如何在Firebase实时数据库（Android开发）中获取特定子级的键值？

Question

如何在Firebase实时数据库（Android开发）中获取特定子级的键值？我想根据所选主题获取特定键的值。假设如果用户选择java，那么我想得到“-MnLZAaa1pmVpRusEG8s”（在蓝色框中）。我试图获取列表中的主题名称和主题唯一键，然后将其与用户选择的字符串进行比较，但不知道为什么会得到空值。我怎么做？

public String getSubjectUuid() {
    List<String> subjectNamesList = new ArrayList<>();
    List<String> subjectUuidList = new ArrayList<>();

    DatabaseReference addedSubjectReference = FirebaseDatabase
            .getInstance()
            .getReference(classCreatorPhoneNo + "/" + "class_list" + "/" + classUuid + "/" + "added_subject");

    addedSubjectReference
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
                    if (snapshot.exists()) {
                        subjectNamesList.add(snapshot.child("subject_name").getValue(String.class));
                        subjectUuidList.add(snapshot.child("subject_uuid").getValue(String.class));
                    }
                }

                @Override
                public void onChildChanged(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
                }

                @Override
                public void onChildRemoved(@NonNull DataSnapshot snapshot) {
                }

                @Override
                public void onChildMoved(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
                }

                @Override
                public void onCancelled(@NonNull DatabaseError error) {
                }
            });

    for (int i = 0; i < subjectNamesList.size(); i++) {
        if (selectedSubject.equals(subjectNamesList.get(i))) {
            classKey = subjectUuidList.get(i);
            break;
        }
    }
    
    return classKey;
}