英文:
What's wrong with my code for the toString() method?
问题
请注意，我是您的中文翻译，以下为翻译好的内容：
请注意，我是一个完全的编程初学者，发现学习Java相当困难。我有一个学校任务，需要我编写一个toString()方法的代码，我已经盲目地按照我的老师在课堂上写/教的内容进行了编写，但由于某种原因，当我运行测试文件时，我的代码似乎无法正常工作？我已经在下面包含了我的代码。
我确实收到一个错误，上面写着：“空指针访问：变量coef在此位置只能为null”，但是当我改变了所有的coef[]变量时，我的代码就变得无意义，因为它只等于null。
非常感谢任何指导！
编辑1：感谢您的回复，我以为我需要在toString()方法内部初始化coef[]作为一个变量（大错误！！）。还要感谢您提供了空指针问题的链接 - 上面有一些非常详细的解释，现在我明白我对coeff[]变量进行了取消引用。
问题a：我已经删除了我的第一行，但现在似乎我的代码在这一行上失败了 return coef[1]+ "x + " + coef[0];
问题b：好奇知道如果类是复数形式为什么是一个不好的迹象？
public class Polynomials {public Double coefficient;public Integer exponent;private int[] coef;public String toString() {String[] coef = null;if (exponent <= -1)return "0";else if (exponent == 0)return "" + coef[0];else if (exponent == 1)return coef[1] + "x + " + coef[0];String s = coef[exponent] + "x^" + exponent;return s;}}
英文:
please note that I'm a complete beginner to programming and have found learning Java quite hard. I have a school task that requires me to write a code for a toString() method and I've blindly followed what my teacher has been writing/teaching in class but for some reason my code doesn't seem to work when I run the test file? I've included my code below.
I do get an error that says the following- "Null pointer access: The variable coef can only be null at this location" but when I change all my coef[] variables, then my code becomes moot because it just equates to null.
Any pointers greatly appreciated!
EDIT 1: Thank you for your responses - I thought I needed to intialise coef[] as a variable inside my toString() method (big oops!!). Also thank you for providing the link to the NullPointer question - it had some really thorough explanations and now I understand that I dereferenced the coeff[] variable.
QUERY a: I've removed my first line but now it seems that my code fails on this line return coef[1]+ "x + " + coef[0];
QUERY b: curious to know why it's a bad sign if the class is plural?
public class Polynomials {public Double coefficient;public Integer exponent;private int[] coef;public String toString() {String[] coef = null;if (exponent <= -1)return "0";else if (exponent == 0)return "" + coef[0];else if (exponent == 1)return coef[1]+ "x + " + coef[0];String s = coef[exponent] + "x^" + exponent;return s;}
答案1
得分: 2
你的Polynomials类有3个字段，其中一个称为coef。在你的toString方法中，你声明了一个同样叫做coef的局部变量。
在Java中，这意味着局部变量“遮蔽”了字段 - 换句话说，在整个方法中，coef指的是String[] coef = null;，而不是int[] coef。
而且这个局部变量始终是空的 - 你创建它，将它初始化为空，然后从未改变过。因此，coef[0]在运行时将保证抛出NullPointerException。
修复方法似乎是... 只需完全删除那一行String[] coef = null;。我不知道你为什么写那行代码或者它想要实现什么。
注：这个类的名字不应该是Polynomial吗？用复数形式命名类通常不是一个好兆头。
英文:
Your Polynomials class has 3 fields, and one is called coef. In your toString method you then declare a local variable, also called coef.
In java, that means the local variable 'shadows' the field - in other words, for that entire method, coef refers to the String[] coef = null; and not the int[] coef.
And that local field is always null - you create it, initialize it to null, and never change it. Thus, choef[0] will guaranteed throw a NullPointerException at runtime.
The fix seems to be to .... just remove that String[] coef = null; line entirely. I have no idea why you wrote that or what it's trying to accomplish.
NB: Shouldn't the class be named Polynomial? Naming a class a plural is usually a bad sign.
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