我在刷卡时得到NullPointerException。有什么问题以及如何纠正？方法onTouchEvent由语句return gestureDetector.onTouchEvent(event);
此活动（List8）在Tabs3类中被称为TabActivity，它也是为了完整而提供的。
package myapp.tabnavui;
import myapp.tabnavui.R;
import android.app.ListActivity;
import android.content.Context;
import android.os.Bundle;
import android.view.GestureDetector;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.view.GestureDetector.SimpleOnGestureListener;
import android.view.ViewGroup.LayoutParams;
import android.widget.BaseAdapter;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.AbsListView;
import android.widget.Toast;
import java.util.ArrayList;
/**
* A list view that demonstrates the use of setEmptyView. This example alos uses
* a custom layout file that adds some extra buttons to the screen.
*/
public class List8 extends ListActivity {
private GestureDetector gestureDetector;
PhotoAdapter mAdapter;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Use a custom layout file
setContentView(R.layout.list_8);
// Tell the list view which view to display when the list is empty
getListView().setEmptyView(findViewById(R.id.empty));
// Set up our adapter
mAdapter = new PhotoAdapter(this);
setListAdapter(mAdapter);
// Wire up the clear button to remove all photos
Button clear = (Button) findViewById(R.id.clear);
clear.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
mAdapter.clearPhotos();
} });
// Wire up the add button to add a new photo
Button add = (Button) findViewById(R.id.add);
add.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
mAdapter.addPhotos();
} });
}
@Override
public boolean onTouchEvent(MotionEvent event) {
return gestureDetector.onTouchEvent(event);
}
/**
* A simple adapter which maintains an ArrayList of photo resource Ids.
* Each photo is displayed as an image. This adapter supports clearing the
* list of photos and adding a new photo.
*
*/
public class PhotoAdapter extends BaseAdapter {
private Integer[] mPhotoPool = {
R.drawable.sample_thumb_0, R.drawable.sample_thumb_1, R.drawable.sample_thumb_2,
R.drawable.sample_thumb_3, R.drawable.sample_thumb_4, R.drawable.sample_thumb_5,
R.drawable.sample_thumb_6, R.drawable.sample_thumb_7};
private ArrayList<Integer> mPhotos = new ArrayList<Integer>();
public PhotoAdapter(Context c) {
mContext = c;
}
public int getCount() {
return mPhotos.size();
}
public Object getItem(int position) {
return position;
}
public long getItemId(int position) {
return position;
}
public View getView(int position, View convertView, ViewGroup parent) {
// Make an ImageView to show a photo
ImageView i = new ImageView(mContext);
i.setImageResource(mPhotos.get(position));
i.setAdjustViewBounds(true);
i.setLayoutParams(new AbsListView.LayoutParams(LayoutParams.WRAP_CONTENT,
LayoutParams.WRAP_CONTENT));
// Give it a nice background
i.setBackgroundResource(R.drawable.picture_frame);
return i;
}
private Context mContext;
public void clearPhotos() {
mPhotos.clear();
notifyDataSetChanged();
}
public void addPhotos() {
int whichPhoto = (int)Math.round(Math.random() * (mPhotoPool.length - 1));
int newPhoto = mPhotoPool[whichPhoto];
mPhotos.add(newPhoto);
notifyDataSetChanged();
}
}
class MyGestureDetector extends SimpleOnGestureListener {
private static final int SWIPE_MAX_OFF_PATH = 200;
private static final int SWIPE_MIN_DISTANCE = 50;
private static final int SWIPE_THRESHOLD_VELOCITY = 200;
@Override
public boolean onDown (MotionEvent e) {
return true;
}
@Override
public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
if (Math.abs(e1.getY() - e2.getY()) > SWIPE_MAX_OFF_PATH)
return false;
// left to right swipe and right to left swipe
if (e1.getX() - e2.getX() > SWIPE_MIN_DISTANCE
&& Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY) {
// left swipe
Toast t = Toast.makeText(List8.this, "Left swipe", Toast.LENGTH_LONG);
t.show();
startActivity(Tabs3.tab1);
return true;
} else if (e2.getX() - e1.getX() > SWIPE_MIN_DISTANCE
&& Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY) {
// right swipe
Toast t = Toast.makeText(List8.this, "Right swipe", Toast.LENGTH_LONG);
t.show();
startActivity(Tabs3.tab3);
return true;
}
return false;
}
}
}
这是Tabs3：
package myapp.tabnavui;
import android.os.Bundle;
import android.util.Log;
import android.view.GestureDetector;
import android.view.GestureDetector.SimpleOnGestureListener;
import android.view.MotionEvent;
import android.view.View;
import android.widget.TabHost;
import android.widget.Toast;
import android.app.TabActivity;
import android.content.Intent;
/**
* An example of tab content that launches an activity via {@link android.widget.TabHost.TabSpec#setContent(android.content.Intent)}
*/
public class Tabs3 extends TabActivity {
private GestureDetector gestureDetector;
// View.OnTouchListener gestureListener;
// Intent go;
public static Intent tab1, tab2, tab3;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
final TabHost tabHost = getTabHost();
tab1 = new Intent(this, List1.class);
tabHost.addTab(tabHost.newTabSpec("tab1")
.setIndicator("list")
.setContent(tab1));
tab2 = new Intent(this, List8.class);
tabHost.addTab(tabHost.newTabSpec("tab2")
.setIndicator("photo list")
.setContent(tab2));
// This tab sets the intent flag so that it is recreated each time
// the tab is clicked.
tab3 = new Intent(this, Controls2.class);
tabHost.addTab(tabHost.newTabSpec("tab3")
.setIndicator("destroy")
.setContent(tab3
.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP)));
}
}
答案 0 :(得分：0)
Tabs3.gestureDetector和List8.gestureDetector永远不会设置，因此它们是null。
这就是为什么当您尝试在NullPointerException
gestureDetector.onTouchEvent(event);
根据this question，您需要在创建时使用GestureDetector进行设置：
@Override
public void onCreate(Bundle savedInstanceState) {
// code
gestureDetector = new GestureDetector(new MyGestureDetector());
// code
}