public PushConnection remove(final String clientId) {
        final PushConnection pc = clientPushConnectionMap.remove(clientId);
        return pc;
    }
@Test
    void testRemove() {
        pushConnectionRegistry.put("clientId1", pushConnection);

        assertEquals(pushConnection, pushConnectionRegistry.remove("clientId1"));
        assertNull(pushConnectionRegistry.get("clientId1"));
    }