Question

我按照这个tutorial创建了一个简单的条形码扫描器。但我的应用程序因下线启动而停止。

scanBtn.setOnClickListener(this);

如果我注释掉这一行，App就不会停止。请让我知道如何解决这个问题。

谢谢。

package com.sample.barcodescanningapp;

import android.app.Activity;
import android.app.Fragment;
import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

import com.google.zxing.integration.android.IntentIntegrator;
import com.google.zxing.integration.android.IntentResult;

public class MainActivity extends Activity implements OnClickListener {

    private Button scanBtn;
    private TextView formatTxt, contentTxt;

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

        if (savedInstanceState == null) {
            getFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment()).commit();
        }

        scanBtn = (Button) findViewById(R.id.scan_button);
        formatTxt = (TextView) findViewById(R.id.scan_format);
        contentTxt = (TextView) findViewById(R.id.scan_content);

        scanBtn.setOnClickListener(this);

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container,
                    false);
            return rootView;
        }

    }

    @Override
    public void onClick(View v) {
        // TODO Auto-generated method stub
        if (v.getId() == R.id.scan_button) {

            IntentIntegrator scanIntegrator = new IntentIntegrator(this);
            scanIntegrator.initiateScan();
        }
    }

    public void onActivityResult(int requestCode, int resultCode, Intent intent) {
        // retrieve scan result
        IntentResult scanningResult = IntentIntegrator.parseActivityResult(
                requestCode, resultCode, intent);

        if (scanningResult != null) {
            // we have a result
            String scanContent  = scanningResult.getContents();
            String scanFormat  = scanningResult.getFormatName();

            formatTxt.setText("FORMAT    : " + scanFormat);
            contentTxt.setText("CONTENT  : " + scanContent);

        } else {
            Toast toast = Toast.makeText(getApplicationContext(),
                    "No scan data received!", Toast.LENGTH_SHORT);
            toast.show();
        }
    }

}

activity_main xml

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.sample.barcodescanningapp.MainActivity"
    tools:ignore="MergeRootFrame" />

fragment_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.sample.barcodescanningapp.MainActivity$PlaceholderFragment" >

    <Button
        android:id="@+id/scan_button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:text="@string/scan" />

    <TextView
        android:id="@+id/scan_format"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_below="@id/scan_button"
        android:text="FORMAT    : "
        android:textIsSelectable="true" />

    <TextView
        android:id="@+id/scan_content"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_below="@id/scan_format"
        android:text="CONTENT  : "
        android:textIsSelectable="true" />

</RelativeLayout>

logcat的

01-27 00:37:05.728: E/AndroidRuntime(3970): FATAL EXCEPTION: main
01-27 00:37:05.728: E/AndroidRuntime(3970): Process: com.sample.barcodescanningapp, PID: 3970
01-27 00:37:05.728: E/AndroidRuntime(3970): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.sample.barcodescanningapp/com.sample.barcodescanningapp.MainActivity}: java.lang.NullPointerException
01-27 00:37:05.728: E/AndroidRuntime(3970):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2190)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2239)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at android.app.ActivityThread.access$800(ActivityThread.java:141)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1202)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at android.os.Handler.dispatchMessage(Handler.java:102)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at android.os.Looper.loop(Looper.java:136)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at android.app.ActivityThread.main(ActivityThread.java:5045)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at java.lang.reflect.Method.invokeNative(Native Method)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at java.lang.reflect.Method.invoke(Method.java:515)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:602)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at dalvik.system.NativeStart.main(Native Method)
01-27 00:37:05.728: E/AndroidRuntime(3970): Caused by: java.lang.NullPointerException
01-27 00:37:05.728: E/AndroidRuntime(3970):     at com.sample.barcodescanningapp.MainActivity.onCreate(MainActivity.java:39)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at android.app.Activity.performCreate(Activity.java:5256)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
01-27 00:37:05.728: E/AndroidRuntime(3970):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2154)
01-27 00:37:05.728: E/AndroidRuntime(3970):     ... 11 more

