java - 在调用方法之前检查非空实际值时出现 NullPointerException

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);


}

}

对于发票类，

    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); 

}

最后是成本计算类

private Project p;

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

NullException 指的是 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 检查过以确保在调用该方法之前这些值具有正确的值？

Answer 1

Costing 实例c 仍然未初始化(默认情况下为null)。这就是当您尝试调用其方法时会收到 NullPointerException 的原因。

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