java - 为什么我要为String数组获取java.lang.NullPointerException？

Question

不确定我为什么要为String数组获取java.lang.NullPointerException吗？我看了几个小时，却找不到任何问题。请帮忙

private String[] stringList;
int j = 0;
StringClass stringRef = new StringClass();

while (//As long as there is string available from what I am reading in)
{
        str = //String read in;
        stringRef.setString(str);
        stringList[j] = stringRef.returnString(); //Shows that this line is the error
        j++;
}

这是类(class):

public class StringClass {

   private String stringNew;

   public void setString(String newStr){
      stringNew = newStr;
   }

   public String returnString(){
      return stringNew;
  }
}

Answer 1

即使它是变量List的名称，也没有使用stringList。列表将动态调整大小，因此您不必在编译时声明初始大小。

这是您的代码以及我建议的更改:

import java.util.ArrayList;
import java.util.List;

...

private List<String> stringList = new ArrayList<String>();
int j = 0;
StringClass stringRef = new StringClass();

while (//As long as there is string available from what I am reading in)
{
        str = //String read in;
        stringRef.setString(str);
        stringList.add(stringRef.returnString()); //Shows that this line is the error
        j++;
}