这是我的代码示例，希望对您有所帮助

loginBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            final String phoneTxt = phone.getText().toString();
            final String passwordTxt = password.getText().toString();

            if (phoneTxt.isEmpty() || passwordTxt.isEmpty()){
                Toast.makeText(Login.this, "Please Enter Your Phone Number Or Password", Toast.LENGTH_SHORT).show();
            }else {

                databaseReference.child("users").addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot snapshot) {
                        if (snapshot.hasChild(phoneTxt)){
                            final String getpassword = snapshot.child(phoneTxt).child("password").getValue(String.class);

                            if (getpassword.equals(passwordTxt)){
                                Toast.makeText(Login.this, "Successfully login! ", Toast.LENGTH_SHORT).show();
                                startActivity(new Intent(Login.this,HomeScreen.class));
                                finish();
                            }
                            else{
                                Toast.makeText(Login.this, "Wrong Password", Toast.LENGTH_SHORT).show();
                            }
                        }
                        else {
                            Toast.makeText(Login.this, "Wrong Phone Number!", Toast.LENGTH_SHORT).show();
                        }
                    }

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

                    }
                });
            }
        }
    });

为了能够检查数据库中是否存在特定用户，您应该只执行get（），而不附加ValueEventListener。假设用户是Firebase实时数据库根目录的直接子级，在代码中，将非常简单：

DatabaseReference db = FirebaseDatabase.getInstance().getReference();
String usuario = Usuario.getText().toString();
DatabaseReference usuarioRef = db.child(usuario);
usuarioRef.get().addOnCompleteListener(new OnCompleteListener<DataSnapshot>() {
    @Override
    public void onComplete(@NonNull Task<DataSnapshot> task) {
        if (task.isSuccessful()) {
            DataSnapshot snapshot = task.getResult();
            if(snapshot.exists()) {
                Log.d("TAG", "User exists.");
            } else {
                Log.d("TAG", "User doesn't exist.");
            }
        } else {
            Log.d("TAG", task.getException().getMessage()); //Never ignore potential errors!
        }
    }
});