Question

我正在编写一个程序，从文件中读取数字并计算它们的平均值。但是，我在运行时收到下面显示的错误。我尝试使用转换来修复它，如代码中所示加倍，但它没有帮助。

java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Double
    at ReadFile.main(ReadFile.java:34)
    at __SHELL40.run(__SHELL40.java:6)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at bluej.runtime.ExecServer$3.run(ExecServer.java:774)
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Double
    at ReadFile.main(ReadFile.java:34)
    at __SHELL41.run(__SHELL41.java:6)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at bluej.runtime.ExecServer$3.run(ExecServer.java:774)
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Double
    at ReadFile.main(ReadFile.java:33)
    at __SHELL42.run(__SHELL42.java:6)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at bluej.runtime.ExecServer$3.run(ExecServer.java:774)
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Double
    at ReadFile.main(ReadFile.java:33)
    at __SHELL43.run(__SHELL43.java:6)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at bluej.runtime.ExecServer$3.run(ExecServer.java:774)

代码：

import java.io.*;
import java.util.Scanner;
import java.util.ArrayList;

public class ReadFile 
{
    public static void main(String[] args) 
    {
        try 
        {
            Scanner input = new Scanner("n.txt");
            File file = new File(input.nextLine());
            input = new Scanner(file);
            ArrayList numbers = new ArrayList();

            int index=1;

            while (input.hasNextLine()) {
                String line = input.nextLine();;
                numbers.add(line);
                System.out.println(index + " : " + line);
                index++;
            }
            input.close();

            double sum = 0.0;
            double average;
            for (int j = 0; j < numbers.size(); j++)  
            {
                sum = sum + (double) numbers.get(j) ;  //My guess is that this line is invoking the error

            }
            average = sum/numbers.size();
            System.out.println(average);
        } 
        catch (Exception ex) 
        {
            ex.printStackTrace();
        }
    }
}

Answer 1

列表numbers包含String个对象，因此将元素强制转换为double会引发异常。如果您已正确使用泛型类型ArrayList，则可以避免此运行时异常：

ArrayList<String> numbers = new ArrayList<String>();

这将允许编译器提前检测到类型不兼容。

要阅读字符串的double值，您可以使用Double#parseDouble(String)：

 sum = sum + Double.parseDouble(numbers.get(j));

或者，您可以将numbers的类型参数更改为Double，并在将该元素添加到列表时转换该值：

ArrayList<Double> numbers = new ArrayList<Double>();

...
numbers.add(Double.parseDouble(line));

Java arraylist访问元素以计算平均值

1 个答案: