在Firebase中检索嵌套数据-Android

我刚刚进入了Firebase和android的世界。我遵循在线教程来设置和获取非嵌套数据，并将其显示在列表视图中，如下所示：

之前的数据：

    mListView = (ListView) findViewById(R.id.ListView) ;
    final ArrayAdapter arrayAdapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, mStudents);
    mListView.setAdapter(arrayAdapter);
    FirebaseDatabase database = FirebaseDatabase.getInstance();
    myRef = database.getReference().child("Student1");
    myRef.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            String value = dataSnapshot.getValue(String.class);
            String key = dataSnapshot.getKey();
            String totl = key + ": " + value;
            mStudents.add(totl);
            arrayAdapter.notifyDataSetChanged();
        }
    });

我向数据库添加了带有子字段的成绩字段。我想显示整个学生1数据以及成绩。当我尝试运行上面的代码时。我收到以下错误。

数据后：

FATAL EXCEPTION: main
              Process: com.adcpnmd.mystudents, PID: 3524
              com.google.firebase.database.DatabaseException: Failed to convert value of type java.util.HashMap to String
                  at com.google.android.gms.internal.zzbqi.zzaD(Unknown Source)
                  at com.google.android.gms.internal.zzbqi.zzb(Unknown Source)
                  at com.google.android.gms.internal.zzbqi.zza(Unknown Source)
                  at com.google.firebase.database.DataSnapshot.getValue(Unknown Source)
                  at com.adcpnmd.mystudents.Main$1.onChildAdded(Main.java:39)
                  at com.google.android.gms.internal.zzblz.zza(Unknown Source)
                  at com.google.android.gms.internal.zzbnz.zzYj(Unknown Source)
                  at com.google.android.gms.internal.zzboc$1.run(Unknown Source)
                  at android.os.Handler.handleCallback(Handler.java:751)
                  at android.os.Handler.dispatchMessage(Handler.java:95)
                  at android.os.Looper.loop(Looper.java:154)
                  at android.app.ActivityThread.main(ActivityThread.java:6077)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

访问全部数据并将其添加到ListView时应该怎么做？还是我构造的数据不正确？

提前致谢。

尝试像这样更改您的firebase代码：

myRef = database.getReference().child("Student1").child("Grades");
myRef.addChildEventListener(new ChildEventListener() {
    @Override
    public void onChildAdded(DataSnapshot dataSnapshot, String s) {
        if (dataSnapshot.exists()) {
            String value = dataSnapshot.getValue(String.class);
            String key = dataSnapshot.getKey();
            String totl = key + ": " + value;
            mStudents.add(totl);
            arrayAdapter.notifyDataSetChanged();
        }
    }
});

更新资料

要访问整个节点Student1，您可以这样做：

首先，我将您的Class字段重命名为Course，因为这Class可能会引起混乱和问题（Android本身会使用它）。因此Student.java，请使用代表您数据的普通对象创建一个Java文件：

public class Student {

    private String Course;
    private String Name;
    private String Percentile;
    private HashMap Grades;

    public Student() {

    }

    public Student(String course, String name, String percentile, HashMap grades) {
        Course = course;
        Name = name;
        Percentile = percentile;
        Grades = grades;
    }

    public String getCourse() {
        return Course;
    }

    public String getName() {
        return Name;
    }

    public String getPercentile() {
        return Percentile;
    }

    public HashMap getGrades() {
        return Grades;
    }
}

接下来，通过以下方式获取数据：

ValueEventListener valueEventListener = new ValueEventListener() {

            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                Student student = dataSnapshot.getValue(Student.class);

                Log.d("TAG","name: "+student.getName());
                Log.d("TAG","percentile: "+student.getPercentile());
                Iterator it = student.getGrades().entrySet().iterator();
                while (it.hasNext()) {
                    Map.Entry pair = (Map.Entry)it.next();
                    Log.d("TAG","grade: "+pair.getKey() +  " = "  + pair.getValue());
                    it.remove(); // avoids a ConcurrentModificationException
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        };

myRef = database.getReference().child("Student1");
myRef.addListenerForSingleValueEvent(valueEventListener);