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

Question

我有一个叫做MainActivity.java的AsyncTask类来调用一个类.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

我不明白如何ImageView在完成工作R.layout.activity_main之后编辑定位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();
    }

}

Connection.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

错误就是这样

public MainActivity MainActivity;

永远不会被初始化,因此指向null.要使代码工作,最小步骤是在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;
    }

但是在onCreate中创建一个任务并传递一个Activity并不是最好的主意.此外,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Activity启动之前不要启动任务,并且不要忘记在Activity停止时取消任务.

public final class MainActivity extends Activity {

    public MainActivity() {}

    private Connection connection;
    private ImageView imageView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        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);
        }

    }