Java中的空指针异常和范围

Question

我试图在Java的上下文中学习面向对象的编程.我正在编写一个相当简单的代码,但是我收到了这个错误:

Exception in thread "main" java.lang.NullPointerException
at Advisor_score.Rating.Score(Rating.java:12)
at Advisor_score.Rating.main(Rating.java:25)

这是我的代码:

package Advisor_score;
public class Rating {
    double [] Ratings;
    double sum;
    double raw_advisor;
    double advisor_score;
    public Rating (double [] x){
        double [] Ratings = x;
    }

    public double Score(){
        for(int i=2;i<Ratings.length;i++){
            sum+=Ratings[i];
        }
        raw_advisor=((sum-(3*(Ratings.length-2)))/4);
        advisor_score= 2.5+(2.5*(1-Math.pow(Math.E, -.5*raw_advisor)));
        return advisor_score;
    }

    public void print(){
        System.out.println(advisor_score);
    }

    public static void main(String[] args){ 
        double p1_1[] = {101,1,1,1.5,.5};
        Rating d = new Rating(p1_1);
        d.Score();
        d.print();
    }
}

我一直在看这几个小时,无法弄清楚代码的问题.我是编程新手可以有人帮助我吗？提前致谢!

Answer 1

public Rating (double [] x){
    double [] Ratings = x;
}

在这里,您将声明一个局部变量Ratings并分配x给它.之后,变量超出了范围.

你想要做的是Ratings = x;,它将实例变量设置Ratings为x而不是创建一个新的和未使用的局部变量.

Answer 2

这是你的代码,格式很好,修复了错误:

package com.yourdomain.advisor.score;

public class Rating{

    double[] ratings;
    double sum;
    double rawAdvisor;
    double advisorScore;

    public Rating(final double[] x){
        this.ratings = x;
    }

    public double score(){
        for(int i = 2; i < this.ratings.length; i++){
            this.sum += this.ratings[i];
        }
        this.rawAdvisor = (this.sum - 3 * (this.ratings.length - 2)) / 4;
        this.advisorScore =
            2.5 + 2.5 * (1 - Math.pow(Math.E, -.5 * this.rawAdvisor));
        return this.advisorScore;
    }

    public void print(){
        System.out.println(this.advisorScore);
    }

    public static void main(final String[] args){
        final double p1_1[] = { 101, 1, 1, 1.5, .5 };
        final Rating d = new Rating(p1_1);
        d.score();
        d.print();
    }
}

您正在为未使用的局部变量而不是字段分配值.

我根据java约定添加了一些更改:

• 包名称仅为小写字母和句点,应反映您的域名
• 类名在TitleCase中
• 方法,字段和局部变量名称在camelCase中

请参阅Sun Java命名约定

另外,帮自己一个忙:使用像Eclipse这样的IDE 并打开代码格式