public static <T> T get(String key) {
        Object result = SINGLE_OBJECT_POOL.get(key);
        return result == null ? null : (T) result;
    }
@Test
    public void assertSingletonGet2() {
        Assert.assertNull(Singleton.get("userName1", () -> null));
        Assert.assertEquals("hippo4j", Singleton.get("userName1", () -> "hippo4j"));
        Assert.assertEquals("123456", Singleton.get("pw", () -> "123456") + "");
        Assert.assertNotEquals("135790", Singleton.get("pw", () -> "135790") + "");
    }