求助list.add()一执行就奔溃


    public static List<student_info> studentlist;
    
       protected void onActivityResult(int requestCode, int resultCode, Intent data) {        if (resultCode == Activity.RESULT_OK) {//是否选择，没选择就不会继续
           Uri uri = data.getData();
           try{
           //1:创建workbook
           Workbook workbook;
workbook = Workbook.getWorkbook(new File(uri.getPath().toString()));
//2:获取第一个工作表sheet
               Sheet sheet=workbook.getSheet(0);
               //3:获取数据
               int j;
               for(int i=1;i<sheet.getRows();i++){  
                int k=0;
                //for(int k=0;k<sheet.getColumns();k++){
                Cell cell=sheet.getCell(k,i);
                Cell cell1=sheet.getCell(k+1,i);
                Cell cell2=sheet.getCell(k+2,i);
                       Toast.makeText(this, cell.getContents()+cell1.getContents()+cell2.getContents(), Toast.LENGTH_SHORT).show();
                       j=i%num;
                       if(j==0)
                       {
                        j=num;
                       }
                       String snum=cell.getContents();
                       String name=cell1.getContents();
                       String sclass=cell2.getContents();
                       student_info st = new student_info(j,snum,name,sclass);
                       studentlist.add(st);  //这行一执行就奔溃
                //}
                    a=i;
               }
                
               //最后一步：关闭资源
               workbook.close();
}
         catch (BiffException e) {
Toast.makeText(this, "fail", Toast.LENGTH_SHORT).show();
} catch (IOException e) {
Toast.makeText(this, "fail", Toast.LENGTH_SHORT).show();
}
        }
           //Toast.makeText(this, "文件路径："+uri.getPath().toString(), Toast.LENGTH_SHORT).show();
        
     }
public class student_info {
private String snum;
private String name;
private String sclass;
private int g; public student_info( int g,String snum, String name, String sclass) {
this.snum = snum;
this.name = name;
this.sclass = sclass;
this.g = g;
}
public String getSnum(){
return snum;
}
public String getName(){
return name;
}
public String getSclass(){
return sclass;
}
public int getg(){
return g;
}
}