扫描仪无法正常工作

Question

Hey everyone i am working on a personal project with java (I am still currently learning the language) and have come here with a question or two.

First Question

In my try block i am allowed to ask the user to input a value to be assigned to a variable and in my catch block, if the user inputs a wrong value or skips the entry, it throws an exception and informs the user that a default value has been set. This is a correct way to do this yes?

Code Example

(Note: I have many of these try/catch statements working the same way as the one below inside a method.)

 try {
        System.out.print("Please enter a value for price of computer: ");
        price = sc.nextFloat();

    } catch (InputMismatchException ex) {    //Catch skip or error
        System.out.println("You entered an invalid value for price, it is assumed to be 0.");
        price = 0F;   //Assign a default value
    }

Second Question

When I compile and execue this it doesn't allow me to enter a variable, but instead skips right to the exception. I am not sure what i am missing, so I would love any advice and thanks again to a great community.

Answer 1

Working code:

import java.util.*;

public class InputTest{
    public static void main(String args[]) {
        float price = -1F;
        try {
            Scanner sc = new Scanner(System.in);
            System.out.println("Please enter a value for price of computer: ");
            if (sc.hasNextFloat()){
                price = sc.nextFloat();
            }
        } catch (InputMismatchException ex) {    //Catch skip or error
            System.out.println("You entered an invalid value for price, it is assumed to be 0.");
            price = 0F;   //Assign a default value
        }catch(Exception err){
            err.printStackTrace();
        }
        System.out.println("price:"+price);

    }
}

output:

D:\Study\Java>java InputTest
Please enter a value for price of computer:
1
price:1.0

D:\Study\Java>java InputTest
Please enter a value for price of computer:
2
price:2.0

D:\Study\Java>java InputTest
Please enter a value for price of computer:
3.3
price:3.3

Change from your code snippet:

I have added sc.hasNextFloat() before assigning value to float variable. Do not enter f or F in input value when you are reading value from console. I have provided sample input value for better understanding.

Answer 2

This code looks correct for the most part. Although, I am assuming your sc variable is the same Scanner Object through multiple inputs (as you stated), so its buffer may not be cleared as you would assume each time you use it by using the sc.nextFloat() method.

You can avoid this by reinitializing the Scanner object before you use it again. If I recall correctly, there is no way to clear the buffer of a Scanner Object explicitly, so it would have to be reinitialized to "clear" it.

You can also use the sc.hasNextFloat() to verify a float value has been entered.

You can also use and Float.parseFloat(sc.nextLine()) to capture the entire input before a new line is entered.

Answer 3

Okay, i have found my error thanks to everyone's help here at stackoverflow.

When i initialized my Scanner variable i did it like this: Scanner sc = new Scanner("System.in")

Which meant i wanted it to read a string and not the input from the user so it would automatically throw my exception.

To fix this i removed the quotes and it is now working properly!

Thanks again to everyone!