Android應用崩潰（媒體播放器）

Question

我正在嘗試制作一個簡單的媒體播放器應用程序，該應用程序具有三個按鈕，播放，暫停和停止，但是當我在模擬器上啟動該應用程序時，如果單擊任何按鈕，應用程序都會崩潰。我無法弄清楚哪里是錯誤。 以下是我的.java和.xml文件...

的.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" />

有人可以找出錯誤所在嗎？ 以下是日志...

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

在創建MediaPlayer mp=MediaPlayer.create(this,R.raw.laugh); 在您onCreate()回調將是本地onCreate()回調

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

您需要刪除MediaPlayer的本地初始化，就好像您在本地對其進行初始化一樣，單擊按鈕時，作用域將為null，因為它不是全局初始化的。

如果您在本地初始化MediaPlayer並嘗試在范圍之外進行MediaPlayer的操作，它將為您提供空對象引用

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
    }
}