Null pointer Exception - findViewById()

问题

Can anyone help me to find out what can be the issue with this program. In the onCreate() method the findViewById() returns null for all ids and this causes a null pointer exception later. I can not figure out why the findViewById() can not find the view. Any suggestions?

This is the main code:

public class MainActivity extends Activity {

    ViewPager pager;
    MyPagerAdapter adapter;
    LinearLayout layout1, layout2, layout3;

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

        layout1 = (LinearLayout) findViewById(R.id.first_View);
        layout2 = (LinearLayout) findViewById(R.id.second_View);
        layout3 = (LinearLayout) findViewById(R.id.third_View);

        adapter = new MyPagerAdapter();
        pager = (ViewPager) findViewById(R.id.main_pager);
        pager.setAdapter(adapter);
    }

    private class MyPagerAdapter extends PagerAdapter
    {

        @Override
        public int getCount() { 
            return 3;
        }

        @Override
        public Object instantiateItem(ViewGroup collection, int position) {

            LinearLayout l = null;

            if (position == 0 )
            {
                l = layout1;
            }
            if (position == 1)
            {
                l = layout2;
            }

            if (position == 2)
            {
                l = layout3;
            }
                collection.addView(l, position);
                return l;
        }

        @Override
        public boolean isViewFromObject(View view, Object object) {
            return (view==object);
        }

         @Override
         public void destroyItem(ViewGroup collection, int position, Object view) {
                 collection.removeView((View) view);
         }
    }
}

And the related XML files:

activity_main layout:

<?xml version=\"1.0\" encoding=\"utf-8\"?>
<LinearLayout xmlns:android=\"http://schemas.android.com/apk/res/android\"
                android:orientation=\"vertical\"
                android:layout_width=\"fill_parent\"
                android:layout_height=\"fill_parent\"
                android:background=\"#a4c639\">


    <android.support.v4.view.ViewPager
                        android:layout_width=\"match_parent\" 
                        android:layout_height=\"match_parent\" 
                        android:id=\"@+id/main_pager\"/>
</LinearLayout>

activity_first layout:

<?xml version=\"1.0\" encoding=\"utf-8\"?>
<LinearLayout xmlns:android=\"http://schemas.android.com/apk/res/android\"
android:layout_width=\"match_parent\"
android:layout_height=\"match_parent\"
android:id=\"@+id/first_View\">

<TextView
    android:layout_width=\"wrap_content\"
    android:layout_height=\"wrap_content\"
    android:text=\"@string/hello_world\" />

<Button
    android:id=\"@+id/button1\"
    style=\"?android:attr/buttonStyleSmall\"
    android:layout_width=\"wrap_content\"
    android:layout_height=\"wrap_content\"
    android:text=\"Button\" />

</LinearLayout>

activity_second layout:

 <?xml version=\"1.0\" encoding=\"utf-8\"?>
 <LinearLayout xmlns:android=\"http://schemas.android.com/apk/res/android\"

android:layout_width=\"match_parent\"
android:layout_height=\"match_parent\"
android:id=\"@+id/second_View\">

<TextView
    android:layout_width=\"wrap_content\"
    android:layout_height=\"wrap_content\"
    android:text=\"@string/hello_world\" />

</LinearLayout>

And the activity_third layout:

<?xml version=\"1.0\" encoding=\"utf-8\"?>
<LinearLayout xmlns:android=\"http://schemas.android.com/apk/res/android\"
android:layout_width=\"match_parent\"
android:layout_height=\"match_parent\"
android:id=\"@+id/third_View\">

<TextView
    android:layout_width=\"wrap_content\"
    android:layout_height=\"wrap_content\"
    android:text=\"@string/hello_world\" />

</LinearLayout>

回答1:

findViewById() returns a View if it exists in the layout you provided in setContentView(), otherwise it returns null and that's what happening to you.

Example if you setContentView(R.layout.activity_first); and then call findViewById(R.id.first_View); it will return a View which is your layout.

But if you call findViewById(R.id.second_View); it will return null since there is not a view in your activity_first.xml layout called @+id/second_View.

回答2:

The views you're trying to get are not defined in your activity_main layout. You need to programmatically inflate the views you're trying to add to the pager.-

@Override
public Object instantiateItem(ViewGroup collection, int position) {
    LinearLayout l = null;

    if (position == 0) {
        l = (LinearLayout) View.inflate(this, R.layout.activity_first, null);
    }
    if (position == 1) {
        l = (LinearLayout) View.inflate(this, R.layout.activity_second, null);
    }
    if (position == 2) {
        l = (LinearLayout) View.inflate(this, R.layout.activity_third, null);
    }

    collection.addView(l, position);
    return l;
}

回答3:

^{Emphasis added}

For those cases within an Activity class.

Activity.findViewById(int id)

Finds a view that was identified by the id attribute from the XML that was processed in onCreate(Bundle).

Otherwise, such as an Fragment, Adapter, a View from a LayoutInflater, etc.

View.findViewById(int id)

Look for a child view with the given id. If this view has the given id, return this view.

Either case,

Returns
The view if found or null otherwise.

Now, re-check your XML files. Make sure you put the right value into setContentView or inflater.inflate.

In the case of an Activity, call findViewById after setContentView.

Then, make sure there is a View you are looking for with android:id="@+id/..." in that layout. Make sure the + is at @+id, which will add the resource to the R.id values to ensure you can find it from Java.

回答4:

add those views to the pager adapter before accessing them.

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

    adapter = new MyPagerAdapter();
    pager = (ViewPager) findViewById(R.id.main_pager);
    pager.setAdapter(adapter);

    layout1 = (LinearLayout) findViewById(R.id.first_View);
    layout2 = (LinearLayout) findViewById(R.id.second_View);
    layout3 = (LinearLayout) findViewById(R.id.third_View);

}

in the pager adapter:

public Object instantiateItem(View collection, int position) {
    if(position == 0){
        View layout = inflater.inflate(R.layout.activity_first, null);

        ((ViewPager) collection).addView(layout);

        return layout;
    } 
    ... and so forth.

}

from here you can access them via findViewById.

回答5:

Sometimes you need clean your project in Eclipse (Project - Clean..).

回答6:

I have gotten this error today and it was so simple to clear, that I "facepalmed".

Just try to add the UI element to your layout xml File in your res/layout-port directory!!!

回答7:

In my case, it was a stupid mistake on my part. I had written code in the OnCreate method but it was above the setContentView line of code. Once I moved my code below this line the application started working fine.

setContentView(R.layout.activity_main);

回答8:

What @Warlock said above is right , you should initial LinearLayout layout1, layout2, layout3 by the right way:

LinearLayout layout1 = (LinearLayout) View.inflate(this, R.layout.first_View, null);
LinearLayout layout2 = (LinearLayout) View.inflate(this, R.layout.second_View, null);
LinearLayout layout3 = (LinearLayout) View.inflate(this, R.layout.third, null);

wish my advise help you

回答9:

In Android, findViewById(R.id.some_id) works when you are finding view in the layout set.

That is, if you have set a layout say:

setContentView(R.layout.my_layout);

Views can be found only in this layout (my_layout).

In your code layout1, layout2, layout3 all are three different layouts and they are not set to the activity.

回答10:

The findViewById method must find what is in the layout (which you called in the setContentView)

来源：https://stackoverflow.com/questions/19078461/null-pointer-exception-findviewbyid

标签

android

nullpointerexception

oncreate

findviewbyid