IO-546: ClosedOutputStream#flush should throw (closes #42)

Author: tjcelaya

src/main/java/org/apache/commons/io/output/ClosedOutputStream.java

@@ -47,4 +47,13 @@ public void write(final int b) throws IOException {
         throw new IOException("write(" + b + ") failed: stream is closed");
     }
 
+    /**
+     * Throws an {@link IOException} to indicate that the stream is closed.
+     *
+     * @throws IOException always thrown
+     */
+    @Override
+    public void flush() throws IOException {
+        throw new IOException("flush() failed: stream is closed");
+    }
 }

src/test/java/org/apache/commons/io/output/ClosedOutputStreamTest.java

@@ -41,4 +41,18 @@ public void testRead() throws Exception {
         }
     }
 
+    /**
+     * Test the <code>flush()</code> method.
+     * @throws Exception
+     */
+    @Test
+    public void testFlush() throws Exception {
+        try (ClosedOutputStream cos = new ClosedOutputStream()) {
+            cos.flush();
+            fail("flush()");
+        } catch (final IOException e) {
+            // expected
+        }
+    }
+
 }