java - 我的函数中出现 NullPointerException

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祝大家有美好的一天

我是 Java 新手，这是第一个包含 4 个类和简单方法的程序之一。

在此程序中，我们从键盘输入交易(我们输入买家、卖家姓名、标题、价格和购买产品的数量)。因此，在我输入 2 笔交易并且程序必须给出输出后，我得到 NullPointerException .

应用程序.java

package ua.lviv.my;
import java.util.Scanner;
public class Application {
  private static Deal [] deal = new Deal[2];
  public static void main(String[] args) {
    new Application().allActions();
  }
  void allActions(){
    input();
    System.out.println("======================");
    output();
  }
  public  void output(){
    for(int i=0; i<deal.length; i++){
      System.out.println("Buyer :" +deal[i].getBuyer().getName());
      System.out.println("Seller :" +deal[i].getSeller().getName());
      for (int j = 0; j < deal[i].getProducts().length; j++) {
        System.out.println("Buys " +deal[i].getProducts()[j].getTitle() +"for " +deal[i].getProducts()[j].getPrice() + " in quantity " +deal[i].getProducts()[j].getQuantity());
      }
    }
  }
  public void input(){
    for (int i=0; i<deal.length; i++){
      deal[i]=inputDeal();
    }
  }
  public Members inputMember(String msg){
    Members members = new Members();
    String memberName = keybordIn(msg);
    members.setName(memberName);
    return members;
  }
  public Product inputProduct(){
    Product product =new Product();
    String titleStrng = keybordIn("Enter product title");
    String priceStrng = keybordIn("Enter product price");
    String quantityStrng = keybordIn("Enter quantity");
    double price=Double.parseDouble(priceStrng);
    int quantity = Integer.parseInt(quantityStrng);
    product.setTitle(titleStrng);
    product.setPrice(price);
    product.setQuantity(quantity);
    product.getCost(price, quantity);
    return product;
  }
  public  Deal inputDeal(){
    Members buyer = inputMember("Enter buyer name :");
    Members seller =inputMember("Enter seller name :");
    Product [] products = new Product[2];
    for(int i=0; i<products.length; i++){
      products [i]=inputProduct();
    }
    Deal deal = new Deal(buyer, seller, products);
    return deal;
  }
  public  String keybordIn(String msg){
    System.out.println(msg);
    Scanner scan = new Scanner(System.in);
    String in = scan.next();
    return in;
  }
}

交易.java

package ua.lviv.my;
import java.util.Date;
public class Deal {
  private Date date = new Date();
  private Members buyer;
  private Members seller;
  private Product[] products = new Product[2];
  public Deal(Members buyer, Members seller, Product[] products) {
  }
  public Date getDate() {
    return date;
  }
  public Members getBuyer() {
    return buyer;
  }
  public Members getSeller() {
    return seller;
  }
  public Product[] getProducts() {
    return products;
  }
  public double inTotal() {
    double summ = 0;
    for (int i = 0; i < products.length; i++) {
      summ += products[i].getCost(products[i].getPrice(),
        products[i].getQuantity());
    }
    return summ;
  }
}

成员.java

package ua.lviv.my;
public class Members {
  String name;
  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  }
}

package ua.lviv.my;
public class Product {
  private String title;
  private double price;
  private int quantity;
  public String getTitle() {
    return title;
  }
  public void setTitle(String title) {
    this.title = title;
  }
  public double getPrice() {
    return price;
  }
  public void setPrice(double price) {
    this.price = price;
  }
  public int getQuantity() {
    return quantity;
  }
  public void setQuantity(int quantity) {
    this.quantity = quantity;
  }
  public double getCost(double price, int quantity){
    double cost = price*quantity;
    return cost;
  }
}

Answer 1

首先查看 Deal 的构造函数...

public Deal(Members buyer, Members seller, Product[] products) {

}

例如，您永远不会将通过构造函数传递的任何值分配给成员字段...

public Deal(Members buyer, Members seller, Product[] products) {
    this.buyer = buyer;
    this.seller = seller;
    this.products = products;
}