值
我试图调试这个问题，但对为什么我得到空指针异常错误感到非常困惑。我在代码中声明 checkBTStates（） 方法的行不断收到错误（是的，我已经尽我所能完成了这个以确保一切正常）
下面是我的片段、活动和错误代码。我希望我能更好地粘贴代码。不好意思。
片段
package edu.umass.ecs.chalkmaster3000;
import android.app.Activity;
import android.app.Fragment;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothSocket;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;
import java.io.IOException;
import java.util.UUID;
public class HomeFragment extends Fragment {
public static final int BT_ACTIVITY = 5;
public int i;
ActivityCommunication activityCallback;
public interface ActivityCommunication{
public void updateColors(int color);
public String getAddress();
public UUID getUUID();
public BluetoothSocket getBTSocket();
public BluetoothAdapter getBTAdapter();
public void setConnectionStatus(boolean status);
public View getView(int viewID);
public Context getContext();
}
@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
try {
activityCallback = (ActivityCommunication) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString()
+ " must implement ActivityCommunication");
}
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){
View V = inflater.inflate(R.layout.home_fragment,container,false);
ImageView robotImage = (ImageView) V.findViewById(R.id.robotStatus);
//checkBTStates();
if(BluetoothAdapter.getDefaultAdapter()==null){
V.setBackgroundColor(getResources().getColor(R.color.orange));
robotImage.setImageResource(R.drawable.red_robot);
activityCallback.updateColors(0);
activityCallback.setConnectionStatus(false);
}
else{
V.setBackgroundColor(getResources().getColor(R.color.red));
robotImage.setImageResource(R.drawable.red_robot);
activityCallback.updateColors(2);
activityCallback.setConnectionStatus(false);
}
final Button button = (Button) V.findViewById(R.id.button);
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
checkBTStates();
}
});
return V;
}
private void checkBTStates() {
BluetoothAdapter btAdapter = activityCallback.getBTAdapter();
View currentView = activityCallback.getView(R.id.home_fragment);
ImageView robotImage = (ImageView) activityCallback.getView(R.id.robotStatus);
// Check for Bluetooth support and then check to make sure it is turned on
if(btAdapter==null) {
Toast toast = Toast.makeText(activityCallback.getContext(), "Fatal Error: Bluetooth doesn't seem to be supported on your device! :(", Toast.LENGTH_LONG);
toast.show();
currentView.setBackgroundColor(getResources().getColor(R.color.orange));
activityCallback.updateColors(0);
activityCallback.setConnectionStatus(false);
robotImage.setImageResource(R.drawable.red_robot);
}
else {
if (btAdapter.isEnabled()) {
connectBT();
}
else {
//Prompt user to turn on Bluetooth
Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
startActivityForResult(enableBtIntent, BT_ACTIVITY);
}
}
}
BluetoothSocket btSocket = activityCallback.getBTSocket();
private void connectBT(){
BluetoothAdapter btAdapter = activityCallback.getBTAdapter();
View currentView = activityCallback.getView(R.id.home_fragment);
ImageView robotImage = (ImageView) activityCallback.getView(R.id.robotStatus);
// Two things are needed to make a connection:
// A MAC address, which we got above.
// A Service ID or UUID. In this case we are using the
// UUID for SPP.
// Discovery is resource intensive. Make sure it isn't going on
// when you attempt to connect and pass your message.
btAdapter.cancelDiscovery();
// Establish the connection. This will block until it connects.
