Added code to write CSV file

src/main/java/edu/unl/cse/csv_io/CSVReaderWriter.java

@@ -5,10 +5,8 @@
 
 package edu.unl.cse.csv_io;
 
-import java.io.BufferedReader;
-import java.io.IOException;
-import java.io.InputStream;
-import java.io.InputStreamReader;
+import java.io.*;
+import java.net.URL;
 import java.util.*;
 
 public class CSVReaderWriter {
@@ -29,6 +27,47 @@ public class CSVReaderWriter {
         return csvSet;
     }
 
+    public static boolean writeCSV(String filename, Set<Map<String, String>> data) {
+        boolean wroteFile = true;
+        ClassLoader classLoader = CSVReaderWriter.class.getClassLoader();
+        URL resource = classLoader.getResource("csv/" + filename);
+        if (resource != null) {
+            File file = new File(resource.getPath());
+            try(OutputStream outputStream = new FileOutputStream(file)) {
+                PrintStream writer = new PrintStream(outputStream);
+                Set<String> fieldNames = null;
+                int number_of_fields = 0;
+                for (Map<String, String> row: data) {
+                    int field_number = 0;
+                    if (fieldNames == null) {
+                        fieldNames = row.keySet();
+                        number_of_fields = fieldNames.size();
+                        for (String field: fieldNames) {
+                            writer.print(field);
+                            writer.print(++field_number<number_of_fields ? "," : "\n");
+                        }
+                    }
+                    field_number = 0;
+                    for (String field: fieldNames) {
+                        String value = row.get(field);
+                        writer.print(value != null ? value : "");
+                        writer.print(++field_number<number_of_fields ? "," : "\n");
+                    }
+                }
+            } catch (FileNotFoundException fileNotFoundException) {
+                System.err.println("Could not open " + filename + "; probably due to a bad pathname.  " + fileNotFoundException);
+                wroteFile = false;
+            } catch (IOException ioException) {
+                System.err.println("Error opening " + filename + ".  " + ioException);
+                wroteFile = false;
+            }
+        } else {
+            System.err.println("csv/" + filename + " not found. Cannot overwrite file unless it exists.");
+            wroteFile = false;
+        }
+        return wroteFile;
+    }
+
     private static Set<Map<String, String>> parseCSV(InputStream inputStream) throws IOException {
         Set<Map<String, String>> csvSet = new HashSet<>();
         BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
@@ -57,6 +96,9 @@ public class CSVReaderWriter {
 
     public static void main(String[] args) {
         Set<Map<String, String>> demo = readCSV("demo.csv");
+        boolean success = writeCSV("out.csv", demo);
+        System.out.println(success ? "Wrote file!" : "Didn't write file");
+        /*
         Set<String> fieldNames = null;
         for (Map<String, String> row: demo) {
             if (fieldNames == null) {
@@ -68,5 +110,6 @@ public class CSVReaderWriter {
             }
             System.out.println();
         }
+        */
     }
 }