在线等候nullpointExecpion

我有如下语句在判断是都用了xx!=null进行判断,但是仍然出错究竟时什么原因

tousu_end_date1=request.getParameter("tousu_end_date");
if (tousu_end_date1 ==null)
tousu_end_date1 ="";
depart1=request.getParameter("depart");
if (depart1 ==null)
depart1 ="";
details1=request.getParameter("details");
if (details1 ==null)
details1 ="";
handle1=request.getParameter("handle");
if (handle1 ==null)
handle1 ="";
person1=request.getParameter("person");
if (person1 ==null)
person1 ="";
tousu_class1=request.getParameter("tousu_class");
if (tousu_class1 ==null)
tousu_class1 ="";
strSQL="select * FROM data where ";

if(!tousu_class1.trim().equals(""))
strSQL=strSQL+" and tousu_class like '%"+tousu_class1+"%' ";
if(!depart1.trim().equals("") )
strSQL=strSQL+" depart = '"+depart1+"' ";
if( !handle1.trim().equals("") )
strSQL=strSQL+" and handle = '"+handle1+"' ";
if( !details1.trim().equals("") )
strSQL=strSQL+" and details like '%"+details1+"%' ";
if( !person1.trim().equals("") )
strSQL=strSQL+" and person = '"+person1+"' ";
if(!tousu_start_date1.trim().equals("") && !tousu_end_date1.trim().equals("") )
strSQL=strSQL+" and tousu_date between #"+tousu_start_date1+"# and #"+tousu_end_date1+"# order by tousu_date desc";