为什么 MediaPlayer.create 在 class 的开头初始化时抛出 NullPointer 异常，但在 OnCreate 方法中初始化时不抛出 NullPointer 异常？

Question

在 class 开头使用上下文和资源初始化 MediaPlayer 对象时抛出 NullPointer 异常，但在 class 开头声明它时（因此它是null) 然后在 onCreate 方法中以相同的方式初始化它，就可以了。这也发生在我的其他对象上，比如视图，我不明白为什么它是以同样的方式初始化的。

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

欢迎使用 Whosebug。

作为错误状态，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 还没有发生。

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

}

为什么 MediaPlayer.create 在 class 的开头初始化时抛出 NullPointer 异常，但在 OnCreate 方法中初始化时不抛出 NullPointer 异常？

Why does MediaPlayer.create throw NullPointer exception when initialised in the beginning of the class but not when initialised in OnCreate method?

java

android

android-mediaplayer