Question

我正在尝试实现一个扫描程序来读取代码，然后在我的应用程序中使用它来显示和过滤数据。应用程序的代码片段如下，取自在线教程。

我在尝试运行应用时出现以下错误

    **FATAL EXCEPTION: main
Process: com.example.barcodescanningapp, PID: 27516
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.barcodescanningapp/com.example.barcodescanningapp.MainActivity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2184)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2233)
at android.app.ActivityThread.access$800(ActivityThread.java:135)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5001)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
at dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime(27516): Caused by: java.lang.NullPointerException
at com.example.barcodescanningapp.MainActivity.onCreate(MainActivity.java:38)
at android.app.Activity.performCreate(Activity.java:5231)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2148)**

我无法识别我的零点异常的来源。请帮忙。

public class MainActivity extends ActionBarActivity 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) {
        getSupportFragmentManager().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);
}

public void onClick(View v){
    //respond to clicks
    if(v.getId()==R.id.scan_button){
        //scan
        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();
    }

}


@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;
    }
}
}

Answer 1

您的按钮和textview采用片段布局，而不是活动布局，因此您必须将部分代码移动到onCreateView，如下所示：

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

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

Answer 2

在我更改了onCreate代码后，工作正常

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.fragment_main);

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

无法启动actvity componentinfo - android app条形码扫描程序

2 个答案: