minor refactor

src/main/java/rocks/zipcode/Document.java

@@ -30,8 +30,7 @@ public class Document implements DocumentInterface {
             e.printStackTrace();
         }
         try {
-            bw.append(contentToBeWritten);
-            bw.close();
+            bw.write(contentToBeWritten);
         } catch (IOException ex) {
             ex.printStackTrace();
         } finally {
@@ -57,23 +56,7 @@ public class Document implements DocumentInterface {
 
     @Override
     public String read(Integer lineNumber) {
-        String result ="";
-        try {
-            BufferedReader br = new BufferedReader(new FileReader(file));
-            String str =br.readLine();
-            int counter = 0;
-            while (str  != null){
-                if(counter==lineNumber){
-                result+= str;}
-                str = br.readLine();
-               counter++;
-            }
-        } catch (FileNotFoundException e) {
-            e.printStackTrace();
-        } catch (IOException e) {
-            e.printStackTrace();
-        }
-        return result;
+        return this.toList().get(lineNumber);
     }
 
     @Override

src/main/java/rocks/zipcode/NumericCharDocument.java

@@ -21,7 +21,7 @@ public class NumericCharDocument extends Document {
     }
 
     private void isNumeric(String s) throws IOException{
-        if(!s.matches("[0-9\\s]*"))
+        if(!s.matches("[\\d\\s]*"))
             throw new IllegalArgumentException();
     }
 }