Java-在Double.parseDouble（）的'catch'块中包含负值

Question

I have a Java client application that can send a message to the server. I am setting up input validation so the client detects whether an acceptable message has been sent on the 'send' button-click.

The messages that should be accepted are anything that can be parsed to a double, or a number of selected strings (recognised by the string.equals() method).

I test for the strings first, and if the condition is not met, I try to parse the message to double ( Double.parseDouble(message) ). At this point, I have a try/catch block that recognises any failed attept to parse to double (meaning it must contain letters and is therefore invalid), that catches the NumberFormatException and allows the user a chance to re-enter the message.

I would now like to make it so that the number cannot be negative, and would like to include this in the same try/catch block so that any negative value can give the user the same opportunity to re-enter.

Here is what I currently have:

    else
    {   
        try
        {
            //convert number to double (monetary value)
            bidAmount = Double.parseDouble(messageToServer);
            //send to server with item code
            output.println(selectedItemCode + bidAmount);
        }
        //item cannot turned to double
        catch (NumberFormatException numfEx)
        {
            //inform user
            fromServer.setText("Invalid Request!"
                    + USER_PROMPT);
        }
    }
    //clear input field for subsequent entry
    toServer.setText("");

Can anyone point me in the direction of how to implement this rule, without code duplication if at all possible?

Thanks, Mark

Answer 1

After bidAmount = Double.parseDouble(messageToServer); , we need to add the following:

if(bidAmount < 0){
  throw new NumberFormatException("Invalid amount");
}

An alternate solution (and probably, best practice) would be to modify catch block to catch IllegalArgumentException and throw IllegalArgumentException if the amount is less than 0 as shown below:

try
        {
            //convert number to double (monetary value)
            bidAmount = Double.parseDouble(messageToServer);
            if(bidAmount < 0){
                throw new IllegalArgumentException("Invalid amount");
            }
            //send to server with item code
            output.println(selectedItemCode + bidAmount);
        }
        //item cannot turned to double
        catch (IllegalArgumentException numfEx)
        {
            //inform user
            fromServer.setText("Invalid Request!"
                    + USER_PROMPT);
        }

Answer 2

Create a separate method which parses the String to a Double and then checks if it is non-negative, returning null in either invalid case.

//convert number to double (monetary value)
bidAmount = getDouble();
if(bidAmount != null) {

    //send to server with item code
    output.println(selectedItemCode + bidAmount);
    toServer.setText("");

} else {
    //item cannot turned to double
    //inform user
    fromServer.setText("Invalid Request!"
            + USER_PROMPT);
}



private Double getDouble() {
    try {
        double d = Double.parseDouble(messageToServer);
        if(d >= 0) {
            return d;
        }
    } catch (NumberFormatException numfEx) {}
    return null;
}

Edit: to say if you are opposed to using null values you could also change the method to return a default negative value, say -1, and then change the condition to check if the double is negative instead of null.

Answer 3

This can be achieved without relying on exceptions to control the flow of your program, which is generally a bad idea - see discussion here

else {   
    // using a tempBidAmount as I don't have enough context to know whether
    // I could just assign the output of parseUserDouble directly to bidAmount
    // without causing issues
    Double tempBidAmount = parseUserDouble(messageToServer);

    if (tempBidAmount != null && tempBidAmount > 0.0) {
        bidAmount = tempBidAmount;
        output.println(selectedItemCode + bidAmount);
    } else {
        fromServer.setText("Invalid Request!" + USER_PROMPT);
    }
}
toServer.setText("");

....

private Double parseUserDouble(Sting userInput) {
    try {
        return Double.parseDouble(userInput);
    }
    catch (NumberFormatException numfEx) {
        // consider logging here        
    }
    return null;
}