Java中的运行时错误：所有控制台输入的一半记录在文本文件中

Question

package addlinenumbers;

import java.util.Scanner;
import java.io.PrintWriter;
import java.io.FileOutputStream;
import java.io.FileNotFoundException;
import java.io.IOException;

public class AddLineNumbers {

    public static void main(String[] args) 
    {

    Scanner input = new Scanner(System.in);
    int i=0;

        try (FileOutputStream fos = new FileOutputStream ("dataInput.txt", true)) {
            try (PrintWriter pw = new PrintWriter(fos)) {
                String userInput = " ";
                do {
                    System.out.print("Enter string to prepend to 'dataInput.txt'; -1 to EXIT: ");
                    userInput = input.nextLine();
                    if (!userInput.equals("-1")) {
                        pw.println(input.nextLine());
                        i++;
                    }
                } while(!(userInput.equals("-1")));
            }
        } catch (FileNotFoundException fnfe) {
            System.out.println("Unable to find dataInput.txt...");
        } catch (IOException ioe) {
            ioe.printStackTrace(System.out);
        } finally { 
                System.out.println("# of objects prepended: " + i);
                System.out.println("Closing file...");
        }
    }
}






****************************OUTPUT**********************************************
    run:
    Please enter strings to prepend to 'dataInput.txt'; -1 to EXIT: 
    David
    Goliath
    Please enter strings to prepend to 'dataInput.txt'; -1 to EXIT: 
    Delilah
    Samson
    Please enter strings to prepend to 'dataInput.txt'; -1 to EXIT: 
    -1
    # of objects prepended: 2
    Closing file...
    BUILD SUCCESSFUL (total time: 18 seconds)

正如你所看到的，它只包含了两个对象＆＃34; Goliath＆＃34;和＆＃34;参孙＆＃34;它们是写入文本文件的唯一字符串。从技术上讲，它应该有4个物体和＃34; David＆＃34;和＃34; Delilah＆＃34; 也应该在文本文件中，但它们不是。

非常感谢任何帮助。