java - 似乎无法解决此 NullPointerException

Question

我得到的错误是:

java.lang.NullPointerException at com.me.app.MainActivity.onCreate(MainActivity.java:34)

MainActivity.java中第34-39行如下:

light_switch.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        turn_torch_on();
    }
});

这让我相信 light_switch 正在返回 null，尽管在​​第 31 行:

light_switch = (ImageButton)findViewById(R.id.light_switch);

我仔细检查了 ID 是否正确。有什么我想念的吗？我不太确定目前我能做些什么来解决这个问题。

异常日志可在此处获得:http://pastebin.com/raw.php?i=yDPRjvry

主 Activity .java

package com.me.app;

import android.app.Activity;
import android.app.AlertDialog;
import android.app.Fragment;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageButton;

public class MainActivity extends Activity {

ImageButton light_switch;

private boolean has_flash;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    if (savedInstanceState == null) {
        getFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }

    // get the light switch
    light_switch = (ImageButton)findViewById(R.id.light_switch);

    // make the light switch work
    light_switch.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            turn_torch_on();
        }
    });

    // check if device has flash capability
    has_flash = getApplicationContext().getPackageManager().hasSystemFeature(PackageManager.FEATURE_CAMERA_FLASH);

    // alert user that their device is not supported
    if ( !has_flash ) {
        AlertDialog alert = new AlertDialog.Builder(MainActivity.this).create();
        alert.setTitle("Sorry :(");
        alert.setMessage("Your device does not have a flash");
        alert.show();
        return;
    }

}

/**
 * turn torch on
 */
private void turn_torch_on() {
    // change the layout to off
    toggle_layout();
}

/**
 * toggle layout for on/off
 */
private void toggle_layout() {
    light_switch.setBackgroundResource(R.drawable.button_on);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        return rootView;
    }
}

}

fragment _main.xml

<RelativeLayout 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:background="@drawable/bg_off"
    tools:context=".MainActivity$PlaceholderFragment">

    <ImageButton
        android:id="@+id/light_switch"
        android:layout_height="100dp"
        android:layout_width="fill_parent"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:src="@drawable/button_off"
        android:contentDescription="On/Off switch" />

</RelativeLayout>

Answer 1

您正在 PlaceholderFragment 的 onCreateView 上膨胀 fragment_main.xml(其中包含您的 ImageButton)。试试这个:

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_main, container, false);
 // get the light switch
    light_switch = (ImageButton)rootView.findViewById(R.id.light_switch);

 // make the light switch work
     light_switch.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        turn_torch_on();
    }
});
    return rootView;
}