我在Recyclerview中显示了从Firebase数据库中检索到的图像。我正在尝试实现字母索引，以便当我单击一个字母时，将检索图像的名称。然后我应用子字符串来获取名称的第一个字符，使用equals（）检查字符是否等于单击的字母，如果不相等则从回收器视图中删除该项。问题是当我应用子字符串时会产生空指针异常。我知道我成功检索了图像的名称，因为我可以使用Toast在屏幕上显示它。当我尝试获取子字符串时，我不知道是什么原因导致空指针异常。任何帮助将不胜感激。
以下是主要活动的代码 - 问题出在updateLogoDisplay（）：
public class AddBrandPage extends AppCompatActivity implements OnClickListener {
//declare variables
private RecyclerView recyclerView;
private RecyclerView.LayoutManager layoutManager;
private RecyclerView.Adapter adapter;
private RecyclerView alpharecyclerView;
private RecyclerView.LayoutManager alphaLayoutManager;
private RecyclerView.Adapter alphaAdapter;
private DatabaseReference myRef;
private DatabaseReference userRef;
private Button btn_skip;
private Button btn_save;
private CheckBox checkbox;
private FirebaseAuth mAuth;
private String t;
List<LogoItems> brandLogo = new ArrayList<>();
//HashMap<String, String> saveBrands = new HashMap<>();
List<LogoItems> removedBrandLogo = new ArrayList<>();
List<String> selectionList = new ArrayList<>();
List<AlphaItems> alphaList = new LinkedList<>();
String [] alphabets = {"All","A","B", "C", "D", "E", "F", "G", "H", "I",
"J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"};
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_add_brand_page);
//initialize variables
btn_skip = (Button) findViewById(R.id.btn_skip);
btn_save = (Button) findViewById(R.id.btn_save);
myRef = FirebaseDatabase.getInstance().getReference().child("/brands");
userRef = FirebaseDatabase.getInstance().getReference().child("/users");
// checkbox = (CheckBox) findViewById(R.id.checkbox);
//calls to load data to arraylists
//alpha = getResources().getStringArray(alphabets);
loadAlpha();
loadLogoImgData();
//set the listener for the buttons click event
btn_skip.setOnClickListener(this);
btn_save.setOnClickListener(this);
}
private void loadAlpha() {
for (String alpha: alphabets) {
alphaList.add(new AlphaItems(alpha));
}
startAlphaRecyclerView();
}
public void loadLogoImgData() {
brandLogo.clear();
myRef.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot brandSnapshot : dataSnapshot.getChildren()) {
LogoItems value = brandSnapshot.getValue(LogoItems.class);
brandLogo.add(value);
}
startLogoRecyclerView();
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
@Override
public void onClick(View view) {
if (view == btn_skip) {
//if skip button clicked close current window and go to user main page
finish();
startActivity(new Intent(getApplicationContext(), UserMainPage.class));
}
if (view == btn_save) {
//selector.addAll(logoadapter.selector);
saveData();
}
}
public void startLogoRecyclerView() {
// set the main recyclerview view for the logo in the layout
recyclerView = (RecyclerView) findViewById(R.id.recylView);
recyclerView.setHasFixedSize(true);
// set the main layoutManager of the recyclerview
layoutManager = new GridLayoutManager(this, 2);
recyclerView.setLayoutManager(layoutManager);
// set the recycler view adapter
adapter = new LogoAdapter(brandLogo, getBaseContext(), AddBrandPage.this);
recyclerView.setAdapter(adapter);
}
public void startAlphaRecyclerView() {
// set the main recyclerview view for the logo in the layout
alpharecyclerView = (RecyclerView) findViewById(R.id.alpharecyclerView);
alpharecyclerView.setHasFixedSize(true);
// set the main layoutManager of the recyclerview
alphaLayoutManager = new LinearLayoutManager(this,LinearLayoutManager.HORIZONTAL,false);
alpharecyclerView.setLayoutManager(alphaLayoutManager);
// set the recycler view adapter
alphaAdapter = new AlphaAdapter(alphaList, getBaseContext(), AddBrandPage.this);
alpharecyclerView.setAdapter(alphaAdapter);
