java - CompareTo 函数问题

Question

帮助我无法弄清楚compareTo函数。这就是我必须做的:编写一个compareTo函数，可用于根据以下内容按顺序放置产品到他们的零件号。也就是说，后面按字母顺序排列的零件号顺序大于按字母顺序排列较早的部件号。这是我的代码:

public class ProductType implements Comparable<ProductType> {
    private String partnum;
    private double price;
    private int stock;

    public ProductType(String partnum, double price, int stock) {
        this.partnum = partnum;
        this.price = price;
        this.stock = stock;
    }

    public ProductType() {
        partnum = "";
        price = 0;
        stock = 0;
    }

    public void setNum(String partnum) {
        this.partnum = partnum;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    public void setStock(int stock) {
        this.stock = stock;
    }

    public String getNum() {
        return partnum;
    }

    public double getPrice() {
        return price;
    }

    public int getStock() {
        return stock;
    }

    public int compareTo(ProductType otherType) throws NullPointerExeption {
        if (otherType == null)
            throw new NullPointerException();
        return (this.getNum().compareTo.otherType.getNum());
    }

    public String toString() {
        String result = "" + this.getNum();
        return result;
    }
}

Answer 1

更改您的返回声明

 return (this.getNum().compareTo.otherType.getNum());

至

 return (this.getNum().compareTo(otherType.getNum()));

因为 compareTo() 是方法。

在调用compareTo()方法之前检查是否

null != this.getNum()

否则你会得到NPE。