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示範

XML

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/t1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/t2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="16dp"
        android:layout_marginRight="16dp"
        app:layout_constraintEnd_toEndOf="parent"
        tools:layout_editor_absoluteY="72dp" />

    <Button
        android:id="@+id/b2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:onClick="b2"
        android:text="刪除"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/t1" />

    <Button
        android:id="@+id/b3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="b3"
        android:text="新增"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/b2" />

    <Button
        android:id="@+id/b4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="b4"
        android:text="更新"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/b3" />
</android.support.constraint.ConstraintLayout>

JAVA

public class MainActivity extends AppCompatActivity {
TextView t1,t2;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        t1=(TextView)findViewById(R.id.t1);
        t2=(TextView)findViewById(R.id.t2);

        final FirebaseDatabase database = FirebaseDatabase.getInstance();//取得資料庫連結
        DatabaseReference myRef= database.getReference("CCCC");

        myRef.addChildEventListener(new ChildEventListener() {//讀取
            @Override
            public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
            }

            @Override
            public void onChildChanged(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
                t1.setText(""+dataSnapshot.getValue());
            }

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

            }

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

            }

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

            }
        });
    }
    public void b2(View v){//刪除
        FirebaseDatabase database = FirebaseDatabase.getInstance();
        DatabaseReference myRef = database.getReference("CCCC");
        myRef.child("ABCD").removeValue();
    }
    public void b3(View v){//新增
        final FirebaseDatabase database = FirebaseDatabase.getInstance();//取得資料庫連結
        DatabaseReference myRef= database.getReference("CCCC");
        myRef.child("ABCD").setValue("224466");
    }
    public void b4(View v){//更新
        FirebaseDatabase database = FirebaseDatabase.getInstance();//取得資料庫連結
        DatabaseReference myRef = null;
        myRef= database.getReference("CCCC");
        Map<String, Object> childUpdates = new HashMap<>();
        childUpdates.put("DCBA","123456777");//前面的字是child後面的字是要修改的value值
        myRef.updateChildren(childUpdates);
    }
}

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