如何修复 Android Studio 上的 NullPointerException？ [复制]

Question

我想在我的应用程序中使用文件选择器。它在我想将选定文本附加到视图的部分崩溃，特别是当我想为 TextView 形成 setOnClickListener 时。以下是应用程序的主要活动；

package de.kai_morich.simple_bluetooth_terminal;

import android.Manifest;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.net.Uri;
import android.os.Build;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
import androidx.fragment.app.FragmentManager;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;

import com.jjoe64.graphview.GraphView;
import com.jjoe64.graphview.series.DataPoint;
import com.jjoe64.graphview.series.LineGraphSeries;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.IOException;

public class MainActivity extends AppCompatActivity implements FragmentManager.OnBackStackChangedListener {

    private static final int PERMISSION_REQUEST_CODE = 100;
    Button read;
    TextView output;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        read = findViewById(R.id.read);
        output = findViewById(R.id.output);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportFragmentManager().addOnBackStackChangedListener(this);
        if (savedInstanceState == null)
            getSupportFragmentManager().beginTransaction().add(R.id.fragment, new DevicesFragment(), "devices").commit();
        else
            onBackStackChanged();
        read.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String state = Environment.getExternalStorageState();
                if (Environment.MEDIA_MOUNTED.equals(state)) {
                    if (Build.VERSION.SDK_INT >= 23) {
                        if (checkPermission()) {
                            File sdcard = Environment.getExternalStorageDirectory();
                            File dir = new File(sdcard.getAbsolutePath() + "/text/");
                            if(dir.exists()) {
                                File file = new File(dir, "sample.txt");
                                FileOutputStream os = null;
                                StringBuilder text = new StringBuilder();
                                try {
                                    BufferedReader br = new BufferedReader(new FileReader(file));
                                    String line;
                                    while ((line = br.readLine()) != null) {
                                        text.append(line);
                                        text.append('\n');
                                    }
                                    br.close();
                                } catch (IOException e) {
                                    //You'll need to add proper error handling here
                                }
                                output.setText(text);
                            }
                        } else {
                            requestPermission(); // Code for permission
                        }
                    } else {
                        File sdcard = Environment.getExternalStorageDirectory();
                        File dir = new File(sdcard.getAbsolutePath() + "/text/");
                        if(dir.exists()) {
                            File file = new File(dir, "sample.txt");
                            FileOutputStream os = null;
                            StringBuilder text = new StringBuilder();
                            try {
                                BufferedReader br = new BufferedReader(new FileReader(file));
                                String line;
                                while ((line = br.readLine()) != null) {
                                    text.append(line);
                                    text.append('\n');
                                }
                                br.close();
                            } catch (IOException e) {
                                //You'll need to add proper error handling here
                            }
                            output.setText(text);
                        }
                    }
                }
            }
        });
    }

    private boolean checkPermission() {
        int result = ContextCompat.checkSelfPermission(MainActivity.this, android.Manifest.permission.READ_EXTERNAL_STORAGE);
        if (result == PackageManager.PERMISSION_GRANTED) {
            return true;
        } else {
            return false;
        }
    }
    private void requestPermission() {
        if (ActivityCompat.shouldShowRequestPermissionRationale(MainActivity.this, android.Manifest.permission.READ_EXTERNAL_STORAGE)) {
            Toast.makeText(MainActivity.this, "Write External Storage permission allows us to read files. Please allow this permission in App Settings.", Toast.LENGTH_LONG).show();
        } else {
            ActivityCompat.requestPermissions(MainActivity.this, new String[]{android.Manifest.permission.READ_EXTERNAL_STORAGE}, PERMISSION_REQUEST_CODE);
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String permissions[], int[] grantResults) {
        switch (requestCode) {
            case PERMISSION_REQUEST_CODE:
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                Log.e("value", "Permission Granted, Now you can use local drive .");
            } else {
                Log.e("value", "Permission Denied, You cannot use local drive .");
            }
            break;
        }
    }


    @Override
    public void onBackStackChanged() {
        getSupportActionBar().setDisplayHomeAsUpEnabled(getSupportFragmentManager().getBackStackEntryCount()>0);
    }

    @Override
    public boolean onSupportNavigateUp() {
        onBackPressed();
        return true;
    }

}

以下是我收到的错误代码，我不确定如何修复 NullPointerException 错误；

2020-09-15 10:41:06.244 9578-9578/de.kai_morich.simple_bluetooth_terminal E/AndroidRuntime: FATAL EXCEPTION: main
    Process: de.kai_morich.simple_bluetooth_terminal, PID: 9578
    java.lang.RuntimeException: Unable to start activity ComponentInfo{de.kai_morich.simple_bluetooth_terminal/de.kai_morich.simple_bluetooth_terminal.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2750)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2811)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1528)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6316)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at de.kai_morich.simple_bluetooth_terminal.MainActivity.onCreate(MainActivity.java:57)
        at android.app.Activity.performCreate(Activity.java:6757)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2703)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2811) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1528) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6316) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762) 

Answer 1

您的堆栈跟踪表明您正在 null 对象上设置 setOnClickListener。您只在 read Button 上设置此侦听器一次，所以它是 null for shure

在onCreate 里面你有这些行：

@Override
protected void onCreate(Bundle savedInstanceState) {
    read = findViewById(R.id.read);
    output = findViewById(R.id.output);
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ...

前两行正在尝试findViewById，但此时根本没有视图，因为稍后会调用setContentView。你应该在第一行调用super（总是），在第二行调用setContentView（对于大多数情况）。设置内容（布局）后，您可能会寻找膨胀的Views

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

    read = findViewById(R.id.read);
    output = findViewById(R.id.output);
    ...

Answer 2

我认为在您的代码中 读取 = findViewById(R.id.buttonName) 不见了