Question

我正在尝试学习使用Java开发Android以处理蓝牙设备。

扫描蓝牙设备时，应用程序崩溃，并出现以下错误：

2020-09-20 00:59:06.684 14049-14049/com.fm.basic_app E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.fm.basic_app, PID: 14049
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
        at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:445)
        at android.widget.ArrayAdapter.getView(ArrayAdapter.java:407)
        at android.widget.AbsListView.obtainView(AbsListView.java:2388)
        at android.widget.ListView.makeAndAddView(ListView.java:2054)
        at android.widget.ListView.fillDown(ListView.java:788)
        at android.widget.ListView.fillFromTop(ListView.java:849)
        at android.widget.ListView.layoutChildren(ListView.java:1800)
        at android.widget.AbsListView.onLayout(AbsListView.java:2187)
        at android.view.View.layout(View.java:19663)
        at android.view.ViewGroup.layout(ViewGroup.java:6075)
        at androidx.constraintlayout.widget.ConstraintLayout.onLayout(ConstraintLayout.java:1915)
        at android.view.View.layout(View.java:19663)
        at android.view.ViewGroup.layout(ViewGroup.java:6075)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:19663)
        at android.view.ViewGroup.layout(ViewGroup.java:6075)
        at androidx.appcompat.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:444)
        at android.view.View.layout(View.java:19663)
        at android.view.ViewGroup.layout(ViewGroup.java:6075)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:19663)
        at android.view.ViewGroup.layout(ViewGroup.java:6075)
        at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791)
        at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635)
        at android.widget.LinearLayout.onLayout(LinearLayout.java:1544)
        at android.view.View.layout(View.java:19663)
        at android.view.ViewGroup.layout(ViewGroup.java:6075)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at com.android.internal.policy.DecorView.onLayout(DecorView.java:761)
        at android.view.View.layout(View.java:19663)
        at android.view.ViewGroup.layout(ViewGroup.java:6075)
        at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2548)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2264)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1444)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6843)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:966)
        at android.view.Choreographer.doCallbacks(Choreographer.java:778)
        at android.view.Choreographer.doFrame(Choreographer.java:713)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:952)
        at android.os.Handler.handleCallback(Handler.java:790)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6518)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

我不确定如何根据上述内容调试应用程序，这是通过以下方式触发蓝牙扫描的方法：

bt_disc = (Button) findViewById(R.id.disc_dev);

完整代码：

package com.fm.basic_app; //package name

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity; //packages import
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;

import android.Manifest;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.content.pm.PackageManager;
import android.graphics.Color;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.Set;

public class MainActivity extends AppCompatActivity { //main activity

    ListView list_view;
    Button bt_on, bt_off, bt_scan, bt_disc; //declare button object
    BluetoothAdapter devBT; //declare BT adapter object
    Intent bt_enabled;
    IntentFilter discover_dev;
    int bt_req_code;
    int lc_coarse_req_code;
    int lc_fine_req_code;

    ArrayList<String> discovered_devices = new ArrayList<String>();
    ArrayAdapter<String> dev_adapt;

    @Override //override the current class
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getWindow().getDecorView().setBackgroundColor(Color.LTGRAY); //change background color

        bt_on = (Button) findViewById(R.id.bt_on); //declare button on view
        bt_off = (Button) findViewById(R.id.bt_off);
        bt_scan = (Button) findViewById(R.id.bt_scan);
        list_view = (ListView) findViewById(R.id.lst_view);
        bt_disc = (Button) findViewById(R.id.disc_dev);

        devBT = BluetoothAdapter.getDefaultAdapter(); //try to get default adapter for BT

        bt_enabled = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);

        bt_req_code = 1;

        lc_coarse_req_code = 1;

        lc_fine_req_code = 1;

        req_perm_method();
        bt_on_method();
        bt_off_method();
        bt_scan_method();
        bt_disc_method();
    }

