex) {
            Logger.getLogger(ModelTabeleIntervencija.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

And here is the ckeckID(ID) method

public boolean CheckID(String ID) throws SQLException {
    String query = "SELECT itemId FROM items WHERE itemID= '"+ID+"'";
    Statement dbStatement = connection.createStatement();
    ResultSet rsItems= dbStatement .executeQuery(query);
        if (rsItems.isEmpty( )== true){
            return false;
            // ID not found - is unique
        } else{
        return true;
            // ID found - is not unique
        }
}

Thanks

Answer 1

While generating unique id is best done in your database, I can help you simplify your code. You shouldn't need to check your database twice.

private final Random r = new Random();
public String getUniqueId() {
    try {
        while (true) {
            int n = r.nextInt(1000 * 1000) + 1000 * 1000;
            String id = ("" + n).substring(1); // number between 000000 and 999999
            if (DatabaseConnection.checkID(id))
                return id;
        }
    } catch (SQLException ex) {
        throw new IllegalStateException("Cannot access database", ex);
    }
}

However, instead of generating a random id, you could just get the next id.

public String getUniqueId() {
    try {
        String maxId = DatabaseConnection.selectMaxId();
        int n = Integer.parseInt(maxId) + 1;
        return ("" + n).substring(1); // number between 000000 and 999999
    } catch (SQLException ex) {
        throw new IllegalStateException("Cannot access database", ex);
    }
}