怎样删除android的gallery中的图片实例说明

/*长按gallery中的图片进行删除该图片的操作*/
gallery.setonitemlongclicklistener(new onitemlongclicklistener(){

@override
public boolean onitemlongclick(adapterview<?> arg0, view arg1,
final int arg2, long arg3) {
// todo auto-generated method stub
new alertdialog.builder(sdcardactivity.this)
.seticon(android.r.drawable.ic_dialog_alert)
.settitle("温馨提示")
.setmessage("您真的要删除这个图片吗")
.setpositivebutton("确定", new dialoginterface.onclicklistener() {

@override
public void onclick(dialoginterface dialog, int which) {
// todo auto-generated method stub
file file=new file(constants.path+imagenames[arg2]);
if(file.exists()){
file.delete();
}
}
})
.setnegativebutton("取消", null)
.show();
return true;
}

});
}