[英]Unable to instantiate activity:Attempt to invoke virtual method android.content.Context.getResources()
我是初學者。
首先說我看過以前的帖子但沒有回答。
我嘗試測試音頻播放器，但是當我嘗試運行該應用程序時，由於空對象引用，它立即崩潰。
這是 Logcat：（此日志說：錯誤在 Java 代碼的第 28 行）
EXCEPTION: main
Process: ir.pluto.mediaplayer, PID: 19100
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{ir.pluto.mediaplayer/ir.pluto.mediaplayer.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3132)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3415)
at android.app.ActivityThread.access$1100(ActivityThread.java:229)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1821)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:7325)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
at android.content.ContextWrapper.getResources(ContextWrapper.java:92)
at android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:81)
at android.view.View.<init>(View.java:4054)
at android.view.View.<init>(View.java:4186)
at android.widget.ProgressBar.<init>(ProgressBar.java:313)
at android.widget.AbsSeekBar.<init>(AbsSeekBar.java:188)
at android.widget.SeekBar.<init>(SeekBar.java:85)
at android.widget.SeekBar.<init>(SeekBar.java:81)
at android.widget.SeekBar.<init>(SeekBar.java:77)
at android.widget.SeekBar.<init>(SeekBar.java:73)
at ir.pluto.mediaplayer.MainActivity.<init>(MainActivity.java:28)
at java.lang.Class.newInstance(Native Method)
at android.app.Instrumentation.newActivity(Instrumentation.java:1096)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3122)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3415)
at android.app.ActivityThread.access$1100(ActivityThread.java:229)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1821)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:7325)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
我的java代碼的一部分是：
import android.app.Activity;
import android.content.Intent;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.os.Handler;
import android.view.View;
import android.widget.ImageButton;
import android.widget.SeekBar;
import android.widget.TextView;
import android.widget.Toast;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Random;
public class MainActivity extends Activity implements MediaPlayer.OnCompletionListener {
ImageButton btnPlay;
ImageButton btnForward;
ImageButton btnBackward;
ImageButton btnNext;
ImageButton btnPrevious;
ImageButton btnPlaylist;
ImageButton btnRepeat;
ImageButton btnShuffle;
SeekBar songProgressBar=new SeekBar(this);
TextView songTitleLabel;
TextView songCurrentDurationLabel;
TextView songTotalDurationLabel;
// Media Player
MediaPlayer mp;
// Handler to update UI timer, progress bar etc,.
Handler mHandler = new Handler();
SongsManager songManager;
Utilities utils;
int seekForwardTime = 5000; // 5000 milliseconds
int seekBackwardTime = 5000; // 5000 milliseconds
int currentSongIndex = 0;
boolean isShuffle = false;
boolean isRepeat = false;
ArrayList<HashMap<String, String>> songsList = new ArrayList<>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btnPlay = (ImageButton) findViewById(R.id.btnPlay);
btnForward = (ImageButton) findViewById(R.id.btnForward);
btnBackward = (ImageButton) findViewById(R.id.btnBackward);
btnNext = (ImageButton) findViewById(R.id.btnNext);
btnPrevious = (ImageButton) findViewById(R.id.btnPrevious);
btnPlaylist = (ImageButton) findViewById(R.id.btnPlaylist);
btnRepeat = (ImageButton) findViewById(R.id.btnRepeat);
btnShuffle = (ImageButton) findViewById(R.id.btnShuffle);
songProgressBar = (SeekBar) findViewById(R.id.songProgressBar);
songTitleLabel = (TextView) findViewById(R.id.songTitle);
songCurrentDurationLabel = (TextView) findViewById(R.id.songCurrentDurationLabel);
songTotalDurationLabel = (TextView) findViewById(R.id.songTotalDurationLabel);
// Media player
mp = new MediaPlayer();
songManager = new SongsManager();
utils = new Utilities();
// Listeners
songProgressBar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
@Override
public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
}...
第 28 行是：
SeekBar songProgressBar=new SeekBar(this);
謝謝您的回答 ...
