Question

我正在尝试在我的应用程序上打开蓝牙，但是当我单击按钮提交时，应用程序崩溃并显示错误为java.lang.nullpointerexception

错误代码

致命的例外：主要

Process: com.example.smartaams, PID: 4094
java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.bluetooth.BluetoothAdapter.isEnabled()' on a null object reference
                      at com.example.smartaams.MarkAttendance.enableDisableBT(MarkAttendance.java:131)
                      at com.example.smartaams.MarkAttendance$1.onClick(MarkAttendance.java:92)
                      at android.view.View.performClick(View.java:5610)
                      at android.view.View$PerformClick.run(View.java:22265)
                      at android.os.Handler.handleCallback(Handler.java:751)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:154)
                      at android.app.ActivityThread.main(ActivityThread.java:6077)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

脚本

package com.example.smartaams;

import android.app.Activity;
import android.app.AlertDialog;
import android.app.DatePickerDialog;
import android.app.Dialog;
import android.app.ProgressDialog;
import android.bluetooth.BluetoothAdapter;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.DatePicker;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.Toast;

import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.util.ByteArrayBuffer;

import java.io.BufferedInputStream;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.Calendar;


public class MarkAttendance extends Activity {

    int year;
    int month;
    int day;
    static final int DATE_DIALOG_ID=999;
    String class_item,sub_item;
    Button submitbutton,backbutton;
    EditText tvDisplayDate,tvTopic;
    Spinner spclassname,spsubject,session;
    HttpPost httppost;
    StringBuffer buffer;
    HttpResponse response;
    HttpClient httpclient;
    ArrayList<NameValuePair> nameValuePairs;
    ProgressDialog dialog = null;


    String mydate=java.text.DateFormat.getDateInstance().format(Calendar.getInstance().getTime());
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_mark_attendance);


        tvDisplayDate = (EditText) findViewById(R.id.currentdate);
        tvTopic = (EditText) findViewById(R.id.topic);



        final Calendar c = Calendar.getInstance();
        int yy = c.get(Calendar.YEAR);
        int mm = c.get(Calendar.MONTH);
        int dd = c.get(Calendar.DAY_OF_MONTH);

        // set current date into textview
        tvDisplayDate.setText(new StringBuilder()
                // Month is 0 based, just add 1
                .append(yy).append(" ").append("-").append(mm + 1).append("-")
                .append(dd));


        submitbutton = (Button) findViewById(R.id.submit);

        mBluetoothAdapter = BluetoothAdapter.getDefaultAdapter();


        submitbutton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Log.d(TAG, "onClick: enabling/disabling bluetooth.");
                enableDisableBT();

            }});
    }


        private static final String TAG = "Attendance";

        BluetoothAdapter mBluetoothAdapter;
        // Create a BroadcastReceiver for ACTION_FOUND
        private final BroadcastReceiver mBroadcastReceiver1 = new BroadcastReceiver() {
            public void onReceive(Context context, Intent intent) {
                String action = intent.getAction();
                // When discovery finds a device
                if (action.equals(mBluetoothAdapter.ACTION_STATE_CHANGED)) {
                    final int state = intent.getIntExtra(BluetoothAdapter.EXTRA_STATE, mBluetoothAdapter.ERROR);

                    switch(state){
                        case BluetoothAdapter.STATE_OFF:
                            Log.d(TAG, "onReceive: STATE OFF");
                            break;
                        case BluetoothAdapter.STATE_TURNING_OFF:
                            Log.d(TAG, "mBroadcastReceiver1: STATE TURNING OFF");
                            break;
                        case BluetoothAdapter.STATE_ON:
                            Log.d(TAG, "mBroadcastReceiver1: STATE ON");
                            break;
                        case BluetoothAdapter.STATE_TURNING_ON:
                            Log.d(TAG, "mBroadcastReceiver1: STATE TURNING ON");
                            break;
                    }
                }
            }
        };

    public void enableDisableBT() {
        if (mBluetoothAdapter == null) {
            Log.d(TAG, "enableDisableBT: Does not have BT capabilities.");
        }
        if (mBluetoothAdapter.isEnabled()) {
            Log.d(TAG, "enableDisableBT: enabling BT.");
            Intent enableBTIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivity(enableBTIntent);

            IntentFilter BTIntent = new IntentFilter(BluetoothAdapter.ACTION_STATE_CHANGED);
            registerReceiver(mBroadcastReceiver1, BTIntent);
        }
        if (mBluetoothAdapter.isEnabled()) {
            Log.d(TAG, "enableDisableBT: disabling BT.");
            mBluetoothAdapter.disable();

            IntentFilter BTIntent = new IntentFilter(BluetoothAdapter.ACTION_STATE_CHANGED);
            registerReceiver(mBroadcastReceiver1, BTIntent);

        backbutton = (Button) findViewById(R.id.back);
        backbutton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Intent i=new Intent(MarkAttendance.this,StudentHome .class);
                startActivity(i);
            }
        });



            spclassname.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener()
            {

                @Override
                public void onItemSelected(AdapterView<?>spclassname, View view, int position, long id) {
                    // On selecting a spinner item
                    class_item =spclassname.getSelectedItem().toString();

                }

                @Override
                public void onNothingSelected(AdapterView<?> parent)
                {
                    // TODO Auto-generated method stub

                }

            });



            spsubject.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener()
            {
                @Override
                public void onItemSelected(AdapterView<?>  spsubject, View view, int position, long id) {
                    // On selecting a spinner item
                    sub_item =  spsubject.getSelectedItem().toString();

                }

                @Override
                public void onNothingSelected(AdapterView<?> parent)
                {
                    // TODO Auto-generated method stub

                }

            });

            session.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> session, View view, int position, long id) {
                // On selecting a spinner item
                sub_item = session.getSelectedItem().toString();

            }

            @Override
            public void onNothingSelected(AdapterView<?> parent) {
                // TODO Auto-generated method stub

            }
        });
        }
    }

}

请帮忙 PS：我对编码非常陌生，我正在使用别人的项目

Answer 1

确保您已在AndroidManifest.xml文件中添加了蓝牙权限，如下所示：

<uses-permission android:name="android.permission.BLUETOOTH" />
<uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />

您需要检查蓝牙是否已启用，如果没有，请求启用蓝牙。因此，您需要检查不条件，而不是检查(mBluetoothAdapter.isEnabled())，而需要调用蓝牙启用意图： (!mBluetoothAdapter.isEnabled())。

此外，如果将BluetoothAdapter的实例设为null，则需要添加return语句。请参阅以下代码：

if (mBluetoothAdapter == null) {
            Log.d(TAG, "enableDisableBT: Does not have BT capabilities.");
            return; // missing statement
        }

我希望这会对你有所帮助。

试图打开蓝牙，然而应用程序崩溃了

1 个答案: