Android应用崩溃（媒体播放器）

Question

I am trying to make a simple media player app which has three buttons play,pause and stop,but when i launch the app on emulator and if i click any of the buttons,app crashes.I am not able to figure out where is the error. below is my .java and .xml file...

.Java

package com.example.android.songdemo;

import android.media.MediaPlayer;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;

public class MainActivity extends AppCompatActivity {


    MediaPlayer mp;

    public void songPlay(View view){
        mp.start();
    }

    public void songPause(View view){
        mp.pause();
    }

    public void songStop(View view){
        mp.stop();
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        MediaPlayer mp=MediaPlayer.create(this,R.raw.laugh);
    }
}

.xml

<?xml version="1.0" encoding="utf-8"?>
<android.widget.RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.android.songdemo.MainActivity">

<Button
    android:id="@+id/button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_toLeftOf="@+id/button2"
    android:layout_toStartOf="@+id/button2"
    android:onClick="songPlay"
    android:text="play"
     />

<Button
    android:id="@+id/button2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentEnd="true"
    android:layout_alignParentRight="true"
    android:layout_below="@+id/button"
    android:layout_marginEnd="65dp"
    android:layout_marginRight="65dp"
    android:layout_marginTop="110dp"
    android:onClick="songPause"
    android:text="pause"
    tools:layout_editor_absoluteX="117dp"
    tools:layout_editor_absoluteY="76dp" />

<Button
    android:id="@+id/button3"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBaseline="@+id/button2"
    android:layout_alignBottom="@+id/button2"
    android:layout_toLeftOf="@+id/button"
    android:layout_toStartOf="@+id/button"
    android:onClick="songStop"
    android:text="stop"
    tools:layout_editor_absoluteX="117dp"
    tools:layout_editor_absoluteY="145dp" />

Can someone find out what is the error. Below is the Log....

java.lang.IllegalStateException: Could not execute method for android:onClick
                                                                              at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
                                                                              at android.view.View.performClick(View.java:6294)
                                                                              at android.view.View$PerformClick.run(View.java:24770)
                                                                              at android.os.Handler.handleCallback(Handler.java:790)
                                                                              at android.os.Handler.dispatchMessage(Handler.java:99)
                                                                              at android.os.Looper.loop(Looper.java:164)
                                                                              at android.app.ActivityThread.main(ActivityThread.java:6494)
                                                                              at java.lang.reflect.Method.invoke(Native Method)
                                                                              at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
                                                                              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
                                                                           Caused by: java.lang.reflect.InvocationTargetException
                                                                              at java.lang.reflect.Method.invoke(Native Method)
                                                                              at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
                                                                              at android.view.View.performClick(View.java:6294) 
                                                                              at android.view.View$PerformClick.run(View.java:24770) 
                                                                              at android.os.Handler.handleCallback(Handler.java:790) 
                                                                              at android.os.Handler.dispatchMessage(Handler.java:99) 
                                                                              at android.os.Looper.loop(Looper.java:164) 
                                                                              at android.app.ActivityThread.main(ActivityThread.java:6494) 
                                                                              at java.lang.reflect.Method.invoke(Native Method) 
                                                                              at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
                                                                              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807) 
                                                                           Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.start()' on a null object reference
                                                                              at com.example.android.songdemo.MainActivity.songPlay(MainActivity.java:16)

Answer 1

As you are creating MediaPlayer mp=MediaPlayer.create(this,R.raw.laugh); in your onCreate() callback it will be local to onCreate() callback

Change:-

package com.example.android.songdemo;

import android.media.MediaPlayer;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;

public class MainActivity extends AppCompatActivity {


    MediaPlayer mp;

    public void songPlay(View view){
        mp.start();
    }

    public void songPause(View view){
        mp.pause();
    }

    public void songStop(View view){
        mp.stop();
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mp=MediaPlayer.create(this,R.raw.laugh);
    }
}

Answer 2

You need to remove the local initialization of MediaPlayer as if you initialize it locally the scope will be null when you click on the buttons as it is not globally initialized.

If you initialize the MediaPlayer locally and you try to do the operations of MediaPlayer outside the scope it will give you null object reference

package com.example.android.songdemo;

import android.media.MediaPlayer;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;

public class MainActivity extends AppCompatActivity {


    private MediaPlayer mp;

    public void songPlay(View view){
        mp.start();
    }

    public void songPause(View view){
        mp.pause();
    }

    public void songStop(View view){
        mp.stop();
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mp=MediaPlayer.create(this,R.raw.laugh); // removed the local initialization
    }
}