java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法'android.view.View .MainActivity.findViewById（int）'

Question

我有一个称为 MainActivity.java
的类，该类称为一个AsyncTask类。最后一个类findViewById()在执行时返回此错误：

java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View <mypackage>.MainActivity.findViewById(int)' on a null object reference

我不明白ImageView在完成工作R.layout.activity_main后如何编辑排名AsyncTask。

MainActivity.java

public class MainActivity extends Activity {

    public MainActivity() {}

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        new Connection().execute();
    }

}

连接.java

public class Connection extends AsyncTask<String, Void, String> {
    public String result;

    //I know, this isn't correct, how can i do?
    public MainActivity MainActivity;

    @Override
    protected String doInBackground(String... arg0) {
        // TODO Auto-generated method stub
                //...

        return "a string";
    }

    protected void onPostExecute(String result) {
        super.onPostExecute(result);
                    //...

            // Where the error is generated
            ImageView image = (ImageView) MainActivity.findViewById(R.id.image);

            //...

    }
}

Answer 1

错误是

public MainActivity MainActivity;

永远不会初始化，因此指向null。为了使您的代码正常工作，最小的步骤是在MainActivity中

new Connection(this).execute();

在连接

public class Connection extends AsyncTask<String, Void, String> {

    public MainActivity MainActivity;

    public Connection(MainActivity activity) {
        MainActivity = activity;
    }

但是无论如何，在onCreate中创建任务并传递Activity并不是最好的主意。另外，字段名称应始终以小写字母开头。

最好的方法是将ImageView传递给AsyncTask。在启动“活动”之前不要启动任务，而且，在“活动”停止后也不要忘记取消任务。

public final class MainActivity extends Activity {

    public MainActivity() {}

    private Connection connection;
    private ImageView imageView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        html" target="_blank">setContentView(R.layout.activity_main);

        imageView = (ImageView) findViewById(R.id.image);
    }

    @Override
    protected void onStart() {
        super.onStart();
        if (connection == null || connection.getStatus() != AsyncTask.Status.RUNNING) {
            connection = new Connection(imageView);
            connection.execute();
        }
    }

    @Override
    protected void onStop() {
        super.onStop();
        if (connection != null && connection.getStatus() == AsyncTask.Status.RUNNING) {
            connection.cancel(true);
        }
    }

}

在Connection.java中，将ImageView存储为WeakReference以避免泄漏。

public final class Connection extends AsyncTask<String, Void, String> {

    private final WeakReference<ImageView> imageViewRef;

    public Connection(ImageView view) {
        imageViewRef = new WeakReference<ImageView>(view);
    }

    @Override
    protected String doInBackground(String... arg0) {
        // TODO Auto-generated method stub
                //...

        return "a string";
    }

    @Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);
                //...

        final ImageView imageView = imageViewRef.get();
        // if the Activity is still alive, the ImageView will not be null
        if (imageView != null) {
            // set an image or whatever you need
            image.setImageResource(666);
        }

    }