Streams are everywhere in Java 8. Just look around and for sure you will find them. It also applies to java.io.BufferedReader. Parsing a file in Java 8 with Stream API is extremely easy.
I have a CSV file that I want to be read. An example below:
username;visited jdoe;10 kolorobot;4
A contract for my reader is to provide a header as list of strings and all records as list of lists of strings. My reader accepts java.io.Reader as a source to read from.
I will start with reading the header. The algorithm for reading the header is as follows:
- Open a source for reading,
- Get the first line and parse it,
- Split line by a separator,
- Get the first line and parse it,
- Convert the line to list of strings and return.
And the implementation:
class CsvReader {
private static final String SEPARATOR = ";";
private final Reader source;
CsvReader(Reader source) {
this(source);
}
List<String> readHeader() {
try (BufferedReader reader = new BufferedReader(source)) {
return reader.lines()
.findFirst()
.map(line -> Arrays.asList(line.split(SEPARATOR)))
.get();
} catch (IOException e) {
throw new UncheckedIOException(e);
}
}
}Fairly simple. Self-explanatory. Similarly, I created a method to read all records. The algorithm for reading the records is as follows:
- Open a source for reading,
- Skip the first line,
- Split line by a separator,
- Apply a mapper on each line that maps a line to a list of strings.
And the implementation:
class CsvReader {
List<List<String>> readRecords() {
try (BufferedReader reader = new BufferedReader(source)) {
return reader.lines()
.substream(1)
.map(line -> Arrays.asList(line.split(separator)))
.collect(Collectors.toList());
} catch (IOException e) {
throw new UncheckedIOException(e);
}
}
}Nothing fancy here. What you could notice is that a mapper in both methods is exactly the same. In fact, it can be easily extracted to a variable:
Function<String, List<String>> mapper
= line -> Arrays.asList(line.split(separator));To finish up, I created a simple test.
public class CsvReaderTest {
@Test
public void readsHeader() {
CsvReader csvReader = createCsvReader();
List<String> header = csvReader.readHeader();
assertThat(header)
.contains("username")
.contains("visited")
.hasSize(2);
}
@Test
public void readsRecords() {
CsvReader csvReader = createCsvReader();
List<List<String>> records = csvReader.readRecords();
assertThat(records)
.contains(Arrays.asList("jdoe", "10"))
.contains(Arrays.asList("kolorobot", "4"))
.hasSize(2);
}
private CsvReader createCsvReader() {
try {
Path path = Paths.get("src/test/resources", "sample.csv");
Reader reader = Files.newBufferedReader(
path, Charset.forName("UTF-8"));
return new CsvReader(reader);
} catch (IOException e) {
throw new UncheckedIOException(e);
}
}
}| Reference: | Parsing a file with Stream API in Java 8 from our JCG partner Rafal Borowiec at the Codeleak.pl blog. |
