线程“主”java.lang.IllegalArgumentException 中的异常：无法将 Object 格式化为数字

Question

I need to display 2 decimal point but I don't get it how to do It.

Exception in thread "main" java.lang.IllegalArgumentException: Cannot format given Object as a Number

import java.text.DecimalFormat;
import java.util.Scanner;
public class ShoppingOrders {
    public static void main(String[] args) {
        double itemPrice, shippingFeeRate, shippingFee,totalCost;
        String nameItem;
        Scanner myScanner= new Scanner (System.in);
        DecimalFormat df = new DecimalFormat("#.00");
        
        nameItem = myScanner.nextLine();
        itemPrice = myScanner.nextDouble();
        shippingFeeRate = myScanner.nextDouble();
        
        shippingFee =shippingFeeRate*itemPrice;
        totalCost = shippingFee+itemPrice;
        
        System.out.println ("Costs for "+nameItem);
        System.out.println (df.format("Item Price: $" +itemPrice));
        System.out.println (df.format("Shipping Fee: $"+ shippingFee));
        System.out.println (df.format("Total Cost: $"+totalCost));
    }
}

Answer 1

You are using DecimalFormat.format(...) incorrectly. You don't pass it a String . You pass it an object that represents a number. (Or a primitive number type which will be boxed to an object that represents a number.)

For example:

df.format(itemPrice)    // itemPrice will be boxed to a Double

So if you want to include a message in the output, you might do this:

System.out.println("Item Price: $" + df.format(itemPrice));

Alternatively, you could use String.format(...) and specify the number of digits after the decimal point in the format specifier.

Answer 2

DecimalFormat.format(...) is for formatting a number. In your case, you added a String to the double, which turns it into a String , which DecimalFormat.format(...) doesn't support. Instead, use:

import java.text.DecimalFormat;
import java.util.Scanner;
public class Main{
public static void main(String[] args) {
    double itemPrice, shippingFeeRate, shippingFee,totalCost;
    String nameItem;
    Scanner myScanner= new Scanner (System.in);
    DecimalFormat df = new DecimalFormat("#.00");
    
    nameItem = myScanner.nextLine();
    itemPrice = myScanner.nextDouble();
    shippingFeeRate = myScanner.nextDouble();
    
    shippingFee =shippingFeeRate*itemPrice;
    totalCost = shippingFee+itemPrice;
    
    System.out.println ("Costs for "+nameItem);
    System.out.println ("Item Price: $" +df.format(itemPrice));
    System.out.println ("Shipping Fee: $"+ df.format(shippingFee));
    System.out.println ("Total Cost: $"+df.format(totalCost));
}
}