Update per latest feedback

src/java.base/share/classes/java/io/ByteArrayOutputStream.java

@@ -34,18 +34,23 @@ import jdk.internal.util.ArraysSupport;
 /**
  * This class implements an output stream in which the data is
  * written into a byte array. The buffer automatically grows as data
- * is written to it.
- * The data can be retrieved using {@code toByteArray()} and
- * {@code toString()}.
+ * is written to it. The data can be retrieved using {@code toByteArray()}
+ * and {@code toString()}.
+ *
  * <p>
  * Closing a {@code ByteArrayOutputStream} has no effect. The methods in
  * this class can be called after the stream has been closed without
  * generating an {@code IOException}.
  *
+ * <p>
+ * Subclasses of this class may override {@code ensureCapacity(int)} in
+ * order to customize the buffer growth behavior. Subclasses which add
+ * additional write methods should either manage buffer growth manually
+ * or invoke {@code ensureCapacity(int)} to grow the buffer.
+ *
  * @author  Arthur van Hoff
  * @since   1.0
  */
-
 public class ByteArrayOutputStream extends OutputStream {
 
     /**
@@ -81,22 +86,6 @@ public class ByteArrayOutputStream extends OutputStream {
         buf = new byte[size];
     }
 
-    /**
-     * Increases the capacity if necessary to ensure that this
-     * {@code ByteArrayOutputStream} can hold at least the number of
-     * elements specified by the {@code minCapacity} argument.
-     * If the {@code minCapacity} argument is nonpositive, this
-     * method takes no action and simply returns.
-     *
-     * @param minCapacity the desired minimum capacity.
-     * @since 27
-     */
-    protected void ensureCapacity(int minCapacity) {
-        if (minCapacity > 0) {
-            ensureCapacityInternal(minCapacity);
-        }
-    }
-
     /**
      * Increases the capacity if necessary to ensure that this
      * {@code ByteArrayOutputStream} can hold at least the number of
@@ -105,10 +94,11 @@ public class ByteArrayOutputStream extends OutputStream {
      * @param  minCapacity the desired minimum capacity.
      * @throws OutOfMemoryError if {@code minCapacity < 0} and
      * {@code minCapacity - buf.length > 0}.  This is interpreted as a
-     * request for the unsatisfiably large capacity.
+     * request for the unsatisfiably large capacity
      * {@code (long) Integer.MAX_VALUE + (minCapacity - Integer.MAX_VALUE)}.
+     * @since 27
      */
-    private void ensureCapacityInternal(int minCapacity) {
+    protected void ensureCapacity(int minCapacity) {
         // overflow-conscious code
         int oldCapacity = buf.length;
         int minGrowth = minCapacity - oldCapacity;
@@ -125,7 +115,7 @@ public class ByteArrayOutputStream extends OutputStream {
      */
     @Override
     public synchronized void write(int b) {
-        ensureCapacityInternal(count + 1);
+        ensureCapacity(count + 1);
         buf[count] = (byte) b;
         count += 1;
     }
@@ -145,7 +135,7 @@ public class ByteArrayOutputStream extends OutputStream {
     @Override
     public synchronized void write(byte[] b, int off, int len) {
         Objects.checkFromIndexSize(off, len, b.length);
-        ensureCapacityInternal(count + len);
+        ensureCapacity(count + len);
         System.arraycopy(b, off, buf, count, len);
         count += len;
     }

test/jdk/java/io/ByteArrayOutputStream/EnsureCapacity.java

@@ -71,6 +71,23 @@ public class EnsureCapacity {
         byte[] actual = out.toByteArray();
         byte[] expected = new byte[] { 1, 2, 3, 4, 5, 6 };
         assertEquals(expected, actual);
+
+        // verify that overriding ensureCapacity() affects behavior of standard write methods
+
+        EvenOutputStream out2 = new EvenOutputStream();
+        assertEquals(0, out2.getBufferLength());
+
+        out2.write(89);
+        assertEquals(2, out2.getBufferLength());
+
+        out2.write(12);
+        assertEquals(2, out2.getBufferLength());
+
+        out2.write(new byte[] { 1, 2, 3 }, 0, 3);
+        assertEquals(6, out2.getBufferLength());
+
+        out2.write(77);
+        assertEquals(6, out2.getBufferLength());
     }
 
     private static void assertAtLeast(int actual, int min) {
@@ -95,6 +112,7 @@ public class EnsureCapacity {
         }
     }
 
+    // provides extra visibility into internals
     private static final class TestOutputStream extends ByteArrayOutputStream {
         public void ensureCapacity(int minCapacity) {
             super.ensureCapacity(minCapacity);
@@ -103,4 +121,21 @@ public class EnsureCapacity {
             return buf.length;
         }
     }
+
+    // starts empty, is always sized to an even number of bytes
+    private static final class EvenOutputStream extends ByteArrayOutputStream {
+        public EvenOutputStream() {
+            super(0);
+        }
+        protected void ensureCapacity(int minCapacity) {
+            int deficit = minCapacity - buf.length;
+            if (deficit > 0) {
+                minCapacity += minCapacity % 2;
+                buf = Arrays.copyOf(buf, minCapacity);
+            }
+        }
+        public int getBufferLength() {
+            return buf.length;
+        }
+    }
 }