Answer 1

添加Button和TextView对象，然后单击PlaceholderFragment中的方法调用，如下所示，

public static class PlaceholderFragment extends Fragment implements OnClickListener  {

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container,
                false);
        scanBtn = (Button) rootView.findViewById(R.id.scan_button);
        formatTxt = (TextView) rootView.findViewById(R.id.scan_format);
        contentTxt = (TextView) rootView.findViewById(R.id.scan_content);

        scanBtn.setOnClickListener(this);

        return rootView;
    }

    @Override
    public void onClick(View v) {
         // TODO Auto-generated method stub
         if (v.getId() == R.id.scan_button) {
              IntentIntegrator scanIntegrator = new IntentIntegrator(this);
             scanIntegrator.initiateScan();
         }
    }
}

Answer 2

您需要在fragment中声明并使用，而不是MainActivity

                package com.sample.barcodescanningapp;

                import android.app.Activity;
                import android.app.Fragment;
                import android.content.Intent;
                import android.os.Bundle;
                import android.view.LayoutInflater;
                import android.view.Menu;
                import android.view.MenuItem;
                import android.view.View;
                import android.view.View.OnClickListener;
                import android.view.ViewGroup;
                import android.widget.Button;
                import android.widget.TextView;
                import android.widget.Toast;

                import com.google.zxing.integration.android.IntentIntegrator;
                import com.google.zxing.integration.android.IntentResult;



                public class MainActivity extends Activity {



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

                        if (savedInstanceState == null) {
                            getFragmentManager().beginTransaction()
                                    .add(R.id.container, new PlaceholderFragment()).commit();
                        }



                    }

                    @Override
                    public boolean onCreateOptionsMenu(Menu menu) {

                        // Inflate the menu; this adds items to the action bar if it is present.
                        getMenuInflater().inflate(R.menu.main, menu);
                        return true;
                    }

                    @Override
                    public boolean onOptionsItemSelected(MenuItem item) {
                        // Handle action bar item clicks here. The action bar will
                        // automatically handle clicks on the Home/Up button, so long
                        // as you specify a parent activity in AndroidManifest.xml.
                        int id = item.getItemId();
                        if (id == R.id.action_settings) {
                            return true;
                        }
                        return super.onOptionsItemSelected(item);
                    }

                    /**
                     * A placeholder fragment containing a simple view.
                     */
                    public static class PlaceholderFragment extends Fragment {
private Button scanBtn;
                private TextView formatTxt, contentTxt;

                        public PlaceholderFragment() {
                        }

                        @Override
                        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                Bundle savedInstanceState) {
                            View rootView = inflater.inflate(R.layout.fragment_main, container,
                                    false);
             scanBtn = (Button) rootView .findViewById(R.id.scan_button);
                        formatTxt = (TextView) rootView .findViewById(R.id.scan_format);
                        contentTxt = (TextView) rootView .findViewById(R.id.scan_content);

                        scanBtn.setOnClickListener(new View.OnClickListener() {

                    @Override
                    public void onClick(View v) {
                        // TODO Auto-generated method stub

                    }
                });
                            return rootView;
                        }

                    }



                    public void onActivityResult(int requestCode, int resultCode, Intent intent) {
                        // retrieve scan result
                        IntentResult scanningResult = IntentIntegrator.parseActivityResult(
                                requestCode, resultCode, intent);

                        if (scanningResult != null) {
                            // we have a result
                            String scanContent  = scanningResult.getContents();
                            String scanFormat  = scanningResult.getFormatName();

                            formatTxt.setText("FORMAT    : " + scanFormat);
                            contentTxt.setText("CONTENT  : " + scanContent);

                        } else {
                            Toast toast = Toast.makeText(getApplicationContext(),
                                    "No scan data received!", Toast.LENGTH_SHORT);
                            toast.show();
                        }
                    }

                }

为什么Android应用程序在使用setOnClickListener（this）时停止了？

2 个答案: