remove public access modifiers on ref solution and initial stub exception for circular buffer

exercises/circular-buffer/.meta/src/reference/java/BufferIOException.java

-public class BufferIOException extends Exception {
+class BufferIOException extends Exception {
 
     BufferIOException(String message) {
         super(message);

exercises/circular-buffer/.meta/src/reference/java/CircularBuffer.java

 import java.util.LinkedList;
 import java.util.Queue;
 
-public class CircularBuffer<T> {
+class CircularBuffer<T> {
 
     private Queue<T> buffer;
     private int capacity;
 
-    public CircularBuffer(int size) {
+    CircularBuffer(int size) {
         this.buffer = new LinkedList<>();
         this.capacity = size;
     }
 
-    public T read() throws BufferIOException {
+    T read() throws BufferIOException {
         if (this.buffer.size() == 0) {
             throw new BufferIOException("Tried to read from empty buffer");
         }
         return this.buffer.remove();
     }
 
-    public void write(T data) throws BufferIOException {
+    void write(T data) throws BufferIOException {
         if (this.buffer.size() == this.capacity) {
             throw new BufferIOException("Tried to write to full buffer");
         }
 
         this.buffer.add(data);
     }
-    public void overwrite(T data) {
+    void overwrite(T data) {
         if (this.buffer.size() == this.capacity) {
             this.buffer.remove();
         }
         this.buffer.add(data);
     }
 
-    public void clear() {
+    void clear() {
         this.buffer.clear();
     }
 }

exercises/circular-buffer/src/main/java/BufferIOException.java

-public class BufferIOException extends Exception {
+class BufferIOException extends Exception {
 
     BufferIOException(String message) {
         super(message);