Java servlet，从文件读取，数组越界

Question

我正在制作一个java servlet，我的任务是获取文件中写入的产品的总成本:

category1 1 10 101 1 good1
category6 2 11 105 2 good5
category1 5 13 103 3 good4
category3 6 14 102 4 good2
category5 3 12 107 2 good1

费用在第 4 栏。我已经写了:

    public int Sum_of_Elements()throws IOException{
    int sum = 0;
    BufferedReader br = new BufferedReader(new FileReader("/Data.txt"));
    String line = "";
    while((line=br.readLine())!=null){
        String[] columns = line.split(" ");
        sum = sum + Integer.parseInt(columns[4]);
    }
    System.out.println(sum);
    return sum;
}

而且它不起作用。当我进入 servlet 页面时，我得到

java.lang.ArrayIndexOutOfBoundsException: 4

怎么了？以及如何修复它？

Answer 1

如果存在以下情况，此代码将失败:文件中的空行，或者格式不同的行，或者空格实际上是制表符(也许还有更多原因)。如果你想进行防御性编程，你应该这样做:

while((line=br.readLine())!=null) {
    String[] columns = line.split(" ");
    if( columns != null && columns.length >= 5 ) {
        sum = sum + Integer.parseInt(columns[4]);
    }
    else {
        // do what you must here; it may be an error to encounter such a line:
        throw new IllegalArgumentException("malformatted line: " + line);
        // or it may be OK to swallow the exceptional case
        // or you may only need to silently log it:
        logger.warn("malformatted line: " + line);
        // etc...
    }
}