how to subtract from database

import java.sql.DriverManager;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.SQLException;

public class UpdateInventory
{
    // set this to some connection protocol like "jdbc:sqlserver://"
    private final String protocol;
    // set this to the name of your database
    private final String dbName;
    private final String username;
    private final String password;
    private final Connection connection;

    public UpdateInventory(String protocol,
            String dbName,
            String username,
            String password)
                throws SQLException
    {
        this.protocol = protocol;
        this.dbName =dbName;
        this.username = username;
        this.password = password;
        this.connection = DriverManager.getConnection(protocol + dbName,
                username, password);
    }

    public int update(String item, int quantity)
            throws SQLException
    {
        // validate
        if (item == null ||
                "".equals(item))
        {
            throw new IllegalArgumentException("Item must be set");
        }
        if (quantity <= 0)
        {
            throw new IllegalArgumentException("Quantity must be greater than zero");
        }

        PreparedStatement updateInventory = this.connection.prepareStatement(
                "UPDATE inventory SET quantity = (quantity - ?) WHERE item = ?");

        updateInventory.setInt(1, quantity);
        updateInventory.setString(2, item);
        //there are similar methods for other SQL types in PerparedStatement

        return updateInventory.executeUpdate();//returns the number of rows changed
    }
}