我想从服务器下载pdf并存储在sdcard上.我尝试像下面的代码,但它不会进入其他条件,因为我没有创建一个文件,仍然会给MSG文件存在.为什么呢

String pdf;
String filenameWithExtension="";

protected void onCreate(Bundle savedInstanceState) {

        Intent intent=getIntent();
        pdf=intent.getStringExtra("pdfurl");
        String PATH2 = Environment.getExternalStorageDirectory()+"/pictures";
        Log.v("log_tag initial path","PATH: " + PATH2);
        File file2 = new File(PATH2);
        if(file2.exists()==true)
        {
        }else
        {   
            Log.v("directory is created","new  dir");
            file2.mkdir();
        }

        /**extracting the pdf filname from url**/
        int slashIndex = pdf.lastIndexOf('/');
        int dotIndex = pdf.lastIndexOf('.');
        filenameWithExtension=pdf.substring(slashIndex + 1);

        DownloadFile downloadFile = new DownloadFile(filenameWithExtension);
        downloadFile.execute(pdf);
}
private class DownloadFile extends AsyncTask<String,Integer,String>{

        String filename="";
        public DownloadFile(String _filename) {
            this.filename=_filename;

        }
        String PATH = Environment.getExternalStorageDirectory()+"/pictures/"+filename;
        File filecheck=new File(PATH);

        @Override
        protected String doInBackground(String... str) {

            Log.v("name of file",this.filename);
            if(filecheck.exists()){
                Log.v("in if condition","file is alredy exist");

            }else{
                Log.v("in else condition","file not present....");
                try {
                    URL url = new URL(str[0]);
                    HttpURLConnection c = (HttpURLConnection)url.openConnection();
                    c.setRequestMethod("GET");
                    c.setDoOutput(true);
                    c.connect();
                    // this will be useful so that you can show a typical 0-100% progress bar
                    int lenghtOfFile = c.getContentLength();

                    // downlod the file
                    // InputStream input = new BufferedInputStream(url.openStream());
                    //OutputStream output = new FileOutputStream("/sdcard/somewhere/nameofthefile.ext");
                    String PATH = Environment.getExternalStorageDirectory()+ "/pictures/";
                    Log.v("log_tag","PATH: " + PATH);
                    File file = new File(PATH);
                    Boolean check=filecheck.createNewFile();
                    Log.v("check file creation..::",check.toString());
                    if(file.mkdirs()==false)
                    {   
                        Log.v("file is alredy exist in sdcard","exist file");
                    }else
                    {   
                        Log.v("file is created in card","new  dir");
                        file.mkdirs();
                    }

                    File outputFile = new File(file,filenameWithExtension);
                    FileOutputStream fos = new FileOutputStream(outputFile);

                    InputStream is = c.getInputStream();
                    long total = 0;

                    byte[] buffer = new byte[1024];
                    int len1 = 0;
                    while ((len1 = is.read(buffer)) != -1) {
                        total +=len1;

                         publishProgress((int)(total*100/lenghtOfFile));

                         fos.write(buffer,len1);
                    }
                    fos.close();
                    is.close();
            } catch (IOException e) {
                Log.d("log_tag of background","Error: " + e);
                }
            }
            Log.v("In aync task filename extension",filenameWithExtension);

            return filenameWithExtension;
        }

您可以使用 File.exists()检查文件是否存在

File f = new File(filePathString);
   if(f.exists())
   { 
     /* do something */ 
   }
   else
   {
      file.mkdirs();
      //And your other stuffs goes here
   }

注意：exists()将为目录返回true

如果你想检查一个特定的文件是否存在,你必须使用File.isFile()

boolean fileExists =  new File("path/to/file.txt").isFile();

新文件(“C：/”).exists()将返回true,但不允许您作为文件打开和读取.

你的代码中的问题是你的文件名是空的.

编辑2：

尝试这个 ：

String filename="";
String PATH=""; 
File fileCheck=null;

public DownloadFile(String _filename) {
        this.filename=_filename;
        PATH = Environment.getExternalStorageDirectory()+"/pictures/"+filename;
        filecheck=new File(PATH);
   }

要么

或者在AsyncTask的onPreExecute()中放置这两个语句

PATH = Environment.getExternalStorageDirectory()+"/pictures/"+filename;
      filecheck=new File(PATH);

我是关于Try / Catch的新手.在下面的代码中,我有一个简单的测试来检查文件是否存在.在我的C#课程中,我必须使用Try / Catch,我不知道如何使用它,我是否仍然在Try部分内部使用if语句,或者是否有更好的方法来检查文件是否存在在里面试试？如果文件是简单的txt文件或序列化文件,有什么区别吗？

if (File.Exists("TextFile1.txt"))
{
   MessageBox.Show("The file don't exist!","Problems!",MessageBoxButtons.OK,MessageBoxIcon.Exclamation);
}

我必须使用Try / Catch方式

try
{
code to check if file exist here
}
catch
{
error message here
}

您已经检查了第一个代码段中是否存在文件.没有任何需要,因为此代码在try / catch块中.

#include<iostream>
#include "unistd.h"
#include "stdio.h"
#include "stdlib.h"

using namespace std;

int main()
{  
    if(access("234.bin",F_OK))
    {
       bool remove=true;
    }

    if(remove)
    {
        cout<strong>结果各种悲剧，无论这个文件是否存在：</strong>
<p><img  src="/inc/test.jsp?url=http%3A%2F%2Fmy.csdn.net%2Fuploads%2F201206%2F07%2F1339078989_9217.jpg&amp;refer=http%3A%2F%2Fblog.csdn.net%2Femaste_r%2Farticle%2Fdetails%2F7643479" alt="C++（五） access函数判断文件是否存在" ></p>
<p><br></p>
<p>事实上，我个人认为这个问题出在这个<ahref="https://www.php.cn/zt/16380.html" target="_blank">access</a>函数的返回值上，它的返回值是</p>
<p>0    如果文件是指定的mode<br></p>
<p>-1   如果出错</p>
<p>所以上述程序，无论是找到文件（0），还是找不到（-1），都是false，所以应该是永远都进不了if(remove)的。。<br></p>
<p>所以应该是：</p>
<p><span>立即学习</span>“<a href="https://pan.quark.cn/s/6e7abc4abb9f"rel="nofollow" target="_blank">C++免费学习笔记（深入）</a>”；</p>
<pre>if(0 == access("234.bin",F_OK))
{
remove = true;
}

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#include<iostream>
#include "unistd.h"
#include "stdio.h"
#include "stdlib.h"
using namespace std;

int file_exist(char *file)
{
    return (access(file,F_OK) == 0);
}
int main()
{
    cout<strong>总结：</strong>
<p>（一）用access函数注意返回值是 0 和-1，都是false<br></p>
<p>（二）remove是个函数名，定义命名的时候注意不要用到系统的东东<br></p>
<br></iostream>

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