assertEquals("foo", sb.toString());  sb.append((String) null);
    public void testAppend_String() {        StrBuilder sb = new StrBuilder();        sb.append("foo");        assertEquals("foo", sb.toString());        sb.append((String) null);        assertEquals("foo", sb.toString());        sb.append("");        assertEquals("foo", sb.toString());        sb.append("bar");        assertEquals("foobar", sb.toString());    }