Using Optional.ofNullable and flatMap chaining to retrieve deeply nested properties without having to write a bunch of null checks.

NullableChain.java

package com.soverby.test;

import org.junit.Test;
import java.util.Optional;
import java.util.function.Function;

import static org.assertj.core.api.Java6Assertions.assertThat;

public class NullableChain {

    // Prevent NPEs when extracting a value from a property graph, without having to manually check every one. 
    public static final Function<DomainType, String> getDeeplyNestedValue = (test) ->
            Optional.ofNullable(test)
                    .flatMap(uwrap -> Optional.ofNullable(uwrap.getPropertyDomainType()))
                    .flatMap(uwrap -> Optional.ofNullable(uwrap.getPropertyDomainType()))
                    .flatMap(uwrap -> Optional.ofNullable(uwrap.getPropertyDomainType()))
                    .flatMap(uwrap -> Optional.ofNullable(uwrap.getValue()))
                    .orElseThrow(() -> new IllegalArgumentException("Get chain may not be null"));

    @Test(expected=IllegalArgumentException.class)
    public void retrievedNestedValue_nullTopLevel() {
        getDeeplyNestedValue.apply(null);
    }

    @Test(expected=IllegalArgumentException.class)
    public void retrievedNestedValue_nullThirdLevel() {
        DomainType one = new DomainType("one");
        DomainType two = new DomainType("two");
        one.setPropertyDomainType(two);
        getDeeplyNestedValue.apply(one);
    }

    @Test
    public void retrievedNestedValue_returnsExpectedValue() {
        DomainType one = new DomainType("one");
        DomainType two = new DomainType("two");
        DomainType three = new DomainType("three");
        DomainType four = new DomainType("four");
        three.setPropertyDomainType(four);
        two.setPropertyDomainType(three);
        one.setPropertyDomainType(two);
        assertThat(getDeeplyNestedValue.apply(one)).isEqualTo("four");
    }

    public static class DomainType {
        private DomainType propertyDomainType;
        private String value;

        public DomainType(String value) {
            this.value = value;
        }

        public DomainType getPropertyDomainType() {
            return this.propertyDomainType;
        }

        public void setPropertyDomainType(DomainType one) {
            this.propertyDomainType = one;
        }

        public String getValue() {
            return this.value;
        }
    }
}