android null 對象引用，盡管存在

Question

我有一個 MainActivity，它基本上有一個框架布局，根據用戶正在做什么，它有不同的片段。

活動_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:orientation="vertical" tools:context=".MainActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="#27b"
        android:layout_weight=".04">
        <FrameLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:id="@+id/activity_main_framelayout">
        </FrameLayout>
    </LinearLayout>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight=".9"
        android:orientation="horizontal">
        <TextView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:background="#1B5F96"
            android:layout_weight=".9"
            android:id="@+id/activity_main_status_title"
            android:text="@string/activity_main_status_title"
            tools:ignore="NestedWeights" />
        <TextView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:background="#AD3333"
            android:layout_weight=".15"
            android:id="@+id/activity_main_status_value"
            android:text="@string/activity_main_status_value"/>
    </LinearLayout>
</LinearLayout>

問題始於一個接口方法void WillYouAccept(final String caller)

@Override
    public void WillYouAccept(final String caller) {
        Connection f = new Connection();
        ShiftView(f);

        runOnUiThread(new Runnable() {
            @Override
            public void run() {
                Connection b = (Connection) getFragmentManager().findFragmentById(R.layout.connection_display);
                b.updateInitiator(caller);
            }
        });
    }

行b.updateInitiator(caller); 是什么引發了空對象引用錯誤。 ShiftView基本上已經為我將片段交換到框架布局中，到目前為止運行良好。

@Override
    public void ShiftView(Object obj) {
        FragmentTransaction ft = getFragmentManager().beginTransaction();
        ft.replace(R.id.activity_main_framelayout, (Fragment) obj);
        ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
        ft.addToBackStack(null);
        ft.commit();
    }

基本上需要從 MainActivity 更新 connection_display.xml(Connection.java) 中的 textview，並且以某種方式initiator = (TextView) view.findViewById(R.id.textView_connection_display_initiator_ID); 從 MainActivity 與之交互時為 null。

連接.java

package "";

import android.app.Activity;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;

public class Connection extends Fragment {
    MiddleMan mCallBack;

    Button accept;
    Button deny;
    TextView initiator;

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        try {
            mCallBack = (MiddleMan) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString() + " must implement ReqestConnect");
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.connection_display, container, false);
        mCallBack.DisplayHome();
        initiator = (TextView) view.findViewById(R.id.textView_connection_display_initiator_ID);
        accept = (Button) view.findViewById(R.id.button_connection_display_ACCEPT);
        accept.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                buttonAccept();
            }
        });
        deny = (Button) view.findViewById(R.id.button_connection_display_DENY);
        deny.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                buttonDeny();
            }
        });
        return view;
    }

    private void buttonAccept() {
        System.out.println("Accept Button Pressed");
    }

    private void buttonDeny() {
        System.out.println("Deny Button Pressed");
    }

    public void updateInitiator(final String s) {
        initiator.setText(s);
    }
}

connection_display.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent" android:layout_height="match_parent"
    android:orientation="vertical">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="@string/textView_connection_display_title"
        android:id="@+id/textView_connection_display_title" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:orientation="horizontal"
        android:layout_marginTop="45dp"
        android:layout_weight=".004">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="65dp"
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:text="@string/textView_connection_display_ID_label"
            android:id="@+id/textView_connection_display_ID_label" />

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textAppearance="?android:attr/textAppearanceMedium"
            android:id="@+id/textView_connection_display_initiator_ID"
            android:text="@string/textView_connection_display_initiator_ID" />
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:orientation="horizontal"
        android:layout_weight=".004">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="15dp"
            android:textAppearance="?android:attr/textAppearanceSmall"
            android:text="@string/textView_connection_display_exp"
            android:id="@+id/textView_connection_display_exp" />

    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:orientation="horizontal"
        android:layout_weight=".04">

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="100dp"
            android:text="@string/button_connection_display_ACCEPT"
            android:id="@+id/button_connection_display_ACCEPT"
            tools:ignore="ButtonStyle" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/button_connection_display_DENY"
            android:id="@+id/button_connection_display_DENY"
            tools:ignore="ButtonStyle" />

    </LinearLayout>

</LinearLayout>

Answer 1

b 肯定會變成空值，但是上面的位總是找到片段，所以我用這個解決了它：

@Override
    public void WillYouAccept(final String caller) {
        final Connection f = new Connection();
        ShiftView(f);
        runOnUiThread(new Runnable() {
            @Override
            public void run() {
                f.updateInitiator(caller);
            }
        });
    }