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我有一个 mainActivity，它有一个名为监视的按钮。当我按下按钮时，它应该显示附近的信标。当我按下按钮时，会出现一个错误，即我的应用程序停止工作。这是按钮活动的代码：

public class MonitoringBeacons extends AppCompatActivity {

    private ProximityManagerContract proximityManager;

    ListAdapter beaconsAdapter;
    ListView listView;
    List<String> beaconsList = new ArrayList<String>();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_monitoring_beacons);
        KontaktSDK.initialize("kasjdhioasjkdoasdjo");

        proximityManager = new ProximityManager(this);
        proximityManager.setEddystoneListener(createEddystoneListener());

        setAdapter();


        listView.setOnItemClickListener(
                new AdapterView.OnItemClickListener() {
                    @Override
                    public void onItemClick(AdapterView<?> adapterView, View view, int position, long id) {
                        String beacon = String.valueOf(adapterView.getItemAtPosition(position));
                        Toast.makeText(MonitoringBeacons.this, beacon, Toast.LENGTH_SHORT).show();
                    }
                }
        );
    }

    protected void setAdapter() {
        if(beaconsList.isEmpty()) {
            Toast.makeText(MonitoringBeacons.this, "No beacons found", Toast.LENGTH_SHORT).show();
            return ;
        }
        beaconsAdapter = new CustomListAdapter(this, beaconsList);
        listView = (ListView) findViewById(R.id.listView);
        listView.setAdapter(beaconsAdapter);
    }

    @Override
    protected void onStart() {
        super.onStart();
        startScanning();
    }

    @Override
    protected void onStop() {
        proximityManager.stopScanning();
        super.onStop();
    }

    @Override
    protected void onDestroy() {
        proximityManager.disconnect();
        proximityManager = null;
        super.onDestroy();
    }

    private void startScanning() {
        proximityManager.connect(new OnServiceReadyListener() {
            @Override
            public void onServiceReady() {
                proximityManager.startScanning();
            }
        });
    }

    private EddystoneListener createEddystoneListener() {
        return new SimpleEddystoneListener() {
            @Override
            public void onEddystoneDiscovered(IEddystoneDevice eddystone, IEddystoneNamespace namespace) {
                beaconsList.add(eddystone.getUniqueId());
                setAdapter();
            }

            @Override
            public void onEddystoneLost(IEddystoneDevice eddystone, IEddystoneNamespace namespace) {
                beaconsList.remove(eddystone.getUniqueId());
                setAdapter();
            }
        };
    }

}

和CustomListAdapter的代码：

public class CustomListAdapter extends BaseAdapter {

    private List<String> beaconsList;
    private Activity activity;
    private LayoutInflater inflater;

    public CustomListAdapter(Activity activity, List<String> beaconsList) {
        this.activity = activity;
        this.beaconsList = beaconsList;
    }

    @Override
    public int getCount() {
        return beaconsList.size();
    }

    @Override
    public Object getItem(int location) {
        return beaconsList.get(location);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        if (inflater == null)
            inflater = (LayoutInflater) activity
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);

        if (convertView == null)
            convertView = inflater.inflate(R.layout.custom_row, null);

        String singleBeaconItem = String.valueOf(getItem(position));
        TextView beaconText = (TextView) convertView.findViewById(R.id.BeaconText);
        ImageView beaconImage = (ImageView) convertView.findViewById(R.id.BeaconImage);

        beaconText.setText(singleBeaconItem);
        beaconImage.setImageResource(R.drawable.tough_beacon_1);

        return convertView;
    }
}

理想情况下，当我按下按钮时，附近的信标应该以列表的形式出现，当发现或丢失信标时，列表应该自动更新。

已编辑

LogCat 中的堆栈跟踪

5417) 在 java.lang.reflect.Method.invoke(Native Method) 在 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 在 com.android.internal.os.ZygoteInit.main( ZygoteInit.java:616) 08-06 17:02:18.087 2559-2559/com.example.panagiotis.beaconsproject I/Process：发送信号。PID：2559 SIG：9 08-06 17:02:22.298 3433-3433/com.example.panagiotis.beaconsproject W/系统：类加载器引用未知路径：/data/app/com.example.panagiotis.beaconsproject-1/lib /x86 08-06 17:02:22.455 3433-3433/com.example.panagiotis.beaconsproject W/系统：
[08-06 17:02:22.718 3433:3433 D/] HostConnection::get() 新主机连接已建立 0xaa9a71c0，tid 3433
102) 在 android.os.Looper.loop(Looper.java:148) 在 android.app.ActivityThread.main(ActivityThread.java:5417) 在 java.lang.reflect.Method.invoke(Native Method) 在 com.android .internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 在 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 08-06 17:02:39.204 3433-3433/com。 example.panagiotis.beaconsproject I/Process：发送信号。PID：

任何想法？

4

1 回答 1

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堆栈跟踪解释了正在发生的事情。

当这行代码执行时：

listView.setOnItemClickListener(...

listView尚未初始化，因此为空。这会导致NullPointerException.

该变量在setAdapter()方法中被初始化，但它可能不是因为如果beaconsList.isEmpty()评估为真，该方法会提前退出。

简单的解决方案是将初始化代码移动listView到该方法的顶部。

于 2016-08-06T16:13:58.483 回答