本文介绍了使用Android从Firebase中检索数据无法正常工作的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
Constants.USER_REF.orderByChild（email ）.equalTo（email）.addListenerForSingleValueEvent（new ValueEventListener（）{
@Override $ b $ public void onDataChange（DataSnapshot dataSnapshot）{$ b $ if（dataSnapshot.exists（）&& dataSnapshot！ null）{
//此时存在用户，将其存储为currentUser变量
用户currentUser = dataSnapshot.getValue（User.class）;
Log.i（ THE_SNAPSHOT_AS_STRING :::，dataSnapshot.toString（））;
//获取下面行的错误！
Log.i（THE_USERS_EMAIL :::，currentUser.getEmail（ ））;
} else {
//此时用户不存在
Toast.makeText（TourContactActivity.this，No user exists。，Toast.LENGTH_SHORT ）.show（）;
}
}
@Override
public void onCancelled（FirebaseError firebaseError）{
$ b}）;
我也在控制台日志中获得以下内容，所以我知道它正在拉入正确的用户。
07-13 15：16：18.391 32277-32277 / thepassapp.thompson.com.tomstours I / THE_SNAPSHOT_AS_STRING :::: DataSnapshot {key = users，value = {00845752-985f-4779-8eff-0c1e6a016ad8 = {tour_director_key = 70af128e-777b-4c12-86f6-2952dbdc9185，last_name = River，[email protected]，location_latitude = 42.4305，tour_id = n1337e，photo = http：//chopu.herokuapp.com/parse/files/2enxs2j21mz9/7304a8917568d366a8ec43f5ab88ac6b_user.jpg，tour_director_name = Chuck Thoms，middle_name =，location_longitude = -73.5103，passenger_id = mcu79，location_updated = 1468360656387，tour_director = f9e3z，first_name = Gene，provider = password}}}
这是我的User.class
@JsonIgnoreProperties（ignoreUnknown = true）
public class User {
private String uid;
私人字符串电子邮件;
private String first_name;
private String last_name;
public User（）{
//空的默认构造函数，Firebase必须能够反序列化用户
}
public User（String uid，String first_name，String last_name，String email）{
this.uid = uid;
this.first_name = first_name;
this.last_name = last_name;
this.email = email;
}
public String getEmail（）{
return email;
public String getFirstName（）{
return first_name;
}
public String getLastName（）{
return last_name;
$ b @Override
public String toString（）{
returnUser {uid ='+ uid +'，email ='+ email +' ，first_name ='+ first_name +'，last_name ='+ last_name +\'};
$ b $ / code $ / pre
$ b $ p我的问题是实际上从User类中检索这些数据。我跟着文档，但不断收到错误：
致命例外：main
过程：passengerapp .veriguide.com.veriguidetours，PID：32277
java.lang.NullPointerException：println需要一个消息
任何想法我做错了？注意：我评论了我获得错误的行。
解决方案不能直接调用 getValue （），因为dataSnapshot有childs，所以你需要在得到值之前迭代子。
@Override $ b $ public void onDataChange（DataSnapshot dataSnapshot）{$ b $ if（dataSnapshot.exists（））{
for（DataSnapshot data：dataSnapshot.getChildren（））{
User currentUser = data.getValue（User.class）;
Log.i（THE_SNAPSHOT_AS_STRIN，data.toString（））;
Log.i（THE_USERS_EMAIL :::，currentUser.getEmail（））;
}
} else {
//此时用户不存在。
$ b 此外，你必须为你的 User class，所以这个值可以设置为类。
public void setEmail（String email）{
this.email = email;
}
public void setFirst_name（String first_name）{
this.first_name = first_name;
}
public void setLast_name（String last_name）{
this.last_name = last_name;
...
希望这有助于：）
I'm attempting to obtain the current user's details by doing so:
Constants.USER_REF.orderByChild("email").equalTo(email).addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
if (dataSnapshot.exists() && dataSnapshot != null) {
// User exists at this point, store it as currentUser variable.
User currentUser = dataSnapshot.getValue(User.class);
Log.i("THE_SNAPSHOT_AS_STRING:::", dataSnapshot.toString());
// GETTING THE ERROR ON THE LINE BELOW!
Log.i("THE_USERS_EMAIL:::", currentUser.getEmail());
} else {
// User does not exist at this point.
Toast.makeText(TourContactActivity.this, "No user exists.", Toast.LENGTH_SHORT).show();
}
}
@Override
public void onCancelled(FirebaseError firebaseError) {
}
});
I also get the following printed in the console log, so I know it's pulling in the proper user.
07-13 15:16:18.391 32277-32277/thepassapp.thompson.com.tomstours I/THE_SNAPSHOT_AS_STRING:::: DataSnapshot { key = users, value = {00845752-985f-4779-8eff-0c1e6a016ad8={tour_director_key=70af128e-777b-4c12-86f6-2952dbdc9185, last_name=River, [email protected], location_latitude=42.4305, tour_id=n1337e, photo=http://chopu.herokuapp.com/parse/files/2enxs2j21mz9/7304a8917568d366a8ec43f5ab88ac6b_user.jpg, tour_director_name=Chuck Thoms, middle_name=, location_longitude=-73.5103, passenger_id=mcu79, location_updated=1468360656387, tour_director=f9e3z, first_name=Gene, provider=password}} }
Here is my User.class
@JsonIgnoreProperties(ignoreUnknown=true)
public class User {
private String uid;
private String email;
private String first_name;
private String last_name;
public User() {
// empty default constructor, necessary for Firebase to be able to deserialize users
}
public User(String uid, String first_name, String last_name, String email) {
this.uid = uid;
this.first_name = first_name;
this.last_name = last_name;
this.email = email;
}
public String getEmail() {
return email;
}
public String getFirstName() {
return first_name;
}
public String getLastName() {
return last_name;
}
@Override
public String toString() {
return "User{uid='"+uid+"', email='"+email+"', first_name='"+first_name+"', last_name='"+last_name+"\'}";
}
}
My problem is actually retrieving this data from the User class. I followed the docs, but keep getting the error:
FATAL EXCEPTION: main
Process: passengerapp.veriguide.com.veriguidetours, PID: 32277
java.lang.NullPointerException: println needs a message
Any ideas on what I'm doing wrong? Note: I commented the line I'm obtaining the error on.
解决方案 You cannot directly call getValue() because the dataSnapshot has childs, so you need to iterate the child before getting the value.
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
if (dataSnapshot.exists()) {
for (DataSnapshot data : dataSnapshot.getChildren()) {
User currentUser = data.getValue(User.class);
Log.i("THE_SNAPSHOT_AS_STRIN", data.toString());
Log.i("THE_USERS_EMAIL:::", currentUser.getEmail());
}
} else {
// User does not exist at this point.
}
}
And also, you have to create setter for your User class so the value can be set to the class.
public void setEmail(String email) {
this.email = email;
}
public void setFirst_name(String first_name) {
this.first_name = first_name;
}
public void setLast_name(String last_name) {
this.last_name = last_name;
}
...
Hope this helps :)
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