我的应用程序一切正常，但是当我按下“后退”按钮（我的意思是退出应用程序）时，应用程序崩溃了。我不知道为什么？我尝试了几种解决方案，但没有任何效果。这是一个声音/铃声应用程序。
我是Android的新手，我需要一些建议，例如“删除此” /“添加此”
这是我的代码
package com.app.trying;
import android.content.Intent;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.os.Environment;
import android.support.design.widget.NavigationView;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.MenuItem;
import android.widget.Toast;
import com.app.trying.tabs.Tab3;
import com.app.trying.tabs.Tab1;
import com.app.trying.tabs.Tab2;
import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.AdView;
import java.io.File;
public class MainActivity extends AppCompatActivity {
public MediaPlayer mp;
DrawerLayout mDrawerLayout;
NavigationView mNavigationView;
FragmentManager mFragmentManager;
FragmentTransaction mFragmentTransaction;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Banner Ad
AdView mAdView = (AdView) findViewById(R.id.adView);
AdRequest adRequest = new AdRequest.Builder().build();
mAdView.loadAd(adRequest);
toolbarTabs();
sidebar();
externalStorageAccess();
}
// Creates sidebar and sets onClickListeners
public void sidebar(){
mNavigationView = (NavigationView) findViewById(R.id.navigationView);
mNavigationView.setItemIconTintList(null);
mNavigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
@Override
public boolean onNavigationItemSelected(MenuItem menuItem) {
mDrawerLayout.closeDrawers();
switch (menuItem.getItemId()){
case R.id.sounds:
FragmentTransaction xfragmentTransaction = mFragmentManager.beginTransaction();
xfragmentTransaction.replace(R.id.containerView,new TabFragment()).commit();
break;
case R.id.share:
Intent shareIntent = new Intent(Intent.ACTION_SEND);
shareIntent.setType("text/plain");
shareIntent.putExtra(Intent.EXTRA_SUBJECT, getText(R.string.app_name));
shareIntent.putExtra(Intent.EXTRA_TEXT, getText(R.string.share_text) + " " + getText(R.string.app_name) + "\n\n" + getText(R.string.playstore_link));
startActivity(Intent.createChooser(shareIntent, getText(R.string.share_via)));
break;
}
return false;
}
});
}
// Is listening which sound on Tab1 has been clicked
public void TabOneItemClicked(int position) {
cleanUpMediaPlayer();
mp=MediaPlayer.create(MainActivity.this, Tab1.soundfiles[position]);
mp.start();
}
// Is listening which sound on Tab2 has been clicked
public void TabTwoItemClicked(int position) {
cleanUpMediaPlayer();
mp=MediaPlayer.create(MainActivity.this, Tab2.soundfiles[position]);
mp.start();
}
// Is listening which sound on Tab3 has been clicked
public void TabThreeItemClicked ( int position){
cleanUpMediaPlayer();
mp=MediaPlayer.create(MainActivity.this, Tab3.soundfiles[position]);
mp.start();
}
// Cleans MediaPlayer
public void cleanUpMediaPlayer() {
if (mp != null) {
try {
mp.stop();
mp.release();
mp = null;
} catch (Exception e) {
e.printStackTrace();
Toast.makeText(MainActivity.this, "Error", Toast.LENGTH_LONG).show();
}
}
}
// Access external storage
public void externalStorageAccess(){
final File FILES_PATH = new File(Environment.getExternalStorageDirectory(), "Android/data/"+ getText(R.string.package_name) +"/files");
if (Environment.MEDIA_MOUNTED.equals(
Environment.getExternalStorageState())) {
if (!FILES_PATH.mkdirs()) {
Log.w("error", "Could not create " + FILES_PATH);
}
} else {
Toast.makeText(MainActivity.this, "Error", Toast.LENGTH_LONG).show();
finish();
}
}
// Creates toolbar Tabs
public void toolbarTabs(){
mFragmentManager = getSupportFragmentManager();
mFragmentTransaction = mFragmentManager.beginTransaction();
mFragmentTransaction.replace(R.id.containerView, new TabFragment()).commit();
mDrawerLayout = (DrawerLayout) findViewById(R.id.drawerLayout);
android.support.v7.widget.Toolbar toolbar = (android.support.v7.widget.Toolbar) findViewById(R.id.toolbar);
ActionBarDrawerToggle mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout, toolbar, R.string.app_name,
R.string.app_name);
mDrawerLayout.setDrawerListener(mDrawerToggle);
mDrawerToggle.syncState();
}
@Override
protected void onPause() {
super.onPause();
stopMediaPlayer();
}
public void stopMediaPlayer()
{
mp.stop();
}
}
这是我的错误日志。
2019-05-09 19:13:22.296 704-704/? E/cnss-daemon: Invalid mac address: 0x555555f110M
2019-05-09 19:13:32.682 704-704/? E/cnss-daemon: Stale or unreachable neighbors, ndm state: 4
2019-05-09 19:13:39.537 443-2253/? E/ANDR-PERF-MPCTL: Invalid profile no. 0, total profiles 0 only
2019-05-09 19:13:40.333 24095-24175/? E/libEGL: validate_display:99 error 3008 (EGL_BAD_DISPLAY)
2019-05-09 19:13:43.035 443-2253/? E/ANDR-PERF-MPCTL: Invalid profile no. 0, total profiles 0 only
2019-05-09 19:13:43.116 24095-24095/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.app.trying, PID: 24095
java.lang.RuntimeException: Unable to pause activity {com.app.trying/com.app.trying.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.stop()' on a null object reference
at android.app.ActivityThread.performPauseActivityIfNeeded(ActivityThread.java:3735)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3701)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3675)
at android.app.ActivityThread.handlePauseActivity(ActivityThread.java:3649)
at android.app.ActivityThread.-wrap16(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1487)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6111)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.stop()' on a null object reference
at com.app.trying.MainActivity.stopMediaPlayer(MainActivity.java:155)
at com.app.trying.MainActivity.onPause(MainActivity.java:150)
at android.app.Activity.performPause(Activity.java:6917)
at android.app.Instrumentation.callActivityOnPause(Instrumentation.java:1323)
at android.app.ActivityThread.performPauseActivityIfNeeded(ActivityThread.java:3724)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3701)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3675)
at android.app.ActivityThread.handlePauseActivity(ActivityThread.java:3649)
at android.app.ActivityThread.-wrap16(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1487)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6111)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
2019-05-09 19:13:43.736 2484-2484/? E/Launcher3: Meminfo, NativeHeapAllocatedSize18887424
2019-05-09 19:13:43.736 2484-2484/? E/Launcher3: Meminfo, NativeHeapSize35651584
2019-05-09 19:13:43.736 2484-2484/? E/Launcher3: Meminfo, getNativeHeapFreeSize16764160
2019-05-09 19:13:51.682 704-704/? E/cnss-daemon: Stale or unreachable neighbors, ndm state: 4
如何解决此错误？任何帮助都会很棒。谢谢。
答案 0 :(得分：0)
您的MediaPlayer实例为null，如堆栈跟踪所示，所以：
public void stopMediaPlayer()
{
if (mp != null) {
mp.stop();
}
}
答案 1 :(得分：-1)
您的MediaPlayer实例永远不会初始化。因此，您必须处理它。