我的应用程序从Firebase数据库中检索数据。首先，我正在将java对象正确地推送到数据库，并且可以使用片段onActivityCreated中的addListenerForSingleValueEvent方法无任何问题地检索数据。总共我有4个片段，使用ViewPager实现。其中三个片段各包含一个ListView，每个片段中都显示不同类型的数据。下载数据并将其添加到每个片段的ListView中。当用户单击ListView项时，将显示包含和编辑文本的对话框。用户输入一个值并单击“保存”。用户输入的值将被推送到数据库，其中包含其他参数，例如date，jobID等作为另一个Java对象（Status.class）当我推送这个对象时，我使用的是DatabaseReference.CompletionListener，而在onComplete方法中我正在执行一些操作。其中一个操作是重新创建Fragment的Parent活动，以允许根据项目操作状态更新UI。我没想到每次ListView中的一个项目被执行时重新创建父活动是非常有效的。所以我决定使用ValueEventListener而不是SingleValueEventListener，它应该更新该项的ListView。 ValueEventListener可以很好地创建ListView，但是当用户第一次单击任何ListView项目，并且他们输入一个值并单击保存时，应用程序随后崩溃并出现以下错误。当应用程序重新启动时，ListView中的项目现在以不同的方式显示，因为它已被操作。单击任何项目并保存从那时起的数据工作正常。仅当用户第一次保存数据时，应用程序才会崩溃。我不知道是什么导致了它。
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
at android.view.LayoutInflater.from(LayoutInflater.java:233)
at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:181)
at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:166)
at com.project.nolan.john.application.IOAdapter.<init>(IOAdapter.java:0)
at com.project.nolan.john.application.Fragment_Sensors.makeListViewFromDataSnapshot(Fragment_Sensors.java:178)
at com.project.nolan.john.application.Fragment_Sensors.access$000(Fragment_Sensors.java:58)
at com.project.nolan.john.application.Fragment_Sensors$1.onDataChange(Fragment_Sensors.java:141)
at com.google.android.gms.internal.zzegf.zza(Unknown Source)
at com.google.android.gms.internal.zzeia.zzbyc(Unknown Source)
at com.google.android.gms.internal.zzeig.run(Unknown Source)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6682)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410)*
databaseReference.addValueEventListener(new ValueEventListener()
{
@Override
public void onDataChange(DataSnapshot dataSnapshot)
{
mPointsList = new ArrayList<>();
for (DataSnapshot ds : dataSnapshot.getChildren())
{
IO_Point io = ds.getValue(IO_Point.class);
if(io != null)
{
if(io.getSiteUID().equals(mSiteUID) && io.getType().equals("S"))
{
mPointsList.add(io);
}
}
}
if(mSensors_IOPointList != null)
{
mListAdapter = new IOAdapter(getActivity(), mPointsList);
}
mListView.setAdapter(mListAdapter);
}
@Override
public void onCancelled(DatabaseError databaseError)
{
}
});
如果您需要更多信息/代码，请告诉我，我将编辑我的问题。
IO_Point.class
public class IO_Point
{
private String type;
private String lan;
private String os;
private String ionum;
private String label;
private String siteUID;
private Boolean actioned;
private String iouid;
public IO_Point()
{
}
public IO_Point(String type, String lan, String os, String ioNum, String label, String siteUID, Boolean actioned, String iouid)
{
this.type = type;
this.lan = lan;
this.os = os;
this.ionum = ioNum;
this.label = label;
this.siteUID = siteUID;
this.actioned = actioned;
this.iouid = iouid;
}
public String getType()
{
return type;
}
public String getLan()
{
return lan;
}
public String getOS()
{
return os;
}
public String getIONum()
{
return ionum;
}
public String getLabel()
{
return label;
}
public String getSiteUID()
{
return siteUID;
}
public Boolean getActioned()
{
return actioned;
}
public String getIOUID()
{
return iouid;
}
}
{
"IO Points" : {
"-L9fzerDpRXR72ilvRht" : {
"actioned" : true,
"ionum" : "S1",
"iouid" : "-L9fzerDpRXR72ilvRht",
"label" : "Sensor One",
"lan" : "1",
"os" : "11",
"siteUID" : "-L9fz_QYz8KFQtByYOxN",
"type" : "S"
},
"-L9fzij0Kyt1x3F_W7Ea" : {
"actioned" : false,
"ionum" : "I1",
"iouid" : "-L9fzij0Kyt1x3F_W7Ea",
"label" : "Digital One",
"lan" : "1",
"os" : "11",
"siteUID" : "-L9fz_QYz8KFQtByYOxN",
"type" : "D"
},
"-L9fzlCaDhagrIzIzTTI" : {
"actioned" : false,
"ionum" : "D1",
"iouid" : "-L9fzlCaDhagrIzIzTTI",
"label" : "Output One",
"lan" : "1",
"os" : "11",
"siteUID" : "-L9fz_QYz8KFQtByYOxN",
"type" : "O"
},
"-L9g-Ppg9noyYepYRLMq" : {
"actioned" : false,
"ionum" : "S1",
"iouid" : "-L9g-Ppg9noyYepYRLMq",
"label" : "Sensor Two",
"lan" : "1",
"os" : "11",
"siteUID" : "-L9g-Hr-v8zgO08zWGTP",
"type" : "S"
},
"-L9g-XOoOSlv15PKiC0e" : {
"actioned" : false,
"ionum" : "I1",
"iouid" : "-L9g-XOoOSlv15PKiC0e",
"label" : "Digital Two",
"lan" : "1",
"os" : "11",
"siteUID" : "-L9g-Hr-v8zgO08zWGTP",
"type" : "D"
},
"-L9g-k0poRsnDi5ZyHjP" : {
"actioned" : false,
"ionum" : "D1",
"iouid" : "-L9g-k0poRsnDi5ZyHjP",
"label" : "Output Two",
"lan" : "1",
"os" : "11",
"siteUID" : "-L9g-Hr-v8zgO08zWGTP",
"type" : "O"
}
},
"Sites" : {
"-L9fz_QYz8KFQtByYOxN" : {
"inProgress" : true,
"siteAddress" : "Site One Address",
"siteName" : "Site One",
"siteUID" : "-L9fz_QYz8KFQtByYOxN"
},
"-L9g-Hr-v8zgO08zWGTP" : {
"inProgress" : false,
"siteAddress" : "Site Two Address",
"siteName" : "Site Two",
"siteUID" : "-L9g-Hr-v8zgO08zWGTP"
}
}
}
答案 0 :(得分：1)
＆＃34;尝试在空对象引用上调用虚方法＆＃34;意味着你试图用它做某事的这个对象没有被初始化。调试你的程序，你必须找到你松散的地方＆＃34;这个 。检查＆＃34; LayoutInflater.java：233＆＃34; !!
中的对象答案 1 :(得分：1)
您已移除Firebase自定义子键并使用默认
DatabaseReference save = FirebaseDatabase.getInstance().getReference("users");
String uid = save.push().getKey();
save.child(uid).setValue(new user(uid,"Milan"));