通過@PavneetSingh 回答更新我的代碼后，同樣的錯誤再次發生，但這次是在第 70 行，它是：
songProgressBar.setOnSeekBarChangeListener(this); // Important
通過第一個答案編輯后的主要活動的完整代碼是：
package ir.pluto.mediaplayer;
import android.app.Activity;
import android.content.Intent;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.os.Handler;
import android.view.View;
import android.widget.ImageButton;
import android.widget.SeekBar;
import android.widget.TextView;
import android.widget.Toast;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Random;
public class MainActivity extends Activity implements MediaPlayer.OnCompletionListener,SeekBar.OnSeekBarChangeListener {
ImageButton btnPlay;
ImageButton btnForward;
ImageButton btnBackward;
ImageButton btnNext;
ImageButton btnPrevious;
ImageButton btnPlaylist;
ImageButton btnRepeat;
ImageButton btnShuffle;
SeekBar songProgressBar;
TextView songTitleLabel;
TextView songCurrentDurationLabel;
TextView songTotalDurationLabel;
// Media Player
MediaPlayer mp;
// Handler to update UI timer, progress bar etc,.
Handler mHandler = new Handler();
SongsManager songManager;
Utilities utils;
int seekForwardTime = 5000; // 5000 milliseconds
int seekBackwardTime = 5000; // 5000 milliseconds
int currentSongIndex = 0;
boolean isShuffle = false;
boolean isRepeat = false;
ArrayList<HashMap<String, String>> songsList = new ArrayList<>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btnPlay = (ImageButton) findViewById(R.id.btnPlay);
btnForward = (ImageButton) findViewById(R.id.btnForward);
btnBackward = (ImageButton) findViewById(R.id.btnBackward);
btnNext = (ImageButton) findViewById(R.id.btnNext);
btnPrevious = (ImageButton) findViewById(R.id.btnPrevious);
btnPlaylist = (ImageButton) findViewById(R.id.btnPlaylist);
btnRepeat = (ImageButton) findViewById(R.id.btnRepeat);
btnShuffle = (ImageButton) findViewById(R.id.btnShuffle);
songProgressBar = (SeekBar) findViewById(R.id.songProgressBar);
songTitleLabel = (TextView) findViewById(R.id.songTitle);
songCurrentDurationLabel = (TextView) findViewById(R.id.songCurrentDurationLabel);
songTotalDurationLabel = (TextView) findViewById(R.id.songTotalDurationLabel);
// Media player
mp = new MediaPlayer();
songManager = new SongsManager();
utils = new Utilities();
// Listeners
songProgressBar.setOnSeekBarChangeListener(this); // Important
mp.setOnCompletionListener(this); // Important
// Getting all songs list
songsList = songManager.getPlayList();
/*
Button Click event for Play list click event
Launches list activity which displays list of songs
*/
btnPlaylist.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View arg0) {
Intent i = new Intent(getApplicationContext(), PlayListActivity.class);
startActivityForResult(i, 100);
}
});
/*
Forward button click event
Forwards song specified seconds
*/
btnForward.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View arg0) {
// get current song position
int currentPosition = mp.getCurrentPosition();
// check if seekForward time is lesser than song duration
if(currentPosition + seekForwardTime <= mp.getDuration()){
// forward song
mp.seekTo(currentPosition + seekForwardTime);
}else{
// forward to end position
mp.seekTo(mp.getDuration());
}
}
});
/*
Backward button click event
Backward song to specified seconds
*/
btnBackward.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View arg0) {
// get current song position
int currentPosition = mp.getCurrentPosition();
// check if seekBackward time is greater than 0 sec
if(currentPosition - seekBackwardTime >= 0){
// forward song
mp.seekTo(currentPosition - seekBackwardTime);
}else{
// backward to starting position
mp.seekTo(0);
}
}
});
/*
Next button click event
Plays next song by taking currentSongIndex + 1
*/
btnNext.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View arg0) {
// check if next song is there or not
if(currentSongIndex < (songsList.size() - 1)){
playSong(currentSongIndex + 1);
currentSongIndex = currentSongIndex + 1;
}else{
// play first song
playSong(0);
currentSongIndex = 0;
}
}
});
/*
Back button click event
Plays previous song by currentSongIndex - 1
*/
btnPrevious.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View arg0) {
if(currentSongIndex > 0){
playSong(currentSongIndex - 1);
currentSongIndex = currentSongIndex - 1;
}else{
// play last song
playSong(songsList.size() - 1);
currentSongIndex = songsList.size() - 1;
}
}
});
/*
* Button Click event for Repeat button
* Enables repeat flag to true
*/
btnRepeat.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View arg0) {
if(isRepeat){
isRepeat = false;
Toast.makeText(getApplicationContext(), "Repeat is OFF", Toast.LENGTH_LONG).show();
btnRepeat.setImageResource(R.drawable.btn_repeat);
}else{
// make repeat to true
isRepeat = true;
Toast.makeText(getApplicationContext(), "Repeat is ON", Toast.LENGTH_LONG).show();
// make shuffle to false
isShuffle = false;
btnRepeat.setImageResource(R.drawable.btn_repeat);
btnShuffle.setImageResource(R.drawable.btn_shuffle);
}
}
});
/*
* Button Click event for Shuffle button
* Enables shuffle flag to true
*/
btnShuffle.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View arg0) {
if(isShuffle){
isShuffle = false;
Toast.makeText(getApplicationContext(), "Shuffle is OFF", Toast.LENGTH_SHORT).show();
btnShuffle.setImageResource(R.drawable.btn_shuffle);
}else{
// make repeat to true
isShuffle= true;
Toast.makeText(getApplicationContext(), "Shuffle is ON", Toast.LENGTH_SHORT).show();
// make shuffle to false
isRepeat = false;
btnShuffle.setImageResource(R.drawable.btn_shuffle);
btnRepeat.setImageResource(R.drawable.btn_repeat);
}
}
});
}
//************Outside of onCreate()**************
/**
* Receiving song index from playlist view
* and play the song
**/
@Override
protected void onActivityResult(int requestCode,int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
if(resultCode == 100){
currentSongIndex = data.getExtras().getInt("songIndex");
// play selected song
playSong(currentSongIndex);
}
}
/**
* Function to play a song
* @param songIndex - index of song
* */
public void playSong(int songIndex){
// Play song
try {
mp.reset();
mp.setDataSource(songsList.get(songIndex).get("songPath"));
mp.prepare();
mp.start();
// Displaying Song title
String songTitle = songsList.get(songIndex).get("songTitle");
songTitleLabel.setText(songTitle);
// Changing Button Image to pause image
btnPlay.setImageResource(R.drawable.btn_pause);
// set Progress bar values
songProgressBar.setProgress(0);
songProgressBar.setMax(100);
// Updating progress bar
updateProgressBar();
} catch (IllegalArgumentException | IllegalStateException | IOException e) {
e.printStackTrace();
}
}
/**
* Update timer on seekbar
**/
public void updateProgressBar() {
mHandler.postDelayed(mUpdateTimeTask, 100);
}
/**
* Background Runnable thread
* */
private Runnable mUpdateTimeTask = new Runnable() {
public void run() {
long totalDuration = mp.getDuration();
long currentDuration = mp.getCurrentPosition();
// Displaying Total Duration time
songTotalDurationLabel.setText(utils.milliSecondsToTimer(totalDuration));
// Displaying time completed playing
songCurrentDurationLabel.setText(utils.milliSecondsToTimer(currentDuration));
// Updating progress bar
int progress = utils.getProgressPercentage(currentDuration, totalDuration);
//Log.d("Progress", ""+progress);
songProgressBar.setProgress(progress);
// Running this thread after 100 milliseconds
mHandler.postDelayed(this, 100);
}
};
/*
* On Song Playing completed
* if repeat is ON play same song again
* if shuffle is ON play random song
*/
@Override
public void onCompletion(MediaPlayer mp) {
// check for repeat is ON or OFF
if(isRepeat){
// repeat is on play same song again
playSong(currentSongIndex);
} else if(isShuffle){
// shuffle is on - play a random song
Random rand = new Random();
currentSongIndex = rand.nextInt((songsList.size() - 1) + 1);
playSong(currentSongIndex);
} else{
// no repeat or shuffle ON - play next song
if(currentSongIndex < (songsList.size() - 1)){
playSong(currentSongIndex + 1);
currentSongIndex = currentSongIndex + 1;
}else{
// play first song
playSong(0);
currentSongIndex = 0;
}
}
}
@Override
public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
}
/*
* When user starts moving the progress handler
*/
@Override
public void onStartTrackingTouch(SeekBar seekBar) {
// remove message Handler from updating progress bar
mHandler.removeCallbacks(mUpdateTimeTask);
}
/*
* When user stops moving the progress hanlder
*/
@Override
public void onStopTrackingTouch(SeekBar seekBar) {
mHandler.removeCallbacks(mUpdateTimeTask);
int totalDuration = mp.getDuration();
int currentPosition = utils.progressToTimer(seekBar.getProgress(), totalDuration);
// forward or backward to certain seconds
mp.seekTo(currentPosition);
// update timer progress again
