Java NullPointerException(Java NullPointerException)

 我正在编写两个类来处理简单的拍卖。 我有一个准备好工作的班级，处理单次拍卖的操作，现在我正在编写另一个类似拍卖行的班级，以跟踪所有可用的拍卖。 在测试课程的以下部分时： 
 
import java.util.ArrayList;

public class AuctionHouse {
    private ArrayList<DutchAuction> huutokaupat;

    public AuctionHouse() {
    }

    public void addAuction(DutchAuction newAuction) {
        huutokaupat.add(newAuction);
    }
}
 
 在一个带有以下代码的main方法中（“kauppa”是一个经过测试和工作的对象变量）： 
 
AuctionHouse talo = new AuctionHouse();
talo.addAuction(kauppa);
 
 我明白了： 
 
 
 
 在ope.auction.dutch.DutchAuctionTest.main（DutchAuctionTest.java:54）的ope.auction.dutch.AuctionHouse.addAuction（AuctionHouse.java:13）中的线程“main”java.lang.NullPointerException中的异常 
 
 
 我该如何解决这个问题？ 

I'm writing two classes to handle simple auctions. I have a class ready and working, which handles the operations for a single auction, and now I'm writing another class like an auction house, to keep track of all the auctions available. When testing the following part of the class:
 
import java.util.ArrayList;

public class AuctionHouse {
    private ArrayList<DutchAuction> huutokaupat;

    public AuctionHouse() {
    }

    public void addAuction(DutchAuction newAuction) {
        huutokaupat.add(newAuction);
    }
}
 
inside a main method with following code("kauppa" is a tested and working object-variable):
 
AuctionHouse talo = new AuctionHouse();
talo.addAuction(kauppa);
 
I get:
 
 
 
Exception in thread "main" java.lang.NullPointerException at ope.auction.dutch.AuctionHouse.addAuction(AuctionHouse.java:13) at ope.auction.dutch.DutchAuctionTest.main(DutchAuctionTest.java:54)
 
 
How can I fix the problem?

原文：https://stackoverflow.com/questions/1582215