Android - openFileInput() NullPointerException

Question

I'm trying to generate a code as string in my android application and save it to a text file. When you press a Share button, the code should be read from the text file and be copied to the clipboard.

I guess I'm not understanding something about acivity/context. Of course I read in the documentation about it, but I can't figure out the problem. Hope you guys could help me!

FileSystem.java

package com.niclasjaksch.shoppinglist;

import android.content.Context;

import androidx.appcompat.app.AppCompatActivity;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

class FileSystem extends AppCompatActivity {

    static String filename = "shoppinglist_code.txt";

    void Create() {
        try {
            FileOutputStream fOut = openFileOutput(filename, Context.MODE_PRIVATE);
            String str = Utility.randomString(10);
            fOut.write(str.getBytes());
            fOut.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    String Read() {
        String result = "";
        try {
            FileInputStream fin = openFileInput(filename);
            int c;
            while( (c = fin.read()) != -1){
                result += c;
            }
            fin.close();
        } catch (Exception e) {
            System.out.println("An error occurred.");
            e.printStackTrace();
        }
        return result;
    }
    void Clear() {
        try {
            File dir = getFilesDir();
            File file = new File(dir, filename);
            file.delete();
        } catch (Exception e) {
            System.out.println("An error occurred.");
            e.printStackTrace();
        }
    }

}

Share.java

package com.niclasjaksch.shoppinglist;

import android.content.ClipData;

import androidx.appcompat.app.AppCompatActivity;

class Share extends AppCompatActivity {

    android.content.ClipboardManager clipboardManager = null;
    FileSystem fs = new FileSystem();

    void Share() {
        String code = fs.Read();
        if(code == "" || code == null) {
            fs.Create();
            code = fs.Read();
        }
        ClipData clipData = ClipData.newPlainText("Source Text", code);
        clipboardManager.setPrimaryClip(clipData);
    }
}

MainActivity.java

    ...

    Share share = new Share();

    ...

    @Override
    public boolean onOptionsItemSelected(@NonNull MenuItem item) {
        switch (item.getItemId()) {
            case R.id.item_share:
                share.Share();
                return true;
            default:
                return super.onOptionsItemSelected(item);
        }
    }

Getting a NullPointerException in this line:

FileOutputStream fOut = openFileOutput(filename, Context.MODE_PRIVATE);

Please share the logcat as well so that we can understand which variable is assigned to null. — Reaz Murshed, Jul 22 '19 at 17:25

score 0 · Accepted Answer · answered Jul 22 '19 at 17:26

Only create subclasses of AppCompatActivity for actual UI work, where you will start that activity using startActivity() or startActivityForResult().

Move those methods to a real activity, such as MainActivity, and get rid of the Share and FileSystem classes.

score -1 · Answer 2 · answered Jul 22 '19 at 17:27

First things first, you shouldn't extend AppCompatActivity for FileSystem and Share.

What's happening is that the openFileOutput is taking the FileSystem object (this) as it's context. When in reality it wouldn't count as a separate activity.

You could either do as CommonWare suggest and remove the classes FileSystem and Share, and move their functions to the MainActivity. Or you could explicitly pass the context from the MainActivity.

Android - openFileInput() NullPointerException

2 Answers2