java为什么当我返回应用程序时这些对象是空的？

Question

我有一个主要活动，有一个表格和几个片段。创建应用程序后，一切正常，但是，当应用程序在后台运行并返回时，会出现nullpointer异常。我在片段的OnCreateView方法中创建的对象为null

这是我的FragmentPagerAdapter课程：

public class SectionsPagerAdapter extends FragmentPagerAdapter {
    private FragmentStrobe fragmentStrobe = null;
    private FragmentFlashLight fragmentFlashLight = null;
    private FragmentDisco fragmentDiscoLight = null;
    private FragmentShaking fragmentShaking = null;
    private FragmentPoliceLights fragmentPoliceLights = null;

    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0:
                if(fragmentDiscoLight == null){
                    fragmentDiscoLight = new FragmentDisco();
                }
                return fragmentDiscoLight;
            case 1:
                if(fragmentStrobe == null){
                    fragmentStrobe = new FragmentStrobe();
                }
                return fragmentStrobe;
            case 2:
                if(fragmentFlashLight == null){
                    fragmentFlashLight = new FragmentFlashLight();
                }
                return fragmentFlashLight;
            case 3:
                if(fragmentShaking == null){
                    fragmentShaking = new FragmentShaking();
                }
                return fragmentShaking;
            case 4:
                if(fragmentPoliceLights == null){
                    fragmentPoliceLights = new FragmentPoliceLights();
                }
                return fragmentPoliceLights;
            default:
                return null;
        }
    }

    @Override
    public int getCount() {
        return 5;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        return null;
    }
}

在我的MainActivity中，我在片段顶部有几个按钮。单击此按钮时，我正在调用片段中的一个方法

private void initializeButtonStart(){
    buttonStart = (Button) findViewById(R.id.start);

    buttonStart.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            myPreferences.setBoolPreferences(MyPreferences.MY_PREFS_START,
                    !myPreferences.getBoolPreferences(MyPreferences.MY_PREFS_START, MyPreferences.MY_PREFS_START_DEFAULT));

            setIcon();

            ((FragmentInterface) mSectionsPagerAdapter.getItem(tabLayout.getSelectedTabPosition())).onRunPermissionChanged();
        }
    });
}

这是一段代码

public class FragmentDisco extends Fragment implements FragmentInterface {

private static final double IMAGE_RATIO = 1.08158;

private View viewBackground;

private View rootView;

private ImageView imageViewDiscoBall;

private Bitmap bitmap;

private Disco disco;

private MyPreferences myPreferences;

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

    disco = new Disco(getContext());

    myPreferences = new MyPreferences(getContext());

    bitmap = BitmapFactory.decodeResource(getContext().getResources(),R.drawable.disco_start);

    initializeView();
    initializeImageView();

    return rootView;
}

private void initializeView(){
    viewBackground = getActivity().findViewById(R.id.viewMain);
}

private void initializeImageView(){
    imageViewDiscoBall = (ImageView) rootView.findViewById(R.id.discoBall);

    imageViewDiscoBall.setImageBitmap(bitmap);

    imageViewDiscoBall.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
        @Override
        public void onGlobalLayout() {
            imageViewDiscoBall.getViewTreeObserver().removeOnGlobalLayoutListener(this);

            int width = imageViewDiscoBall.getWidth();
            int resultHeight = (int)((float) width * IMAGE_RATIO);

            ViewGroup.LayoutParams layoutParams = imageViewDiscoBall.getLayoutParams();
            layoutParams.height = resultHeight;
            imageViewDiscoBall.setLayoutParams(layoutParams);
        }
    });
}


@Override
public void onResume() {
    super.onResume();
}

@Override
public void onStop() {
    super.onStop();

    stopDisco();
}

@Override
public void onTabSelected() {
    if(myPreferences == null || imageViewDiscoBall == null || viewBackground == null || disco == null){
        return;
    }

    if(myPreferences.getBoolPreferences(MyPreferences.MY_PREFS_START, MyPreferences.MY_PREFS_START_DEFAULT) &&
            (myPreferences.getIntPreferences(MyPreferences.MY_PREFS_CURRENT_FRAGMENT, MyPreferences.MY_PREFS_CURRENT_FRAGMENT_DEFAULT) == MainActivity.FRAGMENT_DISCO)){
        disco.start(viewBackground, imageViewDiscoBall);
    }
}

@Override
public void onTabUnselected() {
    stopDisco();
}

@Override
public void onRunPermissionChanged() {
    if(myPreferences == null || imageViewDiscoBall == null || viewBackground == null || disco == null){
        System.out.println("NULLL !!!!!!");
        return;
    }

    if(myPreferences.getBoolPreferences(MyPreferences.MY_PREFS_START, MyPreferences.MY_PREFS_START_DEFAULT)){
        disco.start(viewBackground, imageViewDiscoBall);
    } else {
        stopDisco();
    }
}

private void stopDisco(){
    if(disco == null || imageViewDiscoBall == null || viewBackground == null){
        return;
    }

    disco.stop();

    viewBackground.setBackgroundColor(Color.TRANSPARENT);
    imageViewDiscoBall.setImageBitmap(bitmap);
}
}

大多数时候，当我从后台返回时，片段中调用的方法会打印“NULL！！！”。我怎样才能解决这个问题

Answer 1

我建议将功能从onCreate（）移到onResume（）方法。onCreate（）方法仅在创建活动时调用，而onResume（）始终在返回活动时调用（已销毁的案例除外）。有关更多信息，请参见此https://developer.android.com/guide/components/activities/activity-lifecycle.html

Answer 2

android正在清除内存中的数据以节省内存空间和电源。如果您的设备内存较低，操作系统必须清除后台任务，则这种情况可能会发生得更快（另请参阅，如果您的后台任务限制在设置中->；开发人员设置->；设备上的后台进程限制中）要正确使用前台应用程序，请确保它有足够的内存等资源

所以为了防止这个,

• 仅在需要的范围内创建大型对象，如在将使用它们的位置加载图像对象
• 使用savedInstances可使您的活动恢复到之前的状态
• 要使用savedInstances，必须使对象可序列化