获取现有的Firebase子密钥

Question

我想检查日期键是否已经存在，然后显示Toast否则上载考勤。如果考勤子键已经存在。

DBreference.addChildEventListener(new ChildEventListener() {
               @Override
               public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

                   //i want to check if data is available along with the key which is "date".
                   //then show msg data already exists else upload the attendance

                   date = new SimpleDateFormat("yyyy-MM-dd", Locale.getDefault()).format(new Date());
                   String present = "Present";
                   DBreference.child("Attendance").child(date).setValue(present);
                   Toast.makeText(ProfileAct.this, "Marked as Present", Toast.LENGTH_SHORT).show();
               }

               @Override
               public void onChildChanged(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

               }

               @Override
               public void onChildRemoved(@NonNull DataSnapshot dataSnapshot) {

               }

               @Override
               public void onChildMoved(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

               }

               @Override
               public void onCancelled(@NonNull DatabaseError databaseError) {

               }
           });