Java arraylist accessing an element to calculate average

Question

I am writing a program that reads numbers from a file and calculates the average of them. However, I am getting the error shown below at run-time. I tried to fix it using the casting to double as shown in the code but it did not help.

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)

Code:

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();
        }
    }
}

M A · Accepted Answer · 2014-11-18 16:58:12Z

4

The list numbers contains String objects, so casting the elements to double will throw an exception. You could have avoided this runtime exception if you had used the generic type ArrayList properly:

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

This would allow the compiler to detect ahead of time the type incompatibility.

To read the double value of the string, you can use Double#parseDouble(String):

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

Alternatively you can change the type parameter of numbers to Double and convert the value when adding the element to the list:

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

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

answered Nov 18, 2014 at 16:58

M A

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