@Override
    public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
        // will throw UnsupportedOperationException; delegate anyway for testability
        return underlying().computeIfAbsent(key, mappingFunction);
    }
@Test
    public void testDelegationOfUnsupportedFunctionComputeIfAbsent() {
        final Function<Object, Object> mockFunction = mock(Function.class);
        new PCollectionsHashMapWrapperDelegationChecker<>()
            .defineMockConfigurationForUnsupportedFunction(mock -> mock.computeIfAbsent(eq(this), eq(mockFunction)))
            .defineWrapperUnsupportedFunctionInvocation(wrapper -> wrapper.computeIfAbsent(this, mockFunction))
            .doUnsupportedFunctionDelegationCheck();
    }