Collecting values that could be null

Question

I have the following code:

private static <T> Map<String, ?> getDifference(final T a, final T b, final Map<String, Function<T, Object>> fields) {
    return fields.entrySet().stream()
            .map(e -> {
                final String name = e.getKey();
                final Function<T, Object> getter = e.getValue();
                final Object pairKey = getter.apply(a);
                final Object pairValue = getter.apply(b);
                if (Objects.equals(pairKey, pairValue)) {
                    return null;
                } else {
                    return Pair.of(name, pairValue);
                }
            })
            .filter(Objects::nonNull)
            .collect(Collectors.toMap(Pair::getKey, Pair::getValue));
}

Now, pairValue can be null. In order to avoid the NPE as described here, while "collect"-ing, I wish to ensure that I send only those values that are non-null. If null, I want to send "".

So, I tried replacing the last line with this:

.collect(Collectors.toMap(Pair::getKey,Optional.ofNullable(Pair::getValue).orElse(""));

And other modifications thereof:

.collect(Collectors.toMap(pair -> pair.getKey(), Optional.ofNullable(pair -> pair.getValue()).orElse(""));

Does not compile. I'm not sure what is needed here. Any help?

Can you elaborate on what doesn't work? Does the computer catch fire? — Chetan Kinger, Feb 15 '18 at 04:04
Java 8 NullPointerException in Collectors.toMap https://stackoverflow.com/questions/24630963/java-8-nullpointerexception-in-collectors-tomap Is this the same problem? — Chota Bheem, Feb 15 '18 at 04:05
I'm trying to do a null check to avoid the NPE. The code above doesn't compile. — user2116243, Feb 15 '18 at 04:06
What does the duplicate have to do with this question? i.e. this question has no mention of an array... — Jacob G., Feb 15 '18 at 04:07
Yeah, I have no idea! How do I get rid of the duplicate tag? — user2116243, Feb 15 '18 at 04:09
Unfortunately there's not much that I can do at the moment. I voted to reopen the question and flagged it for moderator intervention, so we'll see what happens. — Jacob G., Feb 15 '18 at 04:10
Does this answer your question? [NullPointerException in Collectors.toMap with null entry values](https://stackoverflow.com/questions/24630963/nullpointerexception-in-collectors-tomap-with-null-entry-values) — Nils-o-mat, Aug 27 '21 at 15:52

Holger · Answer 1 · 2018-02-15T07:54:25.530

You can just collect into a HashMap which allows null values without the need for an Optional:

private static <T> Map<String, Object> getDifference(
        final T a, final T b, final Map<String, Function<T, Object>> fields) {
    return fields.entrySet().stream()
        .map(e -> {
            final Function<T, Object> getter = e.getValue();
            final Object value = getter.apply(b);
            return Objects.equals(getter.apply(a),value)? null: Pair.of(e.getKey(), value);
        })
        .filter(Objects::nonNull)
        .collect(HashMap::new, (m,p) -> m.put(p.getKey(),p.getValue()), Map::putAll);
}

By the way, it is discouraged to use wildcards in return types, they can make the caller’s life unnecessarily hard for no benefit.

For comparison, here the same operation without Stream:

private static <T> Map<String, Object> getDifference(
        final T a, final T b, final Map<String, Function<T, Object>> fields) {
    HashMap<String, Object> result = new HashMap<>();
    fields.forEach((key, getter) -> {
        final Object value = getter.apply(b);
        if(!Objects.equals(getter.apply(a), value)) result.put(key, value);
    });
    return result;
}

Of course, this would also work with optional:

private static <T> Map<String, Optional<Object>> getDifference(
        final T a, final T b, final Map<String, Function<T, Object>> fields) {
    HashMap<String, Optional<Object>> result = new HashMap<>();
    fields.forEach((key, getter) -> {
        final Object value = getter.apply(b);
        if(!Objects.equals(getter.apply(a), value))
            result.put(key, Optional.ofNullable(value));
    });
    return result;
}

But if all you want to do, is replace null with an empty string, you don’t need an Optional:

private static <T> Map<String, Object> getDifference(
        final T a, final T b, final Map<String, Function<T, Object>> fields) {
    HashMap<String, Object> result = new HashMap<>();
    fields.forEach((key,getter) -> {
            final Object value = getter.apply(b);
            if(!Objects.equals(getter.apply(a), value))
                result.put(key, value==null? "": value);
        });
    return result;
}

and well, this substitution would also work out-of-the-box with your original code, if you just do it in the map function instead of the collector:

private static <T> Map<String, ?> getDifference(final T a, final T b, final Map<String, Function<T, Object>> fields) {
    return fields.entrySet().stream()
        .map(e -> {
            final String name = e.getKey();
            final Function<T, Object> getter = e.getValue();
            final Object pairKey = getter.apply(a);
            final Object pairValue = getter.apply(b);
            if (Objects.equals(pairKey, pairValue)) {
                return null;
            } else {
                return Pair.of(name, pairValue==null? "": pairValue);
            }
        })
        .filter(Objects::nonNull)
        .collect(Collectors.toMap(Pair::getKey, Pair::getValue));
}

or

private static <T> Map<String, Object> getDifference(
        final T a, final T b, final Map<String, Function<T, Object>> fields) {
    return fields.entrySet().stream()
        .map(e -> {
            final Function<T, Object> getter = e.getValue();
            final Object pairValue = getter.apply(b);
            return Objects.equals(getter.apply(a), pairValue)? null:
                Pair.of(e.getKey(), pairValue==null? "": pairValue);
        })
        .filter(Objects::nonNull)
        .collect(Collectors.toMap(Pair::getKey, Pair::getValue));
}

Nikolai Shevchenko · Accepted Answer · 2018-02-15T04:37:05.207

9

You have incorrect syntax. toMap()'s second parameter must be lambda, so

.collect(Collectors.toMap(
             pair -> pair.getKey(),
             pair -> Optional.ofNullable(pair.getValue()).orElse("")
));

OR

you can modify map() section as follows

return Pair.of(name, Optional.ofNullable(pairValue).orElse(""));

and use your original collect()

edited Feb 15 '18 at 04:37

answered Feb 15 '18 at 04:27

Nikolai Shevchenko

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Collecting values that could be null

2 Answers2