Question

public class InvoiceApp 
{

public static void main(String[] args) 
{
    //create instances
            Client c = new Client(...);    //some client value
            Vendor v = new Vendor(...);    //some vendor value

    Project p = new Project(...);  //some project value

            //passing in some values for invoice, with p being the project created
    Invoice i = new Invoice(105, p, Calendar.getInstance(), true);


}

}

对于Invoice类，

    private int id;
    private Project p;
    private Calendar iDate;
    private boolean vatApp; //is vat payable on this invoice?
    private Costing c;

public Invoice(int id, Project p, Calendar iDate, boolean vatApp) {

    this.id = id;
    this.p= p;
    this.iDate = iDate;
    this.vatApp= vatApp;


            c.calculateTotal(p, vatApp); 

}

＆安培;最后，成本计算类

private Project p;

public void calculateTotal(Project p, boolean vatA) 
{
System.out.print("HELLO 2");
//actual computation formula
}

NullException引用c.calculateTotal（p，vatApp）;线。知道为什么会发生这种情况，即使我已经使用System.out.println检查以确保在调用方法之前这些值，DOES是否具有正确的值？

Answer 1

成本核算实例c仍为uninitialized（默认为null）。这就是为什么当你尝试在其上调用方法时得到NullPointerException。

private Costing c = new Costing(...); // You need to initialize this also.

Answer 2

私人成本核算c;

c.calculateTotal（p，vatApp）;

未初始化为calculteTotal方法称为

NullPointerException当在调用方法之前检查了非空实际值时

2 个答案: