Java: avoid checking for null in nested classes (Deep Null checking)

Question

Imagine I have a class Family. It contains a List of Person. Each (class) Person contains a (class) Address. Each (class) Address contains a (class) PostalCode. Any "i

Answer 1

I was just looking for the same thing (my context: a bunch of automatically created JAXB classes, and somehow I have these long daisy-chains of .getFoo().getBar().... Invariably, once in a while one of the calls in the middle return null, causing NPE.

Something I started fiddling with a while back is based on reflection. I'm sure we can make this prettier and more efficient (caching the reflection, for one thing, and also defining "magic" methods such as ._all to automatically iterate on all the elements of a collection, if some method in the middle returns a collection). Not pretty, but perhaps somebody could tell us if there is already something better out there:

/**
 * Using {@link java.lang.reflect.Method}, apply the given methods (in daisy-chain fashion)
 * to the array of Objects x.
 * 
 * <p>For example, imagine that you'd like to express:
 * 
 * <pre><code>
 * Fubar[] out = new Fubar[x.length];
 * for (int i=0; {@code i<x.length}; i++) {
 *   out[i] = x[i].getFoo().getBar().getFubar();
 * }
 * </code></pre>
 * 
 * Unfortunately, the correct code that checks for nulls at every level of the
 * daisy-chain becomes a bit convoluted.
 * 
 * <p>So instead, this method does it all (checks included) in one call:
 * <pre><code>
 * Fubar[] out = apply(new Fubar[0], x, "getFoo", "getBar", "getFubar");
 * </code></pre>
 * 
 * <p>The cost, of course, is that it uses Reflection, which is slower than
 * direct calls to the methods.
 * @param type the type of the expected result
 * @param x the array of Objects
 * @param methods the methods to apply
 * @return
 */
@SuppressWarnings("unchecked")
public static <T> T[] apply(T[] type, Object[] x, String...methods) {
    int n = x.length;
    try {
        for (String methodName : methods) {
            Object[] out = new Object[n];
            for (int i=0; i<n; i++) {
                Object o = x[i];
                if (o != null) {
                    Method method = o.getClass().getMethod(methodName);
                    Object sub = method.invoke(o);
                    out[i] = sub;
                }
            }
            x = out;
        }
    T[] result = (T[])Array.newInstance(type.getClass().getComponentType(), n);
    for (int i=0; i<n; i++) {
            result[i] = (T)x[i];
    }
            return result;
    } catch (NoSuchMethodException | IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
            throw new RuntimeException(e);
    }
}

Answer 2

and my favorite, the simple try/catch, to avoid nested null checks...

try {
    if(order.getFulfillmentGroups().get(0).getAddress().getPostalCode() != null) {
        // your code
    } 
} catch(NullPointerException|IndexOutOfBoundsException e) {}

Answer 3

Not such a cool idea, but how about catching the exception:

    try 
    {
        PostalCode pc = people.get(0).getAddress().getPostalCode();
    }
    catch(NullPointerException ex)
    {
        System.out.println("Gotcha");
    }

Answer 4

If, in case, you are using java8 then you may use;

resolve(() -> people.get(0).getAddress().getPostalCode());
    .ifPresent(System.out::println);

:
public static <T> Optional<T> resolve(Supplier<T> resolver) {
    try {
        T result = resolver.get();
        return Optional.ofNullable(result);
    }
    catch (NullPointerException e) {
        return Optional.empty();
    }
}

REF: avoid null checks

Answer 5

If you can use groovy for mapping it will clean up the syntax and codes looks cleaner. As Groovy co-exist with java you can leverage groovy for doing the mapping.

if(family != null) {
    if(family.getPeople() != null) {
        if(family.people.get(0) != null) {
            if(people.get(0).getAddress() != null) {
                if(people.get(0).getAddress().getPostalCode() != null) {
                    //FINALLY MADE IT TO DO SOMETHING!!!
                }
            }
        }
    }
}

instead you can do this

if(family?.people?[0]?.address?.postalCode) {
   //do something
}

or if you need to map it to other object

somobject.zip = family?.people?[0]?.address?.postalCode