安卓应用程序崩溃(媒体播放器)

Question

我正在尝试制作一个简单的媒体播放器应用程序，它有三个按钮播放、暂停和停止，但当我在模拟器上启动应用程序时，如果我单击任何按钮，应用程序就会崩溃。我不知道错误在哪里。下面是我的.java和.xml文件。。。

.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" />

有人能找出错误是什么吗。下面是日志。。。。

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

当您在onCreate()回调中创建MediaPlayer mp=MediaPlayer.create(this,R.raw.laugh);时，它将是onCreate()回调的本地

更改：-

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

您需要删除MediaPlayer的本地初始化，因为如果您在本地初始化它，当您单击按钮时，作用域将为null，因为它没有全局初始化。

如果您在本地初始化MediaPlayer，并尝试在范围外执行MediaPlayer的操作，它将为您提供空对象引用

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
}
}