Question

我正在使用Eclipse上的Java程序。该程序从用户获取“txt”文件名，并打印出该“txt”文件中的行数（“文件中有”x“行”）。如果“txt”文件不存在，则应打印出“该文件不存在”

我试图让程序继续循环，询问文件名，无论最后一个条目是否导致错误，直到用户输入“完成”。

这是我正在编辑的代码。共有2个班级。这是第一个（ProcessFile.java）：

// ProcessFile.java

import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Scanner;

/**
 * 
 * This class will ask the user for a text file
 * name, open that file, and print all the lines
 * of the file and how many lines there are.
 * 
 *
 */

public class ProcessFile {

/**
 * This method prompts the user to enter a
 * file name and returns it.
 * 
 * @return A String containing a file name.
 */
public static String getFileName()
{
    Scanner in = new Scanner( System.in );
    String fileName;
    FileReader reader;

    do
    {
        System.out.println( "Please enter a file name to open: " );
        fileName = in.nextLine();

        try
        {
            reader = new FileReader( fileName );
        }
        catch( FileNotFoundException e )
        {
            System.out.println( "That file does not exist." );
            reader = null;
        }
    }
    while ( reader == null );

    in.close();

    try
    {
        reader.close();
    }
        catch ( IOException e )
    {
            System.out.println( e );
    }

    return fileName;    
}

/**
 * This method takes an ArrayList of Strings and prints each
 * element of the ArrayList, one per line, as well as the
 * number of items in the ArrayList.
 * 
 * @param lines
 */
public static void printInformation( ArrayList<String> lines )
{
    for ( String line : lines )
        System.out.println( line );

    System.out.println( "There are " + lines.size() + " lines in the file." );
}

public static void main( String[] args ) 
{
    String fileName;
    FileManager fileInfo;
    ArrayList<String> lines = new ArrayList<String>();

    fileName = getFileName( );

    fileInfo = new FileManager( fileName );

    try 
    {
        lines = fileInfo.readLines();
    }
    catch( FileNotFoundException e ) 
    {
        System.out.println( e );
    }   

    printInformation( lines );
}
}

这是第二个类（FileManager.java）：

// FileManager.java

import java.io.FileNotFoundException;
import java.io.FileReader;
import java.util.ArrayList;
import java.util.Scanner;

/**
 * This class will manage the interaction with the
 * file for the ProcessFile class.
 * 
 *
 */
public class FileManager 
{
    private String fileName;

    public FileManager( String file )
    {
        fileName = file;
    }

    /**
     * This function will read the file stored in 
     * fileName and return an ArrayList made up of
     * the lines of the file.
     * 
     * @return An ArrayList containing the file's lines.
     */
    public ArrayList<String> readLines( ) throws FileNotFoundException
    {
        ArrayList<String> lines = new ArrayList<String>();
        FileReader fileIn = new FileReader( fileName );
        Scanner in = new Scanner( fileIn );

        while ( in.hasNextLine() )
        {
            String line = in.nextLine();
            lines.add( line );
        }

        in.close();
        return lines;
    }
}

对于“如果用户键入'完成'，结束程序”部分，我查找了一些内容并将其包含在下面的代码中。不确定它是否正确，但是当我输入该部分时我没有收到任何错误。这是我所做的更改（我根据我更改的部分添加了注释）：

import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Scanner;

/**
 * 
 * This class will ask the user for a text file
 * name, open that file, and print all the lines
 * of the file and how many lines there are.
 * 
 *
 */
public class ProcessFile {

 /**
  * This method prompts the user to enter a
  * file name and returns it.
  * 
  * @return A String containing a file name.
  */
 public static String getFileName() // Error: This method must return a result of type String
 {
     Scanner in = new Scanner( System.in );
     String fileName;
     FileReader reader;
     int x = 1;

     if (System.in.equals("Done") || System.in.equals("done")) // This is the part I wasn't sure of (the one I said I looked up)
     {
         System.exit(0);
     }

     else 
     {
         while (x == 1)
         {
             System.out.println( "Please enter a file name to open: " );
             fileName = in.nextLine();

             try
             {
                 reader = new FileReader( fileName );
             }
             catch( FileNotFoundException e )
             {
                 System.out.println( "That file does not exist." );
             }


             in.close();

             try
             {
                 reader.close(); //Error: The local variable reader may not have been initialized
             }
                 catch ( IOException e )
             {
                     System.out.println( e );   
             }
             return fileName;
         }
     }
 }

 /**
  * This method takes an ArrayList of Strings and prints each
  * element of the ArrayList, one per line, as well as the
  * number of items in the ArrayList.
  * 
  * @param lines
  */
 public static void printInformation( ArrayList<String> lines )
 {
     for ( String line : lines )
         System.out.println( line );