    private void req_perm_method() {
        if (ContextCompat.checkSelfPermission(MainActivity.this,
                Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            if (ActivityCompat.shouldShowRequestPermissionRationale(MainActivity.this,
                    Manifest.permission.ACCESS_FINE_LOCATION)) {
                ActivityCompat.requestPermissions(MainActivity.this,
                        new String[]{Manifest.permission.ACCESS_FINE_LOCATION}, lc_fine_req_code);
            } else {
                ActivityCompat.requestPermissions(MainActivity.this,
                        new String[]{Manifest.permission.ACCESS_FINE_LOCATION}, lc_fine_req_code);
            }
        }
        if (ContextCompat.checkSelfPermission(MainActivity.this,
                Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            if (ActivityCompat.shouldShowRequestPermissionRationale(MainActivity.this,
                    Manifest.permission.ACCESS_COARSE_LOCATION)) {
                ActivityCompat.requestPermissions(MainActivity.this,
                        new String[]{Manifest.permission.ACCESS_COARSE_LOCATION}, lc_coarse_req_code);
            } else {
                ActivityCompat.requestPermissions(MainActivity.this,
                        new String[]{Manifest.permission.ACCESS_COARSE_LOCATION}, lc_coarse_req_code);
            }
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        switch (requestCode) {
            case 1: {
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    if (ContextCompat.checkSelfPermission(MainActivity.this,
                            Manifest.permission.ACCESS_FINE_LOCATION ) == PackageManager.PERMISSION_GRANTED) {
                        Toast.makeText(this, "Permission Granted", Toast.LENGTH_SHORT).show();
                    }
                    if (ContextCompat.checkSelfPermission(MainActivity.this,
                            Manifest.permission.ACCESS_COARSE_LOCATION ) == PackageManager.PERMISSION_GRANTED) {
                        Toast.makeText(this, "Permission Granted", Toast.LENGTH_SHORT).show();
                    }
                } else {
                    Toast.makeText(this, "Permission Denied", Toast.LENGTH_SHORT).show();
                }
                return;
            }
        }
    }

    private void bt_disc_method() {
        bt_disc.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(MainActivity.this, "Discovering Devices", Toast.LENGTH_LONG).show();
                devBT.startDiscovery();
            }
        });

        discover_dev = new IntentFilter(BluetoothDevice.ACTION_FOUND);
        registerReceiver(dev_discover, discover_dev);

        dev_adapt = new ArrayAdapter<String>(MainActivity.this, android.R.layout.simple_list_item_1, discovered_devices);
        list_view.setAdapter(dev_adapt);

    }

    BroadcastReceiver dev_discover = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            String bt_action = intent.getAction();
            if(BluetoothDevice.ACTION_FOUND.equals(bt_action)){
                BluetoothDevice bt_dev = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);
                discovered_devices.add(bt_dev.getName());
                dev_adapt.notifyDataSetChanged();
            }
        }
    };

    @Override
    protected void onDestroy() {
        super.onDestroy();
        unregisterReceiver(dev_discover);
    }

    private void bt_scan_method() {
        bt_scan.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(MainActivity.this, "Paired Devices Will Be Shown", Toast.LENGTH_SHORT).show();

                Set<BluetoothDevice> btpoll = devBT.getBondedDevices();
                String[] devicelist = new String[btpoll.size()];
                int index = 0;

                if(btpoll.size()>0){
                    for (BluetoothDevice alldevices: btpoll){
                        devicelist[index] = alldevices.getName();
                        index++;
                    }

                    ArrayAdapter<String> devicearray = new ArrayAdapter<String>(MainActivity.this, android.R.layout.simple_list_item_1, devicelist);
                    list_view.setAdapter(devicearray);
                }
            }
        });
    }

    private void bt_off_method() {
        bt_off.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(devBT.isEnabled()){
                    devBT.disable();
                }
            }
        });
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        if(requestCode == bt_req_code){
            if(resultCode == RESULT_OK){
                Toast.makeText(this, "Bluetooth is Enabled", Toast.LENGTH_LONG).show();
            }else if (resultCode == RESULT_CANCELED){
                Toast.makeText(this, "Bluetooth Enable Cancelled", Toast.LENGTH_SHORT).show();
            }
        }
    }

    private void bt_on_method() {
        bt_on.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(devBT == null){
                    Toast.makeText(MainActivity.this, "Device Does Not Support Bluetooth", Toast.LENGTH_LONG).show();
                }else{
                    if(!devBT.isEnabled()){
                        startActivityForResult(bt_enabled, bt_req_code);
                    }
                }
            }
        });
    }
}

列出配对的设备很好，但是列出发现的设备会使应用程序崩溃....：（

任何见解将不胜感激，谢谢！

Answer 1

当list_view适配器尝试呈现发现的设备名称时，将出现异常。对于蓝牙设备，多次返回的名称为null。因此，您尝试获取设备名称并将其添加到Discovered_devices数组的代码可能包含空条目。尝试添加空检查

      BluetoothDevice bt_dev = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);
      if( bt_dev.getName() != null){
           discovered_devices.add(bt_dev.getName());
      }
      else{
         discovered_devices.add(bt_dev.getAddress()); // show device address instead
      }
      dev_adapt.notifyDataSetChanged();

这应该可以解决崩溃问题。让我知道这是否有帮助

将蓝牙设备扫描到ArrayList会导致NullPointerException

1 个答案: