Question

所以当我编码的时候，我遇到了一些逻辑上无法解释的东西。

public class Player {

    ImageView playerImage;
    Bitmap playerBmp;
    InputStream is;
    Drawable drawable;

    public void draw(){

        PlayActivity activity = new PlayActivity();
        playerImage = (ImageView) activity.findViewById(R.id.playerImage);

        is = activity.getResources().openRawResource(+R.drawable.main_character_drawing);
        playerBmp = BitmapFactory.decodeStream(is);

        drawable = new BitmapDrawable(activity.getResources(), playerBmp);
        playerImage.setImageDrawable(drawable);
    }
}

这是PlayActivity类

public class PlayActivity extends Activity {

    Player player;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_play);
        player = new Player();
        player.draw();

    }
}

这是activity_play的XML

  <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/playerImage"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginTop="100dp"
        android:layout_marginLeft="250dp"/>

但是，当我运行player.draw（）时，我得到一个空指针异常。这是错误日志：

07-07 17:49:05.291  31892-31892/com.example.testGame.henry. E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.testGame.henry./com.example.testGame.henry.PlayActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2070)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2095)
            at android.app.ActivityThread.access$600(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1201)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4849)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:562)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at android.app.Activity.findViewById(Activity.java:2045)
            at com.example.testGame.henry.Player.draw(Player.java:36)
            at com.example.testGame.henry.PlayActivity.onCreate(PlayActivity.java:26)
            at android.app.Activity.performCreate(Activity.java:5244)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1082)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2034)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2095)
            at android.app.ActivityThread.access$600(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1201)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4849)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:562)
            at dalvik.system.NativeStart.main(Native Method)

长期做空，问题似乎是这行代码：

playerImage = (ImageView) activity.findViewById(R.id.playerImage);

是否有某种原因会导致空指针异常？我还发现，如果我更换了使用draw()的{{1}}方法，然后删除该行

draw(Activity activity)

而只是简单地写了PlayActivity activity = new PlayActivity();（player.draw(this)是PlayActivity），它会运行得很好。

没有空指针异常，即使我找到的解决方案与创建PlayActivity实例完全相同。造成这种情况的原因是什么？

我不明白为什么在创建和使用PlayActivity实例时它会给出一个空指针异常，但是当我将PlayActivity传递给一个方法并使用参数变量（即PlayActivity）时，它就不会出现这种情况。 / p>

Answer 1

PlayActivity activity = new PlayActivity();

不会为您提供现有的PlayActivity实例。它给你一些东西，如果有的话，没有有效的上下文。为了使其工作，您需要使用适当的PlayActivity实例来查找视图并对其进行充气。或多或少就像你在文中所描述的那样。

Answer 2

原因是您正在调用Activity的新实例。它没有传播的上下文。看看这是否有效。

public class PlayActivity extends Activity {

    Player player;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_play);
        player = new Player();
        player.draw(this); <-- THIS

    }
}


public class Player {

    ImageView playerImage;
    Bitmap playerBmp;
    InputStream is;
    Drawable drawable;

    public void draw(Activity activity){ <-- THIS

        playerImage = (ImageView) activity.findViewById(R.id.playerImage);

        is = activity.getResources().openRawResource(+R.drawable.main_character_drawing);
        playerBmp = BitmapFactory.decodeStream(is);

        drawable = new BitmapDrawable(activity.getResources(), playerBmp);
        playerImage.setImageDrawable(drawable);
    }
}

Android Studios中的奇怪空指针异常

2 个答案: