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
– Chetan Kinger, Commented Feb 15, 2018 at 4:04
Java 8 NullPointerException in Collectors.toMap stackoverflow.com/questions/24630963/… Is this the same problem? — Chota Bheem
– Chota Bheem, Commented Feb 15, 2018 at 4:05
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.
– Jacob G., Commented Feb 15, 2018 at 4:10

Holger · Accepted Answer · 2018-02-15 07:54:25Z

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 · 2018-02-15 04:37:05Z

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()

Collectives™ on Stack Overflow

Collecting values that could be null

2 Answers 2

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Linked

Related