Question

我在尝试设置和写入对象的ArrayList时遇到空指针异常。根据互联网，最常见的原因是ArrayList永远不会被实例化，但即使我实例化它仍然会得到异常。见下面的代码。

public class Invoice {

    private int invoiceNumber;
    private Date date;
    private int supplierCode;
    private double total;
    private ArrayList<InvoiceLines> lines;
    private String status;


    //getters and setters
    public ArrayList<InvoiceLines> getLines() {
        return this.lines;
    }

    public void setLines(ArrayList<InvoiceLines> lines) {
        this.lines = lines;
    }

    public void addLine(int lineNumber, int qty, int productCode, int invoiceNumber) {
        InvoiceLines il = new InvoiceLines();
        il.setinvoiceNumber(invoiceNumber);
        il.setlineNumber(lineNumber);
        il.setqty(qty);
        il.setproductCode(productCode);
        il.setStatus("insert");
        getLines().add(il);
    }

    // Servlet code

    HttpSession session = request.getSession(false);
    if(session==null) {
        session = request.getSession(true);
        Invoice invoice = new Invoice();
        session.setAttribute("invoice", invoice);

        ArrayList<InvoiceLines> lines = new ArrayList<InvoiceLines>();
        session.setAttribute("lines", lines);
    }

    Invoice invoice = (Invoice) session.getAttribute("invoice");
    session.setAttribute("invoice",invoice);
    ArrayList<InvoiceLines> lines = (ArrayList<InvoiceLines>) session.getAttribute("lines");
    session.setAttribute("lines ",lines);


    // gets the data from the request
    int qty = request.getParamater…;
    int productCode = request.getParamater…;
    int lineNumber = request.getParamater…;
    int invoiceNumber = request.getParamater…;

    if(request.getParameter("addLine")!=null) {
        invoice.setLines(lines); ////// null pointer exception!!!
        invoice.addLine(lineNumber, qty, productCode, invoiceNumber);
    }
}

Answer 1

NullPointerException由invoice null引起，而不是与ArrayList有关。

这可能是因为您有预先存在的会话。如果没有会话，您只需为会话分配新发票。

我建议如下：

if(session==null) {
    session = request.getSession(true);
}
Invoice invoice = (Invoice) session.getAttribute("invoice");
if (invoice == null) {
  invoice = new Invoice();
}
session.setAttribute("invoice", invoice);

// Similarly for lines.

我对会话并不熟悉，但我不清楚您是否总是需要再次设置属性值（如无条件调用session.setAttribute(...) 。您可能只能在创建新Invoice时设置它。

即使实例化，ArrayList也会显示空指针异常

1 个答案: