BestWayToWriteMethod

import junit.framework.AssertionFailedError;

import java.io.IOException;
import java.nio.file.*;
import java.util.logging.Logger;
import java.util.stream.Stream;

public class BestWayToWriteMethod {

    private static final Logger logger = Logger.getLogger("BestWayToWriteMethod");

    private final FileSystem _fileSystem;

    public BestWayToWriteMethod() {
        this(FileSystems.getDefault());
    }

    public BestWayToWriteMethod(final FileSystem fileSystem) {
        _fileSystem = fileSystem;
    }

    public void processFile(final String fileName) throws IOException {
        class LoopVariables {
            int lineNumber = 0;
            int detailRecordCount = 0;
            boolean trailerRecordSeen = false;
        }
        final LoopVariables vars = new LoopVariables();
        try (Stream<String> stream = Files.lines(_fileSystem.getPath(fileName))) {
            stream.forEach(line -> {
                ++vars.lineNumber;
                final String recordType = line.substring(0, 2);

                if (vars.trailerRecordSeen) {
                    // Not allowed to have more records after the trailer.
                    throw new AssertionFailedError("Invalid record after trailer on line " + vars.lineNumber + ": " + line);
                } else if (vars.lineNumber == 1) {
                    // The first record in the file must be the header record.
                    if (!recordType.equals("01")) {
                        throw new AssertionFailedError("First record must be a header record. Found record type '" + recordType + "' instead. Line #1 = " + line);
                    }
                } else {
                    switch (recordType) {

                        case "01":  // Out of place Header Record
                            throw new AssertionFailedError("Duplicate header record found at line " + vars.lineNumber + ": " + line);

                        case "02":  // Detail Record
                            ++vars.detailRecordCount;
                            // TODO: Presumably more "detail record procesing" would be done here.
                            break;

                        case "99":  // Trailer Record
                            vars.trailerRecordSeen = true;
                            final String rawRecordCount = line.substring(12, 21);
                            final int recordCount = Integer.parseInt(rawRecordCount);
                            if (recordCount != vars.detailRecordCount) {
                                throw new AssertionFailedError("Trailer record has incorrect record count. Trailer record has " + recordCount + ". Actual detail record count " + vars.detailRecordCount + ".");
                            }
                            break;

                        default:
                            throw new AssertionFailedError("Unrecognized record type '" + recordType + "' found at line " + vars.lineNumber + ": " + line);
                    }
                }
            });
        }
    }

}