java - 循环内字符串串联并引发不允许的异常 : java. lang.Exception

Question

在下面的代码中

for (UserSelection userSelection : filterList) {
                    String filterName = checkForDataHolder(userSelection
                            .getCriteriaName());
                    csvRow.append(filterName + ":");
                    List<String> list = userSelection.getCriteriaValues();
                    String filterval = "";
                    csvRow.append(DATA_DELIMITER);
                    for (String value : list) {

                        filterval += checkForDataHolder(value) + ", ";
                    }
                    if (filterval.indexOf(",") != -1) {
                        filterval = filterval.substring(0, filterval
                                .lastIndexOf(","));
                    }
                    csvRow.append(DATA_HOLDER + filterval + DATA_HOLDER);
                    csvRow.append(NEW_LINE);

                }

这一行filterval += checkForDataHolder(value) + ", ";

导致循环内的字符串串联，为每个串联创建一个 StringBuffer。当放置在循环中时，这可能会导致创建和收集大量临时对象。如何在进入循环之前创建 StringBuffer 并在循环内附加到它？

也在下面的代码中

final StringBuffer csvRow = new StringBuffer();
        csvRow.append(NEW_LINE);
        csvRow.append(" ");

我想将字符串文字替换为字 rune 字以提高性能

考虑以下代码

public void writeHeader(List<String> headerColList) throws IOException {
        this.header = headerColList;
        String val;
        try {
            final StringBuffer csvRow = new StringBuffer();
            for (int i = 0; i < header.size(); i++) {
                if (i > 0) {
                    csvRow.append(DATA_DELIMITER);
                }
                val = checkForDataHolder(headerColList.get((i)));
                if (!StringUtil.isEmpty(val)) {
                    csvRow.append(DATA_HOLDER + val + DATA_HOLDER);
                }
            }
            if(!isHeaderProcessed) {
                if(this.header != null) {
                    writer.write(csvRow.toString());
                        writer.write(NEW_LINE);
                    }
                    isHeaderProcessed = true;
                }
            } catch (Exception e) {
                log.error(e);
            }
        }

我想考虑捕获更具体的异常类(异常的一个或多个子类)。这是最佳选择吗？

Answer 1

for (String value : list) {

                    filterval += checkForDataHolder(value) + ", ";
                }

if (filterval.indexOf(",") != -1) {
                        filterval = filterval.substring(0, filterval
                                .lastIndexOf(","));
                    }

这是怎么回事？去掉尾随空格似乎效率很低。也许这更快:

bool Appended = false;
for (String value : list) {
                    if(Appended) { filterval += " ";}
                    filterval += checkForDataHolder(value) + ",";
                    Appended = true;
                }

像沿着字符串扫描这样的事情比使用“”而不是“”更耗时。

异常(exception)确实是你的选择。您期待的不仅仅是内存分配错误吗？