そういえば書いてなかったのでメモ。

    /**
     * Hogeクラスのprivateメソッドadd(int a, int b)をテストする
     * @throws SecurityException
     * @throws IllegalArgumentException
     * @throws NoSuchMethodException
     * @throws IllegalAccessException
     * @throws InvocationTargetException
     */
    public void testAdd() throws SecurityException, IllegalArgumentException, NoSuchMethodException, IllegalAccessException, InvocationTargetException {
        Hoge hoge = new Hoge();
        Class<Hoge> clz = Hoge.class;
        Method method = clz.getDeclaredMethod("add", Integer.class, Integer.class);
        method.setAccessible(true);
        int result = (int) method.invoke(hoge, 5, 3);
        asserTrue(result == 8);
    }

■staticメソッド

    /**
     * Hogeクラスのprivate staticメソッドadd(int a, int b)をテストする
     * @throws SecurityException
     * @throws IllegalArgumentException
     * @throws NoSuchMethodException
     * @throws IllegalAccessException
     * @throws InvocationTargetException
     */
    public void testAdd() throws SecurityException, IllegalArgumentException, NoSuchMethodException, IllegalAccessException, InvocationTargetException {
        Class<Hoge> clz = Hoge.class;
        Method method = clz.getDeclaredMethod("add", Integer.class, Integer.class);
        method.setAccessible(true);
        int result = (int) method.invoke(clz, 5, 3);
        asserTrue(result == 8);
    }

■プライベートフィールド

Hoge hoge = new Hoge();
Field f = hoge.getClass().getDeclaredField("f");
f.setAccessible(true);
assertEquals(value, f.get(hoge));
//assertEquals(Piyo.class, f.get(hoge).getClass());