Java android studio FileNotFoundException:（无此类文件或目录）

有人能告诉我为什么会出现这个错误，我应该把

old.txt

文件放在哪里，让代码看到吗？有什么解决办法吗

 W/System.err: java.io.FileNotFoundException: old.txt (No such file or directory)
W/System.err:     at java.io.FileInputStream.open(Native Method)
  W/System.err:     at java.io.FileInputStream.<init>(FileInputStream.java:146)
 W/System.err:     at java.io.FileInputStream.<init>(FileInputStream.java:99)
  W/System.err:     at java.io.FileReader.<init>(FileReader.java:58)
   W/System.err:     at FileUtils.readFileAsString(FileUtils.java:23)
   W/System.err:     at MainActivity.onclick(MainActivity.java:33)
    W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
   W/System.err:     at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
   W/System.err:     at android.view.View.performClick(View.java:5637)
    W/System.err:     at android.view.View$PerformClick.run(View.java:22429)
   W/System.err:     at android.os.Handler.handleCallback(Handler.java:751)
  W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:95)
   W/System.err:     at android.os.Looper.loop(Looper.java:154)
    W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:6121)
   W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
    W/System.err:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889)
W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779)

java

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;


public class FileUtils {

    /**
     * Opens and reads a file, and returns the contents as one String.
     */
    public static String readFileAsString(String filename)
            throws IOException
    {
        BufferedReader reader = new BufferedReader(new FileReader(filename));
        String line;
        StringBuilder sb = new StringBuilder();
        while ((line = reader.readLine()) != null)
        {
            sb.append(line + "\n");
        }
        reader.close();
        return sb.toString();
    }


    /**
     * Save the given text to the given filename.
     * @param canonicalFilename Like /Users/al/foo/bar.txt
     * @param text All the text you want to save to the file as one String.
     * @throws IOException
     */
    public static void writeFile(String canonicalFilename, String text)
            throws IOException
    {
        File file = new File (canonicalFilename);
        BufferedWriter out = new BufferedWriter(new FileWriter(file));
        out.write(text);
        out.close();
    }

}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

<Button
    android:id="@+id/display_button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginLeft="@dimen/activity_vertical_margin"
    android:text="Display"
    android:onClick="onclick"

    />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Result: "
    android:layout_marginTop="32dp"
    android:layout_marginLeft="@dimen/activity_vertical_margin"/>

<TextView
    android:id="@+id/display"
    android:layout_width="match_parent"
    android:layout_height="64dp"
    android:layout_marginLeft="@dimen/activity_horizontal_margin"
    android:layout_marginRight="@dimen/activity_horizontal_margin"
    android:text=""
    android:background="#e3e3e3"
    style="@style/text_size" />

</LinearLayout>

---

您可以从下面的资产文件夹中读取.txt文件。您可以将文件写入设备存储

---

P/s：无法将文件保存到资产文件夹中

请使用完整路径，而不仅仅是文件名。并知道old.txt的位置。@greenapps我已将该文件放在MainActivity的同一文件夹中。java@greenapps路径的开始和结束应该是什么？如果它与MainActivity.java位于同一文件夹中，那么它仍然存在：在您的PC上。并且由于您的应用程序在您的Android设备上很远，它当然找不到该文件。因此，请将改为将文件保存在资产文件夹中。然后更改您的Android代码以从资产读取文件。只需通过谷歌从资产中读取文件。我这样做了，但在路径/data/user/0/packageName/files/中找不到新的.txt文件。您应该告诉我们您是如何找到该文件的。使用文件浏览器应用程序确实是不可能的@阿曼尼。

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

<Button
    android:id="@+id/display_button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginLeft="@dimen/activity_vertical_margin"
    android:text="Display"
    android:onClick="onclick"

    />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Result: "
    android:layout_marginTop="32dp"
    android:layout_marginLeft="@dimen/activity_vertical_margin"/>

<TextView
    android:id="@+id/display"
    android:layout_width="match_parent"
    android:layout_height="64dp"
    android:layout_marginLeft="@dimen/activity_horizontal_margin"
    android:layout_marginRight="@dimen/activity_horizontal_margin"
    android:text=""
    android:background="#e3e3e3"
    style="@style/text_size" />

</LinearLayout>