如何修复Java中的NullPointerException和方法链接？

这是我第一次使用集合（TreeMap和Treeset）。其他部分工作正常，但我有NullPointerException。我试了几次，但都没能修好。我认为问题在于calcTotal、calcSubtotal和calcTax方法。谁能帮我找到我的错误并解决这个严重的问题？我是否以正确的方式链接这些方法

例外情况

Exception in thread "main" java.lang.NullPointerException
    at posPD.Sale.calcTotal(Sale.java:92)
    at posPD.Session.calTotalSession(Session.java:149)
    at posTest.LoadStore.loadTheStore(LoadStore.java:52)
    at posTest.StoreTest.main(StoreTest.java:163)

Sale.java

package posPD;

import java.math.BigDecimal;
import java.math.RoundingMode;
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.util.*;

/**
 * Sale information and services
 */
public class Sale {

    private LocalDateTime dateTime;

    private Boolean taxFree;
    private BigDecimal amountTendered;
    private ArrayList<SaleLineItem> saleLineItems;
    private ArrayList<Payment> payments;

    private LocalDate date;

    public Sale() {
        // TODO - implement Sale.Sale
        //throw new UnsupportedOperationException();
        saleLineItems = new ArrayList<SaleLineItem>();
        payments = new ArrayList<Payment>();
        //setTaxFree(false);
        setDate(date);
        setDateTime(dateTime);

        //dateTime = LocalDateTime.now();


    }


    /**
     * 
     * @param taxFree
     */
    public Sale(Session session, String taxFree) {
        // TODO - implement Sale.Sale
        //throw new UnsupportedOperationException();
        this();
        /*
        Boolean boolTax = Boolean.parseBoolean(taxFree);
        this.setTaxFree(boolTax);

        if (taxFree.equals("N")) {
            setTaxFree(false);
        } else {
            setTaxFree(true);
        }
        */
        if (taxFree == "Y") {
            this.setTaxFree(true);
        }
        else this.setTaxFree(false);
        session.addSale(this);
        //setSession(session);
    }

    /**
     * 
     * @param payment
     */
    public void addPayment(Payment payment) {
        // TODO - implement Sale.addPayment
        //throw new UnsupportedOperationException();
        payments.add(payment);


    }

    /**
     * 
     * @param payment
     */
    public void removePayment(Payment payment) {
        // TODO - implement Sale.removePayment
        //throw new UnsupportedOperationException();
        payments.remove(payment);
    }

    public BigDecimal calcTotal() {
        // TODO - implement Sale.calcTotal
        //throw new UnsupportedOperationException();

        BigDecimal total = new BigDecimal("0.0");

        if (this.taxFree) {
            total = calcSubTotal().setScale(2, BigDecimal.ROUND_HALF_UP);
            return total;
        }
        else {
            total = calcSubTotal().add(this.calcTax().setScale(2, BigDecimal.ROUND_HALF_UP));
            return total;
        }
    }

    public BigDecimal calcSubTotal() {
        // TODO - implement Sale.calcSubTotal
        //throw new UnsupportedOperationException();


        BigDecimal subTotal = new BigDecimal("0");
        for (SaleLineItem sli: saleLineItems) {
            subTotal = subTotal.add(sli.calcSubTotal());

        }
        System.out.println(subTotal);
        return subTotal.setScale(2, RoundingMode.HALF_UP);

    }

    public BigDecimal calcTax() {
        // TODO - implement Sale.calcTax
        //throw new UnsupportedOperationException();

        BigDecimal tax = new BigDecimal ("0");
        if (!getTaxFree()) {
            for (SaleLineItem sli : saleLineItems) {

                tax = tax.add(sli.calcTax());

            }
        }

        return tax.setScale(2, RoundingMode.HALF_UP);


