finished

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

@@ -1,6 +1,8 @@
 package rocks.zipcode;
 
 import java.io.IOException;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 
 /**
  * @author leon on 16/11/2018.
@@ -12,9 +14,12 @@ public class AlphaCharDocument extends Document {
 
     @Override
     public void write(String contentToBeWritten) {
+        if (isAlpha(contentToBeWritten)) {
+            super.write(contentToBeWritten);
+        } else throw new IllegalArgumentException();
     }
 
     private Boolean isAlpha(String s) {
-        return null;
+        return s.matches("[a-zA-Z\\s]*");
     }
 }

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

@@ -12,9 +12,13 @@ public class NumericCharDocument extends Document {
 
     @Override
     public void write(String contentToBeWritten) {
+        if (isNumeric(contentToBeWritten)) {
+            super.write(contentToBeWritten);
+        } else throw new IllegalArgumentException();
     }
 
     private Boolean isNumeric(String s) {
-        return null;
+
+        return s.matches("[0-9]*");
     }
 }

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

@@ -12,9 +12,13 @@ public class SpecialCharDocument extends Document {
 
     @Override
     public void write(String contentToBeWritten) {
+        if (isSpecialCharacters(contentToBeWritten)) {
+            super.write(contentToBeWritten);
+        } else throw new IllegalArgumentException();
     }
 
     private Boolean isSpecialCharacters(String s) {
-        return null;
+
+        return s.matches("[^a-zA-Z0-9]*");
     }
 }