Question

我正在尝试制作一个简单的媒体播放器应用程序，该应用程序具有三个按钮，播放，暂停和停止，但是当我在模拟器上启动该应用程序时，如果单击任何按钮，应用程序都会崩溃。我无法弄清楚错误在哪里。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" />

有人可以找出错误所在吗？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

在MediaPlayer mp=MediaPlayer.create(this,R.raw.laugh);回调中创建onCreate()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，就像您在本地对其进行初始化一样，因为单击时按钮的作用域将为null，因为它尚未全局初始化。

如果您在本地初始化MediaPlayer，并尝试在范围之外进行MediaPlayer的操作，则会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
    }
}

Android应用崩溃（媒体播放器）

2 个答案: