ImageButton findViewById返回null

Question

It should be pretty straightforward but something is wrong.

public class CreditView extends Activity {

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // The activity is being created.
        setContentView(R.layout.creditlayout);

        back = (TableRow) findViewById(R.id.backView);
        back.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                finish();
            }
        });

        fbBtn = (ImageButton) findViewById(R.id.fbBtn);
        fbBtn.setOnClickListener(new OnClickListener() {
....

I am getting a null pointer on fbBtn.

and the layout :

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

    <RelativeLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:background="@drawable/topbar" >

        <TableRow
            android:id="@+id/backView"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_centerVertical="true"
            android:layout_marginLeft="10dip"
            android:clickable="true"
            android:gravity="center" >

            <ImageView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:src="@drawable/back" />

            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginLeft="5dip"
                android:text="Back"
                android:textColor="#ffffff"
                android:textSize="18sp"
                android:textStyle="normal" />
        </TableRow>

        <ImageView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_centerInParent="true"
            android:src="@drawable/roadmania" />
    </RelativeLayout>

    <RelativeLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content" >

        <TableRow
            android:id="@+id/TopLayout"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:orientation = "vertical"
            android:gravity="center"
            android:padding="5dip" >

            <ImageButton
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/twitterBtn"
        android:layout_weight="0.5"
        android:background="@null"
        android:src="@drawable/twitter_200x200"
        android:padding="15dip"
        android:layout_margin="15dip"
         />

            <ImageButton
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/fbBtn"
        android:layout_weight="0.5"
        android:background="@null"
        android:src="@drawable/fb_200x200"
        android:padding="15dip"
        android:layout_margin="15dip"
         />


        </TableRow>


    </RelativeLayout>

Answer 1

You accidentally didn't correctly define the id. You probably copied and pasted, but forget to change layout_below to id .

You have this:

android:layout_below="@+id/fbBtn"

Change that to this:

android:id="@+id/fbBtn"

Answer 2

Change your ImageButtons from:

  <ImageButton
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/twitterBtn"
        android:layout_weight="0.5"
        android:background="@null"
        android:src="@drawable/twitter_200x200"
        android:padding="15dip"
        android:layout_margin="15dip"
         />
  <ImageButton
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/fbBtn"
        android:layout_weight="0.5"
        android:background="@null"
        android:src="@drawable/fb_200x200"
        android:padding="15dip"
        android:layout_margin="15dip"
         />

to

   <ImageButton
        android:id="@+id/twitterBtn"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_weight="0.5"
        android:background="@null"
        android:src="@drawable/twitter_200x200"
        android:padding="15dip"
        android:layout_margin="15dip"
         />
  <ImageButton
        android:id="@+id/fbBtn"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_weight="0.5"
        android:background="@null"
        android:src="@drawable/fb_200x200"
        android:padding="15dip"
        android:layout_margin="15dip"
         />

You have missed the ids and thats why you have a NullPointerException :

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

fbBtn is null!