    }

    public BigDecimal getTotalPayments() {
        // TODO - implement Sale.getTotalPayments
        //throw new UnsupportedOperationException();
        BigDecimal payment = new BigDecimal("0");
        for (Payment p : payments) {
            payment = payment.add(p.getAmount());
        }

        return payment.setScale(2, BigDecimal.ROUND_HALF_UP);
    }

    public Boolean isPaymentEnough() {
        // TODO - implement Sale.isPaymentEnough
        //throw new UnsupportedOperationException();
        return (calcTotal().compareTo(calcAmtTendered()) <= 0);
    }

    /**
     * 
     * @param amountTendered
     */
    public void calcAmount(BigDecimal amountTendered) {
        // TODO - implement Sale.calcAmount
        //throw new UnsupportedOperationException();
        BigDecimal calcAmount = calcTotal().subtract(calcTotal());
        if (calcAmount.compareTo(amountTendered) > 0)
        {
            calcAmount = amountTendered;
        }
        System.out.println("Calc Amt : "+calcAmount.setScale(2, BigDecimal.ROUND_HALF_UP));
        //return calcAmount.setScale(2, BigDecimal.ROUND_HALF_UP);
    }

    public BigDecimal calcChange() {
        // TODO - implement Sale.calcChange
        //throw new UnsupportedOperationException();
        //return amountTendered.subtract(getTotalPayments())
        return getAmountTendered().subtract(getTotalPayments());
    }

    public BigDecimal calcAmtTendered() {
        // TODO - implement Sale.calcAmtTendered
        //throw new UnsupportedOperationException();
        //return amountTendered;
        BigDecimal amtTendered = new BigDecimal("0");
        for (Payment p : payments) {
            amtTendered = amtTendered.add(p.getAmtTendered());

        }
        System.out.println("Payment Amt:"+amtTendered.setScale(2, BigDecimal.ROUND_HALF_UP));
        return amtTendered.setScale(2, BigDecimal.ROUND_HALF_UP);
    }

    /**
     * 
     * @param sli
     */
    public void addSaleLineItem(SaleLineItem sli) {
        // TODO - implement Sale.addSaleLineItem
        //throw new UnsupportedOperationException();
        saleLineItems.add(sli);


    }

    /**
     * 
     * @param sli
     */
    public void removeSaleLineItem(SaleLineItem sli) {
        // TODO - implement Sale.removeSaleLineItem
        //throw new UnsupportedOperationException();
        saleLineItems.remove(sli);

    }

    public BigDecimal getAmountTendered() {
        return amountTendered;
    }

    public void setAmountTendered(BigDecimal amountTendered) {
        this.amountTendered = amountTendered;
    }


    private ArrayList<SaleLineItem> getSaleLineItems() {
        return saleLineItems;
     }


    public String toString() {
        // TODO - implement Sale.toString
        //throw new UnsupportedOperationException();
        String slis = "";
        for (SaleLineItem sli : getSaleLineItems()) {
            slis += "  " +sli.toString() + "\r\n";

        }
        return "Sale : SubTotal = " +calcSubTotal().toPlainString()
        +" Tax = " +calcTax().toPlainString()
        +" Total =" +calcTotal().toPlainString()
        +" Payment =" +getTotalPayments().toPlainString()
        +" Change =" +calcChange().toPlainString() +"\r\n" 
        +slis;
    }

    public LocalDateTime getDateTime() {
        return dateTime;
    }

    public void setDateTime(LocalDateTime dateTime) {
        this.dateTime = dateTime;
    }

    public LocalDate getDate() {
        return date;
    }


    public void setDate(LocalDate date) {
        this.date = date;
    }

    public Boolean getTaxFree() {
        return taxFree;
    }




    public void setTaxFree(Boolean taxFree) {
        this.taxFree = taxFree;
    }

    public ArrayList<Payment> getPayments() {
        return payments;
    }

    public void setPayments(ArrayList<Payment> payments) {
        this.payments = payments;
    }

    public void setSaleLineItems(ArrayList<SaleLineItem> saleLineItems) {
        this.saleLineItems = saleLineItems;
    }




    public BigDecimal calcCash()
    {
        BigDecimal cash = new BigDecimal ("0");
        for (Payment p : payments)
        { if (p.hasCash()) cash = cash.add(p.getAmount());}

        return cash;
    }



}

package posPD;

import java.math.BigDecimal;
import java.time.LocalDate;
import java.time.LocalDateTime;

