Java currencyConverter Java.lang.NumberFormatException:
我在这一行得到一个错误Java currencyConverter Java.lang.NumberFormatException:,java,Java,我在这一行得到一个错误double dollar=double.parseDouble(输入) 代码: /** Converts money using CurrencyConverter */ import java.util.*; public class CurrencyConverterTester { public static void main(String[] args) { //Normally the scanner
double dollar=double.parseDouble(输入)
代码:
/**
Converts money using CurrencyConverter
*/
import java.util.*;
public class CurrencyConverterTester {
public static void main(String[] args) {
//Normally the scanner is based on System.in,
//Scanner scanner = new Scanner(System.in) ;
//but for predictability we set the input to a fixed sequence:
Scanner scanner = new Scanner("0.79447 100 20 88.88 q");
System.out.print("Conversion factor (euros per dollar): ");
//We could use scanner.nextDouble() here, but this is an example
//of using parseDouble, which you need in the next loop.
String input = scanner.next();
double rate = Double.parseDouble(input);
System.out.println(rate);
//----------------Start below here. To do: approximate lines of code = 1
// 1. make the CurrencyConverter object based on the rate
CurrencyConverter converter = new CurrencyConverter(rate);
//----------------------End here. Please do not remove this comment. Reminder: no changes outside the todo regions.
System.out.print("Dollar value (Q to quit): ");
input = scanner.next();
System.out.println(input); //echo the input
//----------------Start below here. To do: approximate lines of code = 8
// 1. write a while loop where the condition is that input is not "Q" or "q" ; 2. use parseDouble to get the dollars amount ; 3. use the converter object to convert the dollars to euros ; 4. print the dollars and euros in the style shown in the Expected region (Hint: use printf); 5. prompt for the next input ; 6. read the next input ; 7. echo the input (i.e., print it out)
while (input != "q" && input != "Q") {
double dollar = Double.parseDouble(input);
double euros = converter.convert(dollar);
System.out.printf("%.2f dollars = %.2f euros\n", dollar, euros);
System.out.print("Dollar value (Q to quit): ");
input = scanner.next();
System.out.println(input);
}
}
}
Conversion factor(euros per dollar): 0.79447
Dollar value(Q to quit): 100
100.00 dollars = 79.45 euros
Dollar value(Q to quit): 20
20.00 dollars = 15.89 euros
Dollar value(Q to quit): 88.88
88.88 dollars = 70.61 euros
Dollar value(Q to quit): q
输入是一个字符串,因此必须使用.equals()
,而不是=
。相等运算符==
确定两个引用是否指向同一对象。其中,.equals()
方法检查它们在逻辑上是否相等
您还希望使用|
而不是&&
。更好的方法是对字符串输入使用相等运算符=
。对于字符串,必须使用其.equals()
方法
因此:
while (input != "q" && input != "Q")
应该是:
while (!(input.equals("q") || input.equals("Q")))
或
使用scanner.next()
时也要小心,最好使用scanner.nextLine()
否则会在输入变量中获得换行符。因此,当您键入“q”时,输入变量可能是String input=“q\n”代码>不要将字符串值与=
或进行比较=代码>。改用
您使用==
所做的是比较对象地址。这不是您想要的,因为您在内存中的某个地方有一个字符串对象,其中包含“q”
,而另一个对象则包含“q”
,位于内存中的另一个位置。它们有不同的地址,这导致使用==
进行比较返回false,而内容在文本上是相同的。!(…)
可能不起作用…“q”。相等信号转移(输入)
将降低空点异常的风险。。。但我只是吹毛求疵…,而(!(cond))
会更好。
while (!(input.equals("q") || input.equals("Q")))
while (!"q".equalsIgnoreCase(input)) // While input not q or Q