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?
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));
}
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