Concatenation of strings within a loop and Disallowed exception thrown: java.lang.Exception

Question

In the following code

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);

                }

This line filterval += checkForDataHolder(value) + ", ";

causes Concatenation of strings within a loop which creates a StringBuffer for each concatenation. When placed in a loop, this can result in the creation and collection of large numbers of temporary objects. How do I create a StringBuffer before entering the loop, and append to it within the loop ?

Also in the following code

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

I want to Replace the string literal with a character literal to improve performance

consider the following code

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);
            }
        }

I want to consider catching a more specific class of exception (one or more subclasses of Exception).which is the best option here ?

user3125280 · Accepted Answer · 2013-12-25 12:14:34Z

1

for (String value : list) {

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

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

What's going on here? seems to be a very inefficient of taking off the trailing space. Maybe this is faster:

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

Things like scanning along a string are more time consuming than using " " instead of ' '.

The exception is really your choice. Are you expecting more than memory allocation errors?

answered Dec 25, 2013 at 12:14

user3125280

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2 Comments

Amit Sharad Over a year ago

Not really , I just want to know if just throwing IOException is worh in this case

user3125280 Over a year ago

did the suggestion help with performance? often these things are negligible, but it depends on size of string an how often a function is called. Don't worry about throwing the exception, it's safer than hoping the code went right. Just make sure to catch the error elsewhere and print a useful (for the user) message before exiting

Ingo · Accepted Answer · 2013-12-25 11:57:47Z

How do I create a StringBuffer before entering the loop, and append to it within the loop?

StringBuilder sb = new StringBuilder();
for ..... { }
String filterval = sb.toString();

I want to Replace the string literal with a character literal to improve performance?

So what keeps you from writing ' ' instead of " "? By the way, it wouldn't probably improve performance in any noticeably way. In the case of NEW_LINE, I am not sure if this is a character or a string constant, but note it may be a string constant with a length greater 1, like "\r\n".

I want to consider catching a more specific class of exception (one or more subclasses of Exception).which is the best option here ?

The best option is to catch the exceptions that may actually occur and that you can handle.

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