/**
 * Items, Quantity details during a sale
 */
public class SaleLineItem {

    private int quantity;
    private Item item;
    //private Sale sale;
    private LocalDate date;
    private LocalDateTime dateTime;

    public SaleLineItem() {
        // TODO - implement SaleLineItem.SaleLineItem
        //throw new UnsupportedOperationException();
        //date = LocalDate.now();
        setDateTime(dateTime);
        setDate(date);


    }

    /**
     * 
     * @param itemNumber
     * @param quantity
     */
    public SaleLineItem(Item item, String quantity) {
        // TODO - implement SaleLineItem.SaleLineItem
        //throw new UnsupportedOperationException();
        //this.itemNumber = Integer.parseInt(itemNumber);
        //this.quantity = Integer.parseInt(quantity);
        //System.out.println(quantity);
        this();
        this.item = item;
        this.quantity =Integer.parseInt(quantity);
    }
    public SaleLineItem(Sale sale, Item item, int quantity)
    {
        this();
        this.sale = sale;
        setQuantity(quantity);
        setItem(item);
        sale.addSaleLineItem(this);
        getItem().addSaleLineItem(this);
        setSale(sale);
        System.out.println(sale);
    }

    /**
     * 
     * @param itemNumber
     * @param quantity
     */
    public SaleLineItem(Store store, Sale sale, String itemNumber, String quantity) {
        // TODO - implement SaleLineItem.SaleLineItem
        //throw new UnsupportedOperationException();
        this();
        setQuantity(Integer.parseInt(quantity));
        setItem(store.findItemForNumber(itemNumber));

        sale.addSaleLineItem(this);
        getItem().addSaleLineItem(this);
        setSale(sale);




    }

    public LocalDateTime getDateTime() {
        return dateTime;
    }

    public void setDateTime(LocalDateTime dateTime) {
        this.dateTime = dateTime;
    }

    public LocalDate getDate() {
        return date;
    }

    public void setDate(LocalDate date) {
        this.date = date;
    }

    public Sale getSale() {
        return sale;
    }

    public void setSale(Sale sale) {
        this.sale = sale;
    }

    private Sale sale;
    public int getQuantity() {
        return this.quantity;
    }
    public void setQuantity(int quantity) {
        this.quantity = quantity;
    }
    public Item getItem() {
        return this.item;
    }
    public void setItem(Item item) {
        this.item = item;
    }

    public BigDecimal calcSubTotal() {

        // TODO - implement SaleLineItem.calcSubTotal
        //throw new UnsupportedOperationException();
        //return getItem().calcAmountForDateQty(date, getQuantity());
        date = getSale().getDate();
        System.out.println(date);

        BigDecimal subTotal = getItem().calcTotal(date, getQuantity()); 

        return subTotal;

    }



    public BigDecimal calcTax() {
        // TODO - implement SaleLineItem.calcTax
        //throw new UnsupportedOperationException();
        BigDecimal amount = calcSubTotal();
        BigDecimal taxRate = new BigDecimal(getItem().getTaxRateForDate(getSale().getDate()).toString());
        BigDecimal tax = amount.multiply(taxRate).setScale(2, BigDecimal.ROUND_HALF_UP);
        return tax;

    }

    public String toString() {
        // TODO - implement SaleLineItem.toString
        //throw new UnsupportedOperationException();
        try {
            LocalDate date =  sale.getDate();
        } catch (Exception e) {
            System.out.println("Exception exists here in SaleLineItem b"+ e.getMessage());
        }

        return getItem().getNumber()
        +" "
        +getItem().getDescription() 
        + " Qty: "+getQuantity()  
        + "Price for Date: "
        +"@$"
        + getItem().getPriceForDate(date)
        +" $"+calcSubTotal().toString();

    }

}

项目类

package posPD;

import java.math.BigDecimal;
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.util.*;

