E/AndroidRuntime：致命例外：请主要

Question

First app I am trying to create. Just to enter values and store these values in to a file. I have been trying to figure it out but having issues. I would love some advise or a solution to my issues.

The coding seems correct but app crashes as soon as I open.

If you can explain to me where my error is or better ways of executing what I am trying to achieve I would appreciate it.

I am also not sure how to get the values as specifics eg weight in kg, bloodsugar as mmol/L etc.

package com.example.mybodyrecord;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class MainActivity extends AppCompatActivity {

    String myBodyRecord = "My_Body_Record.txt";
    Date date = new Date();
    SimpleDateFormat formatter = new SimpleDateFormat("dd-MM-yyyy HH:mm");
    File file = new File(getApplicationContext().getFilesDir(),myBodyRecord);

    Button button;

    EditText weightInput;
    EditText ketoneBloodInput;
    EditText ketoneUrineInput;
    EditText bloodPressureInput;
    EditText bloodSugarInput;

    String weight;
    String ketoneBlood;
    String ketoneUrine;
    String bloodPressure;
    String bloodSugar;


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

….
    }
}

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.mybodyrecord, PID: 26820
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.mybodyrecord/com.example.mybodyrecord.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2843)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
    at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:116)
    at com.example.mybodyrecord.MainActivity.<init>(MainActivity.java:22)
    at java.lang.Class.newInstance(Native Method)
    at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:69)
    at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:43)
    at android.app.Instrumentation.newActivity(Instrumentation.java:1215)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2831)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loop(Looper.java:193) 
    at android.app.ActivityThread.main(ActivityThread.java:6669) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(Run

Answer 1

Put this line in onCreate

File file = new File(getApplicationContext().getFilesDir(), myBodyRecord);

and replace getApplicationContext with this like so:

File file = new File(this.getFilesDir(), myBodyRecord);

Your error is caused by getApplicationContext returning null.

public class MainActivity extends AppCompatActivity {

    String myBodyRecord = "My_Body_Record.txt";
    Date date = new Date();
    SimpleDateFormat formatter = new SimpleDateFormat("dd-MM-yyyy HH:mm");


    Button button;

    EditText weightInput;
    EditText ketoneBloodInput;
    EditText ketoneUrineInput;
    EditText bloodPressureInput;
    EditText bloodSugarInput;

    String weight;
    String ketoneBlood;
    String ketoneUrine;
    String bloodPressure;
    String bloodSugar;


    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        File file = new File(getApplicationContext().getFilesDir(),myBodyRecord);


        //Setting where to get the data from, ie what fields
        weightInput = findViewById(R.id.weight);

        ....................
    }
}

Answer 2

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
    at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:116)
    at com.example.mybodyrecord.MainActivity.<init>(MainActivity.java:22)

The above three lines tells you what is the problem and where it is.

Problem: The problem is you are using something that is not initialized yet means null . In your case getApplicationContext() is responsible for this exception means when you call it gives you a null reference.

Where: com.example.mybodyrecord.MainActivity.<init>(MainActivity.java:22) this line tells you where does your exception occurs. It clearly says that the exception is in your MainActivity line number 22 .

Cause: Before onCreate getApplicationContext is always null .

Solution: You should initialize your file inside the onCreate() like the following.

public class MainActivity extends AppCompatActivity {

    String myBodyRecord = "My_Body_Record.txt";
    Date date = new Date();
    SimpleDateFormat formatter = new SimpleDateFormat("dd-MM-yyyy HH:mm");
    File file; // decleare here and initialize inside onCreate.

    Button button;

    EditText weightInput;
    EditText ketoneBloodInput;
    EditText ketoneUrineInput;
    EditText bloodPressureInput;
    EditText bloodSugarInput;

    String weight;
    String ketoneBlood;
    String ketoneUrine;
    String bloodPressure;
    String bloodSugar;


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

        //activity has a context now you can use getApplicationContext or this
        file = new File(this.getFilesDir(),myBodyRecord); // here you should initialize your file

        //Setting where to get the data from, ie what fields
        weightInput = findViewById(R.id.weight);
        ketoneBloodInput = findViewById(R.id.ketoneblood);
        ketoneUrineInput = findViewById(R.id.ketoneurine);
        bloodPressureInput = findViewById(R.id.bp);
        bloodSugarInput = findViewById(R.id.bslvl);

        button = findViewById(R.id.submit);

        // ......
}