all test passing, refactoring

src/main/java/rocks/zipcode/AlphaCharDocument.java

 package rocks.zipcode;
 
+import java.io.BufferedWriter;
 import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 
 /**
  * @author leon on 16/11/2018.
 
     @Override
     public void write(String contentToBeWritten) {
+        if (isAlpha(contentToBeWritten)) {
+            try {
+                BufferedWriter out = Files.newBufferedWriter(Paths.get(getFile().getAbsolutePath()));
+                out.write(contentToBeWritten);
+                out.close();
+            } catch(IOException e) {
+                e.printStackTrace();
+            }
+        } else {
+            throw new IllegalArgumentException();
+        }
     }
 
     private Boolean isAlpha(String s) {
-        return null;
+        Pattern p = Pattern.compile("[^a-zA-Z ]");
+        Matcher m = p.matcher(s);
+        return !m.find();
     }
 }

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

 package rocks.zipcode;
 
+import java.io.BufferedWriter;
 import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 
 /**
  * @author leon on 16/11/2018.
 
     @Override
     public void write(String contentToBeWritten) {
+        if (isNumeric(contentToBeWritten)) {
+            try {
+                BufferedWriter out = Files.newBufferedWriter(Paths.get(getFile().getAbsolutePath()));
+                out.write(contentToBeWritten);
+                out.close();
+            } catch(IOException e) {
+                e.printStackTrace();
+            }
+        } else {
+            throw new IllegalArgumentException();
+        }
     }
 
     private Boolean isNumeric(String s) {
-        return null;
+        Pattern p = Pattern.compile("[\\D]");
+        Matcher m = p.matcher(s);
+        return !m.find();
     }
 }

src/main/java/rocks/zipcode/SpecialCharDocument.java

 package rocks.zipcode;
 
-import java.io.IOException;
+import sun.nio.cs.UTF_32;
+
+import java.io.*;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import static java.nio.charset.StandardCharsets.UTF_16;
 
 /**
  * @author leon on 18/11/2018.
 
     @Override
     public void write(String contentToBeWritten) {
+        if (isSpecialCharacters(contentToBeWritten)) {
+            try {
+                BufferedWriter out = Files.newBufferedWriter(Paths.get(getFile().getAbsolutePath()));
+                out.write(contentToBeWritten);
+            } catch (IOException io) {
+                System.out.println("An unexpected IO exception has occurred: " + io.getMessage());
+            }
+        } else {
+            throw new IllegalArgumentException();
+        }
     }
 
     private Boolean isSpecialCharacters(String s) {
-        return null;
+        Pattern p = Pattern.compile("[\\w&&[^_]]");
+        Matcher m = p.matcher(s);
+        return !m.find();
     }
 }