try {
btSocket.connect();
Toast toast = Toast.makeText(activityCallback.getContext(), "Connection established and data link opened...", Toast.LENGTH_LONG);
toast.show();
currentView.setBackgroundColor(getResources().getColor(R.color.blue));
activityCallback.updateColors(1);
robotImage.setImageResource(R.drawable.green_robot);
activityCallback.setConnectionStatus(true);
} catch (IOException e) {
try {
currentView.setBackgroundColor(getResources().getColor(R.color.red));
activityCallback.updateColors(2);
robotImage.setImageResource(R.drawable.red_robot);
activityCallback.setConnectionStatus(false);
btSocket.close();
} catch (IOException e2) {
Toast toast = Toast.makeText(activityCallback.getContext(),"Fatal Error in onResume() and unable to close socket during connection failure" + e2.getMessage() + ".", Toast.LENGTH_LONG);
toast.show();
}
}
}
@Override
public void onActivityResult(int requestCode, int resultCode, Intent data)
{
super.onActivityResult(requestCode, resultCode, data);
View currentView = activityCallback.getView(R.id.home_fragment);
ImageView robotImage = (ImageView) activityCallback.getView(R.id.robotStatus);
switch(requestCode){
case BT_ACTIVITY:
if(resultCode == -1){
i = 50; //Stop asking the user to enable bluetooth
connectBT();
Toast.makeText(activityCallback.getContext(), "Bluetooth is now on!", Toast.LENGTH_SHORT).show();
}
else if(resultCode == 0){
currentView.setBackgroundColor(getResources().getColor(R.color.red));
activityCallback.updateColors(2);
robotImage.setImageResource(R.drawable.red_robot);
activityCallback.setConnectionStatus(false);
Toast toast = Toast.makeText(activityCallback.getContext(), "This application requires Bluetooth...Maybe try to turn it on?! ;)", Toast.LENGTH_LONG);
toast.show();
}
break;
}
}
}
活动
package edu.umass.ecs.chalkmaster3000;
public class MainActivity extends FragmentActivity implements HomeFragment.ActivityCommunication , GalleryFragment.ActivityCommunication {
private String[] viewTitles;
private DrawerLayout drawerLayout;
private ListView drawerList;
private CharSequence currentTitle;
private CharSequence drawerTitle;
private ActionBarDrawerToggle drawerToggle;
public boolean connectionStatus = false;
@TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
@Override
public void onCreate(Bundle savedInstanceStates){
super.onCreate(savedInstanceStates);
setContentView(R.layout.activity_main);
currentTitle=drawerTitle=getTitle();
viewTitles = getResources().getStringArray(R.array.view_names);
drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
drawerList = (ListView) findViewById(R.id.left_drawer);
// set a custom shadow that overlays the gallery content when the drawer opens
drawerLayout.setDrawerShadow(R.drawable.drawer_shadow, GravityCompat.START);
drawerList.setAdapter(new ArrayAdapter<String>(this,R.layout.drawer_list_item, viewTitles));
drawerList.setOnItemClickListener(new DrawerItemClickListener());
// enable ActionBar app icon to behave as action to toggle nav drawer
getActionBar().setDisplayHomeAsUpEnabled(true);
getActionBar().setHomeButtonEnabled(true);
drawerToggle = new ActionBarDrawerToggle(
this,
drawerLayout,
R.drawable.ic_drawer,
R.string.drawer_open,
R.string.drawer_close
) {
/* Called when a drawer has settled in a completely closed state. */
public void onDrawerClosed(View view) {
getActionBar().setTitle(currentTitle);
invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
}
/* Called when a drawer has settled in a completely open state. */
public void onDrawerOpened(View drawerView) {
getActionBar().setTitle(drawerTitle);
invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
}
};
/* Set the drawer toggle as the DrawerListener */
drawerLayout.setDrawerListener(drawerToggle);
if (savedInstanceStates == null) {
selectItem(0);
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.main, menu);
return super.onCreateOptionsMenu(menu);
}
/* Called whenever we call invalidateOptionsMenu() */
@Override
public boolean onPrepareOptionsMenu(Menu menu) {
// If the nav drawer is open, hide action items related to the content view
boolean drawerOpen = drawerLayout.isDrawerOpen(drawerList);
menu.findItem(R.id.action_websearch).setVisible(!drawerOpen);
return super.onPrepareOptionsMenu(menu);
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Pass the event to ActionBarDrawerToggle, if it returns
// true, then it has handled the app icon touch event
if (drawerToggle.onOptionsItemSelected(item)) {
return true;
}
// Handle action buttons
switch(item.getItemId()) {
case R.id.action_websearch:
// create intent to perform web search for this planet
Intent intent = new Intent(Intent.ACTION_WEB_SEARCH);
intent.putExtra(SearchManager.QUERY, getActionBar().getTitle());
// catch event that there's no activity to handle intent
if (intent.resolveActivity(getPackageManager()) != null) {
startActivity(intent);
} else {
