空指针异常

Question

package my.photo;import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.BaseAdapter;
import android.widget.GridView;
import android.widget.ImageButton;
import android.widget.ImageView;public class HelloGridViewActivity extends Activity {
private ImageButton img = null;
private Intent intent = new Intent();
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
GridView gridview = (GridView) findViewById(R.id.gridview);
img = (ImageButton)findViewById(R.id.img);
gridview.setAdapter(new ImageAdapter(this));
gridview.setOnItemClickListener(new OnItemClickListener() {
public void onItemClick(AdapterView<?> parent, View v,
int position, long id) {
//单击GridView的相应的Item时候讲img的图片换成相应的图片
switch(position){
case 0:{
img.setImageResource(R.drawable.sample_0);
intent.setClass(HelloGridViewActivity.this, other.class);
startActivity(intent);};break;
case 1:{
intent.setClass(HelloGridViewActivity.this, other.class);
img.setImageResource(R.drawable.sample_1);
startActivity(intent);};break;
case 2:{
intent.setClass(HelloGridViewActivity.this, other.class);
img.setImageResource(R.drawable.sample_2);
startActivity(intent);};break;
case 3:{
intent.setClass(HelloGridViewActivity.this, other.class);
img.setImageResource(R.drawable.sample_3);
startActivity(intent);};break;
case 4:{
intent.setClass(HelloGridViewActivity.this, other.class);
img.setImageResource(R.drawable.sample_4);
startActivity(intent);};break;
case 5:{
intent.setClass(HelloGridViewActivity.this, other.class);
img.setImageResource(R.drawable.sample_5);
startActivity(intent);};break;
case 6:{
intent.setClass(HelloGridViewActivity.this, other.class);
img.setImageResource(R.drawable.sample_6);
startActivity(intent);};break;
case 7:{
intent.setClass(HelloGridViewActivity.this, other.class);
img.setImageResource(R.drawable.sample_7);
startActivity(intent);};break;
}
}
});
} public class ImageAdapter extends BaseAdapter {
private Context mContext; public ImageAdapter(Context c) {
mContext = c;
} public int getCount() {
return mThumbIds.length;
} public Object getItem(int position) {
return null;
} public long getItemId(int position) {
return 0;
}
    // create a new ImageView for each item referenced by the Adapter
public View getView(int position, View convertView, ViewGroup parent) {
ImageView imageView;
if (convertView == null) { // if it's not recycled, initialize some attributes
imageView = new ImageView(mContext);
imageView.setLayoutParams(new GridView.LayoutParams(85, 85));
imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
imageView.setPadding(8, 8, 8, 8);
} else {
imageView = (ImageView) convertView;
}
imageView.setImageResource(mThumbIds[position]);
return imageView;
}
    // references to our images
private Integer[] mThumbIds = { R.drawable.sample_2,
R.drawable.sample_3, R.drawable.sample_4, R.drawable.sample_5,
R.drawable.sample_6, R.drawable.sample_7, R.drawable.sample_0,
R.drawable.sample_1, R.drawable.sample_2, R.drawable.sample_3,
R.drawable.sample_4, R.drawable.sample_5, R.drawable.sample_6,
R.drawable.sample_7, R.drawable.sample_0, R.drawable.sample_1,
R.drawable.sample_2, R.drawable.sample_3, R.drawable.sample_4,
R.drawable.sample_5, R.drawable.sample_6, R.drawable.sample_7 };
}
}我是想实现一个图片应用程序，点击当前Activity中GridView中的对应图片，会转到other.java中在other.java中的ImageButton中显示这张图片，报错是在gridview.setOnItemClickListener(new OnItemClickListener()）方法中
但是我没发现是空指针啊 ，资源都有啊 ，
这是other.xml文件<?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:orientation="vertical" >
    <ImageButton
        android:id="@+id/img"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:src="@drawable/ic_launcher" />
    
    </LinearLayout>
这是R.java文件/* AUTO-GENERATED FILE.  DO NOT MODIFY.
 *
 * This class was automatically generated by the
 * aapt tool from the resource data it found.  It
 * should not be modified by hand.
 */package my.photo;public final class R {
    public static final class attr {
    }
    public static final class drawable {
        public static final int ic_launcher=0x7f020000;
        public static final int sample_0=0x7f020001;
        public static final int sample_1=0x7f020002;
        public static final int sample_2=0x7f020003;
        public static final int sample_3=0x7f020004;
        public static final int sample_4=0x7f020005;
        public static final int sample_5=0x7f020006;
        public static final int sample_6=0x7f020007;
        public static final int sample_7=0x7f020008;
    }
    public static final class id {
        public static final int gridview=0x7f050000;
        public static final int img=0x7f050001;
    }
    public static final class layout {
        public static final int main=0x7f030000;
        public static final int other=0x7f030001;
    }
    public static final class string {
        public static final int app_name=0x7f040001;
        public static final int hello=0x7f040000;
    }
}

Answer 1

main.xml文件
<?xml version="1.0" encoding="utf-8"?>
<GridView xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/gridview"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:columnWidth="90dp"
        android:gravity="center"
        android:horizontalSpacing="10dp"
        android:numColumns="auto_fit"
        android:stretchMode="columnWidth"
        android:verticalSpacing="10dp" />

Answer 2

当然是空指针啊setContentView(R.layout.main)
这个调用的是main布局，而这个布局里面没有imgimg = (ImageButton)findViewById(R.id.img);
这个img只有在other布局里面才有楼主可以通过Intent的方式将要显示的图片资源，例如R.drawable.sample_0传到Other.class里面再做处理。

Answer 3

main.xml中没有对象的控件，所以空指针。

Answer 4

大侠 R文件中已经有了img的声明啦，我应该不管在哪儿用img都应该是作用于这个ImageButton的啊 ，为什么不行呢  ，必须在布局里面有这个控件才行呢  

Answer 5

楼主理解错了，img = (ImageButton)findViewById(R.id.img);你这样写是直接从main.xml文件里面找img，LayoutInflater factory = LayoutInflater.from(youActivity.this);    
                 final View view = factory.inflate(R.layout.other,null); 
img = (ImageButton)view.findViewById(R.id.img);你必须从other这个xml才能取出img.