各位这段代码怎么理解，还有里面的空指向异常是为什么
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.*;
import java.util.Vector;

public class VectorDemo{
public static void main(String a[]) throws Exception {
//File f = File("E:" + File.separator + "data.txt");
String fileName = "data.txt";

Vector<String> vector = getContentVector(fileName);

for(int i = 0; i < vector.size(); i++){

System.out.print(vector.get(i) + " ");

if(i % 7 == 0 && i != 0){
System.out.println();
}
}

}

private static Vector<String> getContentVector(String fileName) throws Exception {
Vector<String> vector = new Vector<String>();
BufferedReader bf = new BufferedReader(new FileReader(fileName));
String content = "";
StringBuilder sb = new StringBuilder();

while(content != null){
content = bf.readLine();

if(content == null){
break;
}

if(!content.trim().equals("")){
sb.append(content.trim().toUpperCase());
}
}

for(String str: sb.toString().split("\\s+")){
vector.add(str);
}
return vector;
}
}


[解决办法]
这个就是把data.txt里的内容转换成大写打印在控制台上，如果你没有data.txt当然会有错。
在E盘建个data.txt，里面写一些字母。改一下String fileName = "E://data.txt";