GAE應用程序開發：返回hashmap時的NullPointerException

Question

基本上用戶提交所選年份，然後servlet返回相應的年度報表。 然後用戶可以修改相應的信息。那麼servlet保存修改的信息。

假設用戶所尋找的螺柱始終存在於em中。問題是： 如果用戶之前未存儲年度報告信息，並且用戶提交了一年（即2009年），則所有內容都運行正常。用戶修改信息並且GPAInfoServlet將存儲修改的信息。

然而，在完成上述動作，這意味着前一個年度報告的信息存儲在用戶，用戶（指我）得到錯誤NullPointerException異常過程中提交每年評選

at tcndata.Stud.jdoGetannualReport(Stud.java) 
at tcndata.Stud.getAnnualReport(Stud.java:33) 
at servlet.YearSelectedServlet.doPost(YearSelectedServlet.java:35) 

選擇後年份：

public class YearSelectedServlet extends HttpServlet { 
public void doPost(HttpServletRequest req, HttpServletResponse resp) 
     throws IOException { 
    int yearselected = Integer.parseInt(req 
      .getParameterValues("yearselected")[0]); 
    long studid = Long.parseLong(req.getSession().getAttribute("studID") 
      .toString()); 

    EntityManager em; 
    em = EMF.get().createEntityManager(); 

    Stud stud = em.find(Stud.class, studid); 

    if (stud != null) { 
     Map<Integer, AnnualReport> annualreport; 
     if (stud.getAnnualReport() == null) 
      annualreport = new HashMap<Integer, AnnualReport>(); 
     else 
      annualreport = stud.getAnnualReport(); 

     if (annualreport.containsKey(yearselected)) { 
      AnnualReport thatyear = stud.getAnnualReport() 
        .get(yearselected); 
      req.getSession().setAttribute("thatyear", thatyear); 
      req.getSession().setAttribute("annualReport", thatyear); 
     } 
    } else { 
     req.getSession().setAttribute("Error", "alumni not found"); 
     resp.sendRedirect("Error.jsp"); 
    } 
    req.getSession().setAttribute("yearselected", 
      req.getParameterValues("yearselected")[0]); 
    req.getSession().setAttribute("SearchTag", "GPAInfo"); 
    resp.sendRedirect("Search.jsp"); 

} 

}

保存信息修改：

public class GPAInfoServlet extends HttpServlet { 
public void doPost(HttpServletRequest req, HttpServletResponse resp) 
     throws IOException { 

    AnnualReport thatyear = new AnnualReport(); 

    thatyear.setAttendSchool(AttendSchool.valueOf(req 
      .getParameterValues("AttendSchool")[0])); 
    ..... 

    int yearselected = Integer.parseInt(req.getSession() 
      .getAttribute("yearselected").toString()); 
    long studid = Long.parseLong(req.getSession().getAttribute("studID") 
      .toString()); 

    EntityManager em; 
    em = EMF.get().createEntityManager(); 
    Stud stud = em.find(Stud.class, studid); 

    if (stud != null) { 
     Map<Integer, AnnualReport> annualreport; 
     if (stud.getAnnualReport() == null) 
      annualreport = new HashMap<Integer, AnnualReport>(); 
     else 
      annualreport = stud.getAnnualReport(); 

     annualreport.put(yearselected, thatyear); 

     try { 
      stud.setAnnualReport(annualreport); 
      em.merge(stud); 
     } finally { 
      em.close(); 
     } 

    } else { 
     req.getSession().setAttribute("Error", "alumni not found"); 
     resp.sendRedirect("Error.jsp"); 
    } 

    req.getSession().setAttribute("SearchTag", "GPAInfo"); 
    resp.sendRedirect("/Search.jsp"); 

} 

}

梭哈類：

@Entity(name = "Stud") 
public class Stud{ 
    @Id 
@GeneratedValue(strategy = GenerationType.IDENTITY) 
private Long studID; 

private Map<Integer,AnnualReport>annualReport = new HashMap<Integer,AnnualReport>(); 

public Map<Integer, AnnualReport> getAnnualReport() { 
    return annualReport; 
} 

public void setAnnualReport(Map<Integer, AnnualReport> annualReport) { 
    this.annualReport = annualReport; 
} 

**我沒有任何想法會發生什麼。

我已經在年報@Embeddable：

@Embeddable 

公共類年報{

@Basic 
private AttendSchool attendSchool; 
private String attendSchoolNote; 

...... 

}

我怎樣才能得到NullPointerException異常在**如果（stud.getAnnualReport（ ）== null）**

if (stud != null) { 
     Map<Integer, AnnualReport> annualreport; 
     if (stud.getAnnualReport() == null) 
      annualreport = new HashMap<Integer, AnnualReport>(); 
     else 
      annualreport = stud.getAnnualReport(); 

     .... 
    } else { 
     .... 
    } 

NullPointerException？

需要幫助.... 我要死了......

Answer 1

你肯定行annualreport = stud.getAnnualReport();要看你的NPE？

因爲幾行降低你的代碼，肯定會產生NPE如果stud.getAnnualReport()是null：

AnnualReport thatyear = stud.getAnnualReport() 
       .get(yearselected);