我正在尝试从我的 firebase 中检索 2 个数据分支,但我得到了一个 nullpointException,我不知道为什么
这是数据库
获取 cooking 时间和类别的代码
FirebaseDatabase.getInstance().getReference().child( "recipes" )
.addListenerForSingleValueEvent( new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
/*Recipe recipe = snapshot.getValue( Recipe.class );*/
DatabaseReference recipes = FirebaseDatabase.getInstance().getReference("recipes");
Recipe recipe_test = snapshot.child( "recipe_1" ).getValue(Recipe.class);
category.setText( recipe_test.getCategory() );
cookinTime.setText( recipe_test.getCookingtime() );
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
} );
}
配方类
package com.example.myapplicationdatatest;
import java.lang.reflect.Array;
import java.util.List;
import java.util.Map;
public class Recipe {
private String category;
private long cookingtime;
private String description;
private Map<String, Object> amountOfIngredients;
private String nameOfRecipe;
private Map<String, Object> Ingredients;
public Recipe() {};
public Recipe(String category, long cookingtime, String description,Map<String, Object> amountOfIngredients, String nameOfRecipe, Map<String, Object> Ingridients) {
this.category=category;
this.cookingtime=cookingtime;
this.description=description;
this.amountOfIngredients=amountOfIngredients;
this.nameOfRecipe=nameOfRecipe;
this.Ingredients=Ingridients;
}
public String getCategory() {
return category;
}
public String getCookingtime() {
long ct = cookingtime;
String result = String.valueOf( ct );
return result;
}
public String getDescription() {
return description;
}
public Map<String, Object> getAmountOfIngredients() {
return amountOfIngredients;
}
public String getNameOfRecipe() {
return nameOfRecipe;
}
public Map<String, Object> getIngredients() {
return Ingredients;
}
}
和异常
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.myapplicationdatatest, PID: 20846
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.myapplicationdatatest.Recipe.getCategory()' on a null object reference
at com.example.myapplicationdatatest.MainActivity$1.onDataChange(MainActivity.java:73)
at com.google.firebase.database.Query$1.onDataChange(com.google.firebase:firebase-database@@19.1.0:179)
at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@19.1.0:75)
at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@19.1.0:63)
at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@19.1.0:55)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
I/Process: Sending signal. PID: 20846 SIG: 9
Disconnected from the target VM, address: 'localhost:8611', transport: 'socket'
我为数据库创建了一个有效的食谱类,因为我之前在数据库中只有一个食谱时尝试过
谢谢你的帮助
最佳答案
使用.getValue()
时,注意:
This method is used to marshall the data contained in this snapshot into a class of your choosing. The class must fit 2 simple constraints:
- The class must have a default constructor that takes no arguments
- The class must define public getters for the properties to be assigned. Properties without a public getter will be set to their default value when an instance is deserialized
我建议在定义类变量后,只需在 Android Studio 中执行 Alt+Insert
即可生成公共(public) getters-setters。
此外,正如弗兰克在他的回答中指出的那样,您将通过执行以下操作在“食谱”下收到所有食谱:
FirebaseDatabase.getInstance().getReference().child( "recipes" )
.addListenerForSingleValueEvent( new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Recipe recipe_test = snapshot.getValue( Recipe.class );
}
});
现在在 for-each 循环中,您需要使用这些值。例如,当您想要具有 key = "recipe_2"的快照时,您需要这样做:
if( snapshot.getKey().equals("recipe_2")) {
//do your stuff
}
完整代码:
FirebaseDatabase.getInstance().getReference().child( "recipes" )
.addListenerForSingleValueEvent( new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
if( snapshot.getKey().equals("recipe_2")) {
//do your stuff
Recipe recipe_2 = snapshot.getValue( Recipe.class );
}
}
});
关于android - 从android中的firebase数据库中检索 child ,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/57925507/