     System.out.println( "There are " + lines.size() + " lines in the file." );
 }

 public static void main( String[] args ) 
 {
     String fileName;
     FileManager fileInfo;
     ArrayList<String> lines = new ArrayList<String>();

     fileName = getFileName( );

     fileInfo = new FileManager( fileName );

     try 
     {
         lines = fileInfo.readLines();
     }
     catch( FileNotFoundException e ) 
     {
         System.out.println( e );
     }  

     printInformation( lines );
     getFileName(); // <--- Return to the top and get the user input again.
 }
}

我认为我接近这个。任何一点帮助将不胜感激。非常感谢。

**编辑代码（@μTheory）

public static String getFileName()
{
    Scanner in = new Scanner( System.in );
    String fileName;
    FileReader reader;


    do
    {
        System.out.println( "Please enter a file name to open: " );
        fileName = in.nextLine();

        if ( in.equals("Done") || in.equals("done") )
        {
            in.close();
            System.exit(0);
        }

        else
        {
            try
            {
                reader = new FileReader( fileName ); //Error: The local variable reader may not have been initialized.
            }
            catch( FileNotFoundException e )
            {
                System.out.println( "That file does not exist." );
            }
        }

        in.close();

        try
        {
            reader.close();
        }
            catch ( IOException e )
        {
            System.out.println( e );    
        }
        return fileName;
    }
    while ( reader == null );
}

Answer 1

首先，System.in引用InputStream，因此您对System.in.equals("Done");的调用会尝试比较String和InputStream，并且显然会返回假。而是在fileName = in.nextLine();语句之前调用if，然后检查是否filename.equals("Done")。并将if语句放在while循环中。

现在你已经创建了一个无限循环：while ( x == 1)永远不会停止，因为你实例化x=1并且你永远不会改变循环中的值。

我建议您在之前编程时将循环更改为while( reader == null)。并取出while声明并将其放在行

之后

         in.close();

         try
         {
             reader.close();//No more error
         }
             catch ( IOException e )
         {
                 System.out.println( e );   
         }
         return fileName;

为什么？因为，当reader为null时，您可以不关闭允许您在循环的每次迭代中调用Scanner的{{1}}。当你想要在fileName = in.nextLine();不为空时结束循环时，你可以不调用reader，因为根据定义，你的reader.close();对象将为null并抛出一个reader。

在致电NullPointerException之前，请先考虑关闭所有开放流，例如System.exit(0);。

修改

Scanner

所以这是正确的代码。您仍然不明白，public static String getFileName() { Scanner in = new Scanner( System.in ); String fileName; FileReader reader; do { System.out.println( "Please enter a file name to open: " ); fileName = in.nextLine(); if ( fileName.equals("Done") || fileName.equals("done") ) { in.close(); System.exit(0); } else { try { reader = new FileReader( fileName ); //Error: The local variable reader may not have been initialized. } catch( FileNotFoundException e ) { System.out.println( "That file does not exist." ); } } } while ( reader == null ); in.close(); try { reader.close(); } catch ( IOException e ) { System.out.println( e ); } return fileName; }您试图将in.equals("Done") obejct（in和Scanner的实例进行比较，如上所述，显然是假的。因此，我将String替换为in代表用户输入的行。

然后我提取了块：

fileNmae

在in.close(); try { reader.close(); } catch ( IOException e ) { System.out.println( e ); } return fileName;循环之外，如上所述。如果您仍在使用它们或者尚未实现它们，则无法关闭流。

Answer 2

您需要从Scanner获取用户输入并将其与“完成”进行比较，而不是System.in本身。 System.in只是流，而非实际输入，您需要从该流读取才能进行比较。

Scanner s = new Scanner(System.in)

String inp = s.nextLine()

if(inp.equals("Done"))
{
    //Code
}

而且，正如@DeiAndrei在他的回答中指出的那样，你可以使用equalsIgnoreCase来比较它而不区分大小写。忘记了这一点，并为了完整性而添加了这个。

Answer 3

您应该更换您不确定的部分：

if(in.nextLine().equalsIgnoreCase("done")){

    System.exit(1);
}

您可以使用equalsIgnoreCase方法进行不区分大小写的比较。要从控制台读取，只需使用Scanner.nextLine（）或您认为适合Scanner类的任何其他方法。

如果您不想接受如下输入：

"DONE", "DoNe", etc

然后只需将输入字符串与

进行比较

"Done" and "done"

和以前一样。

当键入“完成”一词时，尝试让程序结束

3 个答案: