问题
i am a beginner.
First of all say that I have read previous posts but did not answer.
I try to test a audio player,But when I try to run the app, it instantly crashes because of a null object reference.
This is Logcat: (this log say: Error is in Line 28 of java code)
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)
And part of my java code is:
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) {
}...
and line 28 is:
SeekBar songProgressBar=new SeekBar(this);
Thank you for your answers ...
After Update my code by @PavneetSingh answer,The same error occurred again but this time on line 70 and it is:
songProgressBar.setOnSeekBarChangeListener(this); // Important
Complete code of Main Activty after edit by first answer is:
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();
}
}
And complete error detail is(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)
Design is under this File:
Player.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
>
回答1:
this doesn't mean anything because before onCreate it will be null because Activity gets it's context(which you are referring as this ) from Application when onCreate get called so move your songProgressBar=new SeekBar(this); inside Oncreate
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);
Update : you are initializing the songProgressBar reference from XML using findViewById so you don't need this part at all
songProgressBar=new SeekBar(this);
so do this
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
Update 2 : Use setContentView(R.layout.Player); instead of setContentView(R.layout.activity_main); although it's you must rename XML file to lowercase letters
来源：https://stackoverflow.com/questions/42444928/unable-to-instantiate-activityattempt-to-invoke-virtual-method-android-content