}
public List<String> prepareSelection(View v, int position) {
checkbox = (CheckBox) v;
//check if user selected checkbox and add or remove from list
//if (checkbox.isChecked()) {
selectionList.add(brandLogo.get(position).getName());
//} else {
// selectionList.remove(brandLogo.get(position).getLogo());
//}
return selectionList;
}
public void updateLogoDisplay(String letter) {
if (!(letter.equals("All"))) {
//Iterator<LogoItems> iter = brandLogo.iterator();
for (Iterator<LogoItems> iter = brandLogo.iterator(); iter.hasNext();){
//while (iter.hasNext()) {
LogoItems r = iter.next();
String c = r.getName();
String t = c.substring(0,1);
if (c != null){
if (letter.equals(t)) {
//brandLogo.remove(r);
removedBrandLogo.add(r);
iter.remove();
}
}else {
}
}
brandLogo.removeAll(removedBrandLogo);
adapter.notifyDataSetChanged();
}
}
public void saveData() {
final FirebaseAuth mAuth;
final DatabaseReference userRef;
mAuth = FirebaseAuth.getInstance();
userRef = FirebaseDatabase.getInstance().getReference().child("users");
DatabaseReference curUser = userRef.child(mAuth.getCurrentUser().getUid());
curUser.child("brands").setValue(selectionList);//save selected items to the database
}
}
以下是从中调用updateLogoDisplay（）的recycler视图适配器的代码。
public class AlphaAdapter extends RecyclerView.Adapter<AlphaAdapter.AlphaViewHolder> {
//declare variables
List<AlphaItems> alphaList = new ArrayList<>();
private AddBrandPage addBrandPage;
private Context context;
//the constructor
public AlphaAdapter (List<AlphaItems> alphaList, Context context, AddBrandPage addBrandPage){
this.alphaList = alphaList;
this.context = context;
this.addBrandPage = addBrandPage;
}
@Override
public AlphaAdapter.AlphaViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.alpha_items, parent, false);
AlphaViewHolder alphaViewHolder = new AlphaViewHolder(view,addBrandPage);
return alphaViewHolder;
}
@Override
public void onBindViewHolder(AlphaAdapter.AlphaViewHolder holder, int position) {
holder.txt_alpha.setText(alphaList.get(position).getLetter());
}
@Override
public int getItemCount() {
return alphaList.size();
}
public class AlphaViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener{
//declare variables
private TextView txt_alpha;
private AddBrandPage addBrandPage;
//the constructor
public AlphaViewHolder (View itemView, AddBrandPage addBrandPage){
super(itemView);
this.addBrandPage = addBrandPage;
//initialize variables
txt_alpha = (TextView) itemView.findViewById(R.id.txt_alpha);
//set click listener
txt_alpha.setOnClickListener(this);
}
@Override
public void onClick(View v) {
int position = getAdapterPosition();
String letter = alphaList.get(position).toString();
addBrandPage.updateLogoDisplay(letter);
}
}
}
这是logcat错误：
06-15 15:42:10.306 30577-30577/com.test.test D/AndroidRuntime: Shutting down VM
06-15 15:42:10.307 30577-30577/com.test.test E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.test.test, PID: 30577
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.substring(int)' on a null object reference
at com.test.test.AddBrandPage.updateLogoDisplay(AddBrandPage.java:253)
at com.test.test.AlphaAdapter$AlphaViewHolder.onClick(AlphaAdapter.java:78)
at android.view.View.performClick(View.java:5637)
at android.view.View$PerformClick.run(View.java:22429)
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:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
06-15 15:46:57.570 30577-30626/com.test.test W/DynamiteModule: Local module descriptor class for com.google.firebase.auth not found.
答案 0 :(得分：0)
我能够解决问题。当迭代到达结束时，变量c变为空。这就是产生空指针异常的原因。为了避免null错误，我在String t = ... line之前添加了if（c！= null）。