I have a hashmap with a String key and String value. It contains a large number of keys and their respective values.
For example:
key | value
abc | aabbcc
def | ddeeff
I would like to write this hashmap to a csv file such that my csv file contains rows as below:
abc,aabbcc
def,ddeeff
I tried the following example here using the supercsv library: http://javafascination.blogspot.com/2009/07/csv-write-using-java.html. However, in this example, you have to create a hashmap for each row that you want to add to your csv file. I have a large number of key value pairs which means that several hashmaps, with each containing data for one row need to be created. I would like to know if there is a more optimized approach that can be used for this use case.
Something like this should do the trick:
String eol = System.getProperty("line.separator");
try (Writer writer = new FileWriter("somefile.csv")) {
for (Map.Entry<String, String> entry : myHashMap.entrySet()) {
writer.append(entry.getKey())
.append(',')
.append(entry.getValue())
.append(eol);
}
} catch (IOException ex) {
ex.printStackTrace(System.err);
}
Using the Jackson API, Map or List of Map could be written in CSV file. See complete example here
/**
* @param listOfMap
* @param writer
* @throws IOException
*/
public static void csvWriter(List<HashMap<String, String>> listOfMap, Writer writer) throws IOException {
CsvSchema schema = null;
CsvSchema.Builder schemaBuilder = CsvSchema.builder();
if (listOfMap != null && !listOfMap.isEmpty()) {
for (String col : listOfMap.get(0).keySet()) {
schemaBuilder.addColumn(col);
}
schema = schemaBuilder.build().withLineSeparator(System.lineSeparator()).withHeader();
}
CsvMapper mapper = new CsvMapper();
mapper.writer(schema).writeValues(writer).writeAll(listOfMap);
writer.flush();
}
As your question is asking how to do this using Super CSV, I thought I'd chime in (as a maintainer of the project).
I initially thought you could just iterate over the map's entry set using CsvBeanWriter
and a name mapping array of "key", "value"
, but this doesn't work because HashMap
's internal implementation doesn't allow reflection to get the key/value.
So your only option is to use CsvListWriter
as follows. At least this way you don't have to worry about escaping CSV (every other example here just joins with commas...aaarrggh!):
@Test
public void writeHashMapToCsv() throws Exception {
Map<String, String> map = new HashMap<>();
map.put("abc", "aabbcc");
map.put("def", "ddeeff");
StringWriter output = new StringWriter();
try (ICsvListWriter listWriter = new CsvListWriter(output,
CsvPreference.STANDARD_PREFERENCE)){
for (Map.Entry<String, String> entry : map.entrySet()){
listWriter.write(entry.getKey(), entry.getValue());
}
}
System.out.println(output);
}
Output:
abc,aabbcc
def,ddeeff
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