Toast.makeText(this, "notavail", Toast.LENGTH_LONG).show();
}
return true;
default:
return super.onOptionsItemSelected(item);
}
}
/* The click listener for ListView in the navigation drawer */
private class DrawerItemClickListener implements ListView.OnItemClickListener {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
selectItem(position);
}
}
private void selectItem(int position){
Fragment fragment;
/* Clicked Home Button*/
if(position == 0){
fragment = new HomeFragment();
}
/* Clicked Canvas Button*/
else if(position == 1){
fragment = new CanvasFragment();
}
/* Clicked Canvas Button*/
else if(position == 2){
fragment = new GalleryFragment();
}
/* Clicked Pre-Made Button*/
else if(position == 3){
fragment = new PreMadeFragment();
}
/* Clicked Robot Status Button*/
else{
fragment = new RoboStatusFragment();
}
/* Update Main Activity with new Fragment*/
FragmentManager fragmentManager = getFragmentManager();
fragmentManager.beginTransaction()
.replace(R.id.content_frame, fragment)
.commit();
/* Highlight the icon selected, Close the Navigation Drawer, and Update the Title */
drawerList.setItemChecked(position,true);
setTitle(viewTitles[position]);
drawerLayout.closeDrawer(drawerList);
}
public void updateColors(int color){
if(color == 0)
findViewById(R.id.content_frame).getRootView().setBackgroundColor(getResources().getColor(R.color.orange));
else if(color == 1)
findViewById(R.id.content_frame).getRootView().setBackgroundColor(getResources().getColor(R.color.blue));
else
findViewById(R.id.content_frame).getRootView().setBackgroundColor(getResources().getColor(R.color.red));
}
public UUID getUUID(){
//Well known SPP UUID
final UUID MY_UUID = UUID.fromString("00001101-0000-1000-8000-00805F9B34FB");
return MY_UUID;
}
public String getAddress(){
//MAC Address of the Server BT Adapter
String address = "B8:F6:B1:13:57:B8";
return address;
}
public BluetoothAdapter getBTAdapter(){
BluetoothAdapter btAdapter = BluetoothAdapter.getDefaultAdapter();
return btAdapter;
}
public BluetoothSocket getBTSocket(){
// Set up a pointer to the remote node using it's address.
BluetoothDevice device = this.getBTAdapter().getRemoteDevice(this.getAddress());
BluetoothSocket btSocket = null;
try {
btSocket = device.createRfcommSocketToServiceRecord(this.getUUID());
} catch (IOException e) {
Toast toast = Toast.makeText(this, "Fatal Error in onResume() and socket create failed: " + e.getMessage() + ".", Toast.LENGTH_LONG);
toast.show();
}
return btSocket;
}
public void setConnectionStatus(boolean status){
connectionStatus = status;
}
public boolean getConnectionStatus(){
return connectionStatus;
}
public View getView(int viewID){
return findViewById(viewID);
}
@Override
public void setTitle(CharSequence newTitle){
currentTitle = newTitle;
getActionBar().setTitle(currentTitle);
}
@Override
protected void onPostCreate(Bundle savedInstanceState) {
super.onPostCreate(savedInstanceState);
// Sync the toggle state after onRestoreInstanceState has occurred.
drawerToggle.syncState();
}
@Override
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
drawerToggle.onConfigurationChanged(newConfig);
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data)
{
super.onActivityResult(requestCode, resultCode, data);
}
}
错误
Caused by: java.lang.NullPointerException
at edu.umass.ecs.chalkmaster3000.HomeFragment.<init>(HomeFragment.java:84)
at edu.umass.ecs.chalkmaster3000.MainActivity.selectItem(MainActivity.java:153)
at edu.umass.ecs.chalkmaster3000.MainActivity.onCreate(MainActivity.java:90)
at android.app.Activity.performCreate(Activity.java:5372)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1104)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2267)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2359)
at android.app.ActivityThread.access$700(ActivityThread.java:165)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1326)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:5455)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:525)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1187)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
at dalvik.system.NativeStart.main(Native Method)
感谢您的任何帮助，节日快乐！
您必须
在使用它之前初始化 viewTitles。
viewTitles = new String[];
必须为你工作的currentTitle=drawerTitle=getTitle();？
如果片段与活动分离，则可以在调用getActivity（）时获得nullPointerException
。公共最终活动获取活动 （） 在 API 级别 11 中添加
返回此片段当前关联的活动。
参考这个答案：
安卓获取活动返回空
是的，这是真的。但是你的错误不在Java上，而是来自XML。来自安卓（文档）[http://developer.android.com/training/implementing-导航/导航抽屉.html]
1: The main content view (the FrameLayout above) must be the first child in the
DrawerLayout because the XML order implies z-ordering and the drawer must be
on top of the content.
2: The main content view is set to match the parent view's width and height, because
it represents the entire UI when the navigation drawer is hidden.
因此，从本文档中，您的 XML 应包含抽屉布局，框架布局应包含您的活动主视图
，其顺序是R.id.Content_frame
应为活动布局的视图组或父视图。