/**
 * information about the general details of items in the store
 */
public class Item {


    private String number;
    private String description;
    //private SaleLineItem saleLineItems;
    private TaxCategory taxCategory;
    private ArrayList<SaleLineItem> saleLineItems;
    private TreeMap<String, UPC> upcs;
    private TreeSet<Price> prices;

    public Item() {
        // TODO - implement Item.Item
        //throw new UnsupportedOperationException();
        upcs = new TreeMap<String, UPC>();
        prices = new TreeSet<Price>();
    }

    /**
     * 
     * @param number
     * @param description
     */
    public Item(String number, String description) {
        // TODO - implement Item.Item
        //throw new UnsupportedOperationException();
        this();
        this.setItemNumber(number);
        this.setItemDescription(description);
    }

    public String getItemNumber() {
        return this.number;
    }
    public void setItemNumber(String number) {
        this.number = number;
    }

    public String getItemDescription() {
        return this.description;
    }
    public void setItemDescription(String description) {
        this.description = description;
    }

    /*
    public SaleLineItem getSaleLineItems() {
        return saleLineItems;
    }

    public void setSaleLineItems(SaleLineItem saleLineItems) {
        this.saleLineItems = saleLineItems;
    }
    */
    public ArrayList<SaleLineItem> getSaleLineItems() {
        return saleLineItems;
    }

    public void setSaleLineItems(ArrayList<SaleLineItem> saleLineItems) {
        this.saleLineItems = saleLineItems;
    }

    public TaxCategory getTaxCategory() {
        return taxCategory;
    }



    public void setTaxCategory(TaxCategory taxCategory) {
        this.taxCategory = taxCategory;
    }

    /**
     * 
     * @param price
     */
    public void addPrice(Price price) {
        // TODO - implement Item.addPrice
        //throw new UnsupportedOperationException();
        prices.add(price);
    }

    /**
     * 
     * @param price
     */
    public void removePrice(Price price) {
        // TODO - implement Item.removePrice
        //throw new UnsupportedOperationException();
        prices.remove(price);
    }

    /**
     * 
     * @param upc
     */
    public void addUPC(UPC upc) {
        // TODO - implement Item.addUPC
        //throw new UnsupportedOperationException();
        upcs.put(upc.getUPC(), upc);
    }

    /**
     * 
     * @param upc
     */
    public void removeUPC(UPC upc) {
        // TODO - implement Item.removeUPC
        //throw new UnsupportedOperationException();
        upcs.remove(upc.getUPC());
    }

    public void addSaleLineItem (SaleLineItem sli)
    {
        if (sli != null)
        {
            getSaleLineItems().add(sli);
        }
    }

    public void removeSaleLineItem (SaleLineItem sli)
    {
        if (sli != null)
        {
            getSaleLineItems().remove(sli);
        }
    }
    /**
     * 
     * @param date
     */
    public BigDecimal getPriceForDate(LocalDate date) {
        // TODO - implement Item.getPriceForDate
        //throw new UnsupportedOperationException();
        Price currentPrice = new Price() ;
        for (Price price : getPrices()) {
            if (price.isEffective(date)) {
                currentPrice = price;
            }
        }
        return currentPrice.getPrice();

    }

    /**
     * 
     * @param date
     */
    public BigDecimal getTaxRateForDate(LocalDate date) {
    //public String getTaxRateForDate(LocalDate date) {
        //return null;
        // TODO - implement Item.getTaxRateForDate
        //throw new UnsupportedOperationException();
        //date = LocalDate.now();

        return  getTaxCategory().getTaxRateForEffectiveDate(date);

