尝试在 Android 活动中执行简单命令时出错

Question

I am learning to read and write simple files using Android apps written in Java. But I am not able to resolve this initial error! I think I am getting this error because of this in place of context. The app is compiling successfully but it is not opening after installing in my device.

I have not tried much but I am providing the code here. I am trying to accept a simple text through edit text view and then saving it to a file in Android using the button save.

package com.example.filemaketest;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Context;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;

import java.io.File;
import java.io.FileOutputStream;

public class MainActivity extends AppCompatActivity {
    String filename = "Testing-app-file.txt";
    File path = this.getFilesDir();

    //    File file = new File(path, filename);
//    FileOutputStream outputStream;
    public void save(View view) {
        EditText edit = (EditText) findViewById(R.id.infoText);
        String info = edit.getText().toString();
        Log.i("info", info);
        Toast.makeText(this, info + " button Pressed", Toast.LENGTH_LONG).show();
        Toast.makeText(this, " Saving", Toast.LENGTH_LONG).show();
//        try {
//            outputStream = openFileOutput(filename, this.MODE_PRIVATE);
//            outputStream.write(info.getBytes());
//            outputStream.close();
//        } catch (Exception e) {
//            e.printStackTrace();
//        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

Answer 1

The first thing that springs out to me is this line:

public class MainActivity extends AppCompatActivity {
    String filename ="Testing-app-file.txt";
    File path= this.getFilesDir();           // <<< This line

You are defining path inline within the class definition, which is equivalent to setting it within the MainActivity() constructor. This is before the Activity Lifecycle has begun, therefore this as a Context does not exist yet. You will need to define path later on in the activity lifecycle, for example during onCreate() :

public class MainActivity extends AppCompatActivity {
    String filename ="Testing-app-file.txt";
    File path;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        path = this.getFilesDir();
    }

    ...