updateProgressBar();
}
}
完整的錯誤細節是（MainActivity.java:70）：
EXCEPTION: main
Process: ir.pluto.mediaplayer, PID: 28285
java.lang.RuntimeException: Unable to start activity ComponentInfo{ir.pluto.mediaplayer/ir.pluto.mediaplayer.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.SeekBar.setOnSeekBarChangeListener(android.widget.SeekBar$OnSeekBarChangeListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3319)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3415)
at android.app.ActivityThread.access$1100(ActivityThread.java:229)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1821)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:7325)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.SeekBar.setOnSeekBarChangeListener(android.widget.SeekBar$OnSeekBarChangeListener)' on a null object reference
at ir.pluto.mediaplayer.MainActivity.onCreate(MainActivity.java:70)
at android.app.Activity.performCreate(Activity.java:6904)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1136)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3266)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3415)
at android.app.ActivityThread.access$1100(ActivityThread.java:229)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1821)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:7325)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
設計在此文件下：
播放器.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@drawable/player_background">
<!--another codes-->
<!-- Progress Bar/Seek bar -->
<SeekBar
android:id="@+id/songProgressBar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_above="@id/player_footer_bg"
android:layout_marginBottom="1dp"
android:layout_marginLeft="20dp"
android:layout_marginRight="20dp"
android:paddingLeft="6dp"
android:paddingRight="6dp"
android:progressDrawable="@drawable/seekbar_progress"
android:thumb="@drawable/seekbar_thumb" />
<!--another codes-->
</RelativeLayout
>
this並不意味着什么，因為在onCreate之前它將為null因為當onCreate被調用時， Activity從Application獲取它的context （你指的是this ）所以移動你的songProgressBar=new SeekBar(this); 在Oncreate里面
public class MainActivity extends Activity implements MediaPlayer.OnCompletionListener {
//..
SeekBar songProgressBar;
//..
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
songProgressBar=new SeekBar(this);
更新：您正在使用findViewById從XML初始化songProgressBar引用，因此您根本不需要這部分
songProgressBar=new SeekBar(this);
所以這樣做
public class MainActivity extends Activity implements MediaPlayer.OnCompletionListener {
// .. code
SeekBar songProgressBar;
// .. code
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// .. code
songProgressBar = (SeekBar) findViewById(R.id.songProgressBar);
// .. code
更新 2：使用setContentView(R.layout.Player); 而不是setContentView(R.layout.activity_main); 盡管您必須將XML文件重命名為小寫字母
嘗試在 null object 參考上調用虛擬方法“android.content.Context.getResources()”
[英]Attempt to invoke virtual method 'android.content.Context.getResources()' on a null object reference
例外：嘗試在空對象引用上調用虛擬方法“android.content.res.Resources android.content.Context.getResources()”
[英]Exception: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
android.content.Context.getResources() 在空對象引用上
[英]android.content.Context.getResources() on a null object reference
嘗試在空對象引用上調用虛擬方法'android.content.Context android.content.Context.getApplicationContext（）'
[英]Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
嘗試調用虛擬方法'android.content.Context android.support.v4.app.FragmentActivity.getApplicationContext（）'android
[英]Attempt to invoke virtual method 'android.content.Context android.support.v4.app.FragmentActivity.getApplicationContext()' android
嘗試在 Android Studio 中的 null object 參考上調用虛擬方法 'android.content.Context.getApplicationInfo()'
[英]Attempt to invoke virtual method 'android.content.Context.getApplicationInfo()' on a null object reference in Android Studio
嘗試調用虛擬方法'java.lang.String android.content.Context.getPackageName（）
[英]Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()
嘗試調用虛擬方法 'java.lang.Object android.content.Context
[英]Attempt to invoke virtual method 'java.lang.Object android.content.Context
由於嘗試調用虛擬方法'java.lang.String android.content.Context.getPackageName（），因此意圖失敗
[英]Intent fails because of Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()
無法啟動活動：嘗試調用虛擬方法 (NullPointerException)
[英]Unable to start activity: Attempt to invoke virtual method (NullPointerException)
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