检查Java中是否存在文件

Question

So the following program should take in an input and output file as command line arguments.

class FileCopy
{
public static void main(String[] args) throws IOException
{
    String infile = null;
    String outfile = null;
    BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));    

    if (args.length >= 2) //both files given via command line
    {
        infile = args[0];
        if (fileExists(infile) == false)
        {
            infile = getInputFile();
        }
        outfile = args[1];
    }
    else if (args.length == 1) //input file given via command line
    {
        infile = args[0];
        outfile = getOutputFile(infile);
    }
    else //no files given on command line
    {
        infile = getInputFile();
        outfile = getOutputFile(infile);
    }

    //create file objects to use
    File in = new File(infile);
    File out = new File(outfile);

    /*
     *rest of code
     */
}

//get the input file from the user if given file does not exist
public static String getInputFile() //throws IOException
{
    BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
    String fileName = null;
    boolean haveFile = false;

    while(haveFile == false)
    {
        System.out.println("Enter a valid filename for input:");
        System.out.print(">> ");
        try
        {
            fileName = stdin.readLine();
        }
        catch (IOException e)
        {
            System.out.println("Caught exception: " + e);
        }
        haveFile = fileExists(fileName);
    }

    return fileName;    
}

//get the output file and test things
public static String getOutputFile(String infile)
{
    BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
    File input = new File(infile);
    String filename = null;
    boolean more = true;
    while(more)
    {
        System.out.println("Enter a valid filename for output:");
        System.out.print(">> ");
        try
        {
            filename = stdin.readLine();
        }
        catch (IOException e)
        {
            System.out.println("Caught exception: " + e);
        }
        File output = new File(filename);
        if (output.exists())
        {
            more = false;
        }
        if (filename == infile)
        {
            int selection;
            String inputString = null;

            System.out.println("The output file given matches the input file. Please choose an option:");
            System.out.println("1) Enter new filename");
            System.out.println("2) Overwrite existing file");
            System.out.println("3) Backup existing file");
            System.out.print(">> ");
            try
            {
                inputString = stdin.readLine();
            }
            catch (IOException e)
            {
                System.out.println("Caught exception: " + e);
            }
            selection = Integer.valueOf(inputString);
            switch (selection)
            {
                case 1: //new filename
                case 2: //overwrite
                case 3: //backup
                default: System.exit(0);
            }
        }
    }
    return null;
}

//check the given file to see if it exists in the current working directory
public static boolean fileExists(String n)
{
    return (new File(n)).exists();
}
}

Answer 1

One detail that I believe you have missed:

When your program has only one argument ( args.length == 1 ), ie when only the input file is defined, fileExists() is not called at all; infile is set to args[0] with no validation at all. You should probably add a specific check as you have done for the two-argument case.

Answer 2

I've ran into a similar problem too. I was working under eclipse, and had to specify "src/file.txt" with my current directory having a file named "file" in the src directory.

Note: It was not named "file.txt" (this causes the string to be interpreted as "file.txt.txt"!).

Try testing against this program here assuming you have a file named "file" in your "src" directory:

import java.io.File;

public class FileChecker {

public static boolean Exists( String file ) 
{ 
    System.out.println("File being checked: " + file);
    return( (file.length()) > 0 && (new File(file).exists()) );
}

public static void main( String[] args ) 
{
    File dir = new File("src");

    System.out.println("List of files in source directory: ");
    if( dir.isDirectory()){
        File[] filenames = dir.listFiles();
        for( Object file : filenames ) {
            System.out.println(file.toString());
        }

    }
    else
        System.out.println("Directory does not exist.");

    if(FileChecker.Exists("src/file.txt"))
        System.out.println("File exists");
    else
        System.out.println("File does not exist");
}

}

It will print out the current files in source directory so you can see whether the file is really there or not, then you can test if it actually exists. Works on my end.