    }

    public BigDecimal calcTotal(LocalDate saleDate, int quantity) {
        // TODO Auto-generated method stub
        BigDecimal total;

        total = getPriceForDate(saleDate).multiply(new BigDecimal(quantity));
        return total;
    }
    /**
     * 
     * @param date
     * @param quantity
     */
    public BigDecimal calcAmountForDateQty(LocalDate date, int quantity) {
        // TODO - implement Item.calcAmountForDateQty
        //throw new UnsupportedOperationException();
        BigDecimal total;

        total = getPriceForDate(date).multiply(new BigDecimal(quantity));
        return total;
    }
    public TreeMap<String, UPC> getUpcs() {
        return upcs;
    }

    public void setUpcs(TreeMap<String, UPC> upcs) {
        this.upcs = upcs;
    }

    public TreeSet<Price> getPrices() {
        return prices;
    }

    public void setPrices(TreeSet<Price> prices) {
        this.prices = prices;
    }

    public String toString() {
        // TODO - implement Item.toString
        //throw new UnsupportedOperationException();
        String up="";
        //if (upcs.isEmpty()) System.out.println("EmptyXXXX");
        for(Map.Entry<String, UPC> entry : upcs.entrySet()) {
              //String key = entry.getKey();
              UPC value = entry.getValue();
              //up += key +": " + value;
              up += value;
              //System.out.println(key + " => " + value);
            }
        // for prices from tree set
        String pp = "";
        for (Price p : getPrices()) {
            pp += p;
        }
        return "\n"+"Item No: "+getItemNumber()+ " " + "Item: " + getItemDescription() + " Category: " + getTaxCategory() + " UPC: " + up  + " Price: " + pp +"\n";
    }

    public String getNumber() {
        return number;
    }

    public void setNumber(String number) {
        this.number = number;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }   

}

---

您是如何创建

Sale

实例的？您使用了哪个构造函数，

Sale（Session Session，String taxFree）

或

Sale（）

？如果您使用

Sale（）

，则您的属性

布尔免税

可能未初始化，当您调用时，它可能会抛出

NullPointerException

：

 public BigDecimal calcTotal() {
    // TODO - implement Sale.calcTotal
    //throw new UnsupportedOperationException();

    BigDecimal total = new BigDecimal("0.0");

    if (this.taxFree) { // <--- NullPointerException 's here
        total = calcSubTotal().setScale(2, BigDecimal.ROUND_HALF_UP);
        return total;
    }
    else {
        total = calcSubTotal().add(this.calcTax().setScale(2, BigDecimal.ROUND_HALF_UP));
        return total;
    }
}

public BigDecimal calcTotal（）{
//TODO-实现Sale.calcTotal
//抛出新的UnsupportedOperationException（）；
BigDecimal总计=新的BigDecimal（“0.0”）；
如果（this.taxFree）{/@cricket_007，你是指下一行中的setScale？BigDecimal tax=amount.multiply（taxRate）.setScale（2，BigDecimal.ROUND_HALF_）；如果是的话，它是BigDecimal的一部分，我导入了它。那么，Sales类的第92行是什么。是的，我声明私有布尔免税；使用
boolean
而不是
boolean
。您使用的是
boolean
。这样做会产生更多的异常。线程“main”中的异常posPD.Sale.SaleLineItem.calcSubTotal中的java.lang.NullPointerException在posPD.Sale.calcSubTotal（Sale.java:110）在posPD.Sale.calcTotal（Sale.java:98）在posPD.Session.calTotalSession（Session.java:149）在posTest.LoadStore.loadTheStore（LoadStore.java:52）在posTest.StoreTest.main（StoreTest.java:163）中你的代码在变量声明和初始化方面有很多问题。你需要验证你的代码以避免
NullPointerException
，调试器很容易找到它们。我使用了布尔值，以前的异常消失了，但得到了新的异常。我知道我的小计和总计变量为null，这导致了新的异常例外。你能告诉我为什么小计和总计上有空值吗？
 public BigDecimal calcTotal() {
    // TODO - implement Sale.calcTotal
    //throw new UnsupportedOperationException();

    BigDecimal total = new BigDecimal("0.0");

    if (this.taxFree) { // <--- NullPointerException 's here
        total = calcSubTotal().setScale(2, BigDecimal.ROUND_HALF_UP);
        return total;
    }
    else {
        total = calcSubTotal().add(this.calcTax().setScale(2, BigDecimal.ROUND_HALF_UP));
        return total;
    }
}