java - 为什么 MediaPlayer.create 在类的开头初始化时会抛出 NullPointer 异常，而在 OnCreate 方法中初始化时不会抛出 NullPointer 异常？

Question

当在类的开头使用上下文和资源初始化 MediaPlayer 对象时，它会抛出 NullPointer 异常，但是当在类的开头声明它时(因此它是 null)，然后以相同的方式初始化它在onCreate方法中，它起作用了。对于其他对象(例如 View )，这种情况也发生在我身上，我无法理解为什么，因为它是以相同的方式初始化的。

public class MainActivity extends AppCompatActivity {

//Commented code is how it is written to run without problems

//  private MediaPlayer player;

    private MediaPlayer player = MediaPlayer.create(this, R.raw.test); //Throws NullPointer Exception

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
//        player = MediaPlayer.create(this, R.raw.test);

        setContentView(R.layout.activity_main);
    }

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.android.musicplayer, PID: 17008
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.musicplayer/com.example.android.musicplayer.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:3066

Answer 1

欢迎来到 Stackoverflow。

由于错误发生，create 方法需要上下文。此上下文仅在 Activity 的 onCreate 方法上创建。也就是说，当您创建变量时，没有上下文，因为 onCreate 尚未发生。

进行以下更改:

public class MainActivity extends AppCompatActivity {

//Commented code is how it is written to run without problems

//  private MediaPlayer player;

private MediaPlayer player; //Remove the MediaPlayer.create from here

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
//        player = MediaPlayer.create(this, R.raw.test);

    setContentView(R.layout.activity_main);

    player = MediaPlayer.create(this, R.raw.test); //and put it here

}