I want to have a null-safe comparator, but it does not work:
Comparator<Item> sort_high = (i1, i2)-> Double.compare(i2.getUser().getValue(), i1.getUser().getValue());
items.sort(Comparator.nullsFirst(sort_high));
However, I get a NPE, if item.getUser().getValue() (or item.getUser()) is null.
at Item.lambda$1(Item.java:270)
at java.util.Comparators$NullComparator.compare(Comparators.java:83)
at java.util.TimSort.countRunAndMakeAscending(TimSort.java:355)
at java.util.TimSort.sort(TimSort.java:220)
at java.util.Arrays.sort(Arrays.java:1438)
at java.util.List.sort(List.java:478)
What is wrong?
nullsFirst will take care of null Items ("i1"), but once two valid objects are found, your Comparator is invoked and you need to handle internal null references.
In your case, you could use something like:
items.sort(
Comparator.nullsFirst(
Comparator.comparing(Item::getUser,
Comparator.nullsFirst(Comparator.comparingDouble(User::getValue))
)
)
);
(Assuming getValue() returns a double)
But I hardly recommend such convoluted code.
Comparator.nullsFirst() doesn't mean that any NullPointerException thrown during the compare() method will be caught.
It says rather that a null compared object with always be less than a non null compared object and that if both are null, they are considered equal.
<T> Comparator<T> java.util.Comparator.nullsFirst(Comparator<? super
T> comparator)
Returns a null-friendly comparator that considers null to be less than
non-null. When both are null, they are considered equal. If both are
non-null, the specified Comparator is used to determine the order. If
the specified comparator is null, then the returned comparator
considers all non-null values to be equal.
In your case :
Comparator<Item> sort_high = (i1, i2)-> Double.compare(i2.getUser().getValue(), i1.getUser().getValue());
Compared objects are not null, that is i1 or i2 but fields of them are: i1.getUser() or i2.getUser().
You don't have a good use case for using directly Comparator.nullsFirst().
As workaround, you could use it to compare the User field in your compare() implementation as soon as one of two compared objects present a null User field.
I think that it is a good trade off between handling yourself null case and readability of the code :
private Comparator<User> comparatorUserNullFirst = Comparator.nullsFirst(Comparator.comparingDouble(User::getValue));
Comparator<Item> sortHigh = (i1, i2) -> {
if (i1.getUser() == null || i2.getUser() == null) {
return comparatorUserNullFirst.compare(i1.getUser(), i2.getUser());
}
return Double.compare(i2.getUser().getValue(), i1.getUser().getValue());
};
Another possibility, that is slightly more readable but does not distinguish between an item being null and an item having a user that is null, is the following:
Comparator<Item> sortLow = Comparator.comparing(item -> Optional.ofNullable(item)
.map(Item::getUser)
.map(User::getValue)
.orElse(null),
Comparator.nullsFirst(Comparator.naturalOrder())
);
Related
I have an EdmPortfolio type that has id (int), name (String) and tags which is a map as its members.
EdmPortfolio say has an id 1, and name as Portfolio1 and Map could have the following values,
analyst, John
region, US
I have a List, At the end i want to have a Map which has all the map values combined from all portfolios. Another EdmPortfolio might have the id as 2, name as Portfolio2 and Map with values analyst, Smith
region , UK
I want to have a combined Map with Values
region 'US','UK'
analyst 'John','Smith'
analyst and region are the keys to the map.
It has both the maps combined. I have the following code but i am a little lost
List<Map<Tag,String>> portfolioTagsList = new ArrayList<>();
for (EdmPortfolio edmPortfolio : edmPortfolioList) {
Map<Tag,String> portfolioTags = edmPortfolio.getPortfolioTags().entrySet()
.stream()
.filter(e -> (e.getValue() != ""|| e.getValue() !=null))
.collect(Collectors.toMap(
Map.Entry::getKey,
Map.Entry::getValue
));
portfolioTagsList.add(portfolioTags);
}
Map<Tag,String> finalTags = portfolioTagsList.stream()
.flatMap(m -> m.entrySet().stream())
.collect(Collectors.groupingBy(Map.Entry::getKey, Collectors.joining(",",Map.Entry::getValue)));
I am getting nonstatic method cannot be referenced from static context. How do i achieve this?
Thanks for your time
I'd do something like:
Map<Tag,String> finalTags = edmPortfolioList.stream()
.flatMap(edmPortfolio -> edmPortfolio.getPortfolioTags().entrySet().stream())
.filter(e -> (e.getValue() != "" || e.getValue() !=null)) // this seems weird, but just keeping
.collect(Collectors.groupingBy(e -> e.getKey(),
Collectors.mapping(ev -> ev.getValue(),
Collectors.joining(",")));
BTW: Any reason to break that in two streams?
Unfortunately, “nonstatic method cannot be referenced from static context” is the error that javac often ends up when type inference fails, especially in the context of method references or lambda expressions. The actual cause could be a wrong generic type parameter, a forgotten import statement, the use of a lambda expression or method reference at an inappropriate place, or even a misplaced or forgotten brace somewhere.
In your case, it’s Collectors.joining(",",Map.Entry::getValue) as that collector has no parameter of a function type that could accept a method reference.
You should use Collectors.mapping(Map.Entry::getValue, Collectors.joining(",")) instead.
Besides that, follow BrunoJCM’s suggestion to use a single stream operation and pay attention to the filter condition. e.getValue() != "" || e.getValue() !=null makes no sense as a reference can never be both, a reference to "" and the null reference, hence, this condition is always fulfilled. You most likely wanted to express that the string must not be null and not an empty string. In this regard, you should not compare strings by reference but use equals. For testing for an empty string, there is the alternative to use isEmpty() instead of equals(""). So the condition should be e.getValue()!=null && !e.getValue().isEmpty(). Mind the order, as we can invoke isEmpty() only after having verified that it is not null.
I am trying to create a method that will take a list of items with set weights and choose 1 at random. My solution was to use a Hashmap that will use Integer as a weight to randomly select 1 of the Keys from the Hashmap. The keys of the HashMap can be a mix of Object types and I want to return 1 of the selected keys.
However, I would like to avoid returning a null value on top of avoiding mutation. Yes, I know this is Java, but there are more elegant ways to write Java and hoping to solve this problem as it stands.
public <T> T getRandomValue(HashMap<?, Integer> VALUES) {
final int SIZE = VALUES.values().stream().reduce(0, (a, b) -> a + b);
final int RAND_SELECTION = ThreadLocalRandom.current().nextInt(SIZE) + 1;
int currentWeightSum = 0;
for (Map.Entry<?, Integer> entry : VALUES.entrySet()) {
if (RAND_SELECTION > currentWeightSum && RAND_SELECTION <= (currentWeightSum + entry.getValue())) {
return (T) entry.getKey();
} else {
currentWeightSum += entry.getValue();
}
}
return null;
}
Since the code after the loop should never be reached under normal circumstances, you should indeed not write something like return null at this point, but rather throw an exception, so that irregular conditions can be spotted right at this point, instead of forcing the caller to eventually debug a NullPointerException, perhaps occurring at an entirely different place.
public static <T> T getRandomValue(Map<T, Integer> values) {
if(values.isEmpty())
throw new NoSuchElementException();
final int totalSize = values.values().stream().mapToInt(Integer::intValue).sum();
if(totalSize<=0)
throw new IllegalArgumentException("sum of weights is "+totalSize);
final int threshold = ThreadLocalRandom.current().nextInt(totalSize) + 1;
int currentWeightSum = 0;
for (Map.Entry<T, Integer> entry : values.entrySet()) {
currentWeightSum += entry.getValue();
if(threshold <= currentWeightSum) {
return entry.getKey();
}
}
// if we reach this point, the map's content must have been changed in-between
throw new ConcurrentModificationException();
}
Note that the code fixes some other issues of your code. You should not promise to return an arbitrary T without knowing the actual type of the map. If the map contains objects of different type as key, i.e. is a Map<Object,Integer>, the caller can’t expect to get anything more specific than Object. Besides that, you should not insist of the parameter to be a HashMap when any Map is sufficient. Further, I changed the variable names to adhere to Java’s naming convention and simplified the loop’s body.
If you want to support empty maps as legal input, changing the return type to Optional<T> would be the best solution, returning an empty optional for empty maps and an optional containing the value otherwise (this would disallow null keys). Still, the supposed-to-be-unreachable code point after the loop should be flagged with an exception.
Why does this throw a java.lang.NullPointerException?
List<String> strings = new ArrayList<>();
strings.add(null);
strings.add("test");
String firstString = strings.stream()
.findFirst() // Exception thrown here
.orElse("StringWhenListIsEmpty");
//.orElse(null); // Changing the `orElse()` to avoid ambiguity
The first item in strings is null, which is a perfectly acceptable value. Furthermore, findFirst() returns an Optional, which makes even more sense for findFirst() to be able to handle nulls.
EDIT: updated the orElse() to be less ambiguous.
The reason for this is the use of Optional<T> in the return. Optional is not allowed to contain null. Essentially, it offers no way of distinguishing situations "it's not there" and "it's there, but it is set to null".
That's why the documentation explicitly prohibits the situation when null is selected in findFirst():
Throws:
NullPointerException - if the element selected is null
As already discussed, the API designers do not assume that the developer wants to treat null values and absent values the same way.
If you still want to do that, you may do it explicitly by applying the sequence
.map(Optional::ofNullable).findFirst().flatMap(Function.identity())
to the stream. The result will be an empty optional in both cases, if there is no first element or if the first element is null. So in your case, you may use
String firstString = strings.stream()
.map(Optional::ofNullable).findFirst().flatMap(Function.identity())
.orElse(null);
to get a null value if the first element is either absent or null.
If you want to distinguish between these cases, you may simply omit the flatMap step:
Optional<String> firstString = strings.stream()
.map(Optional::ofNullable).findFirst().orElse(null);
System.out.println(firstString==null? "no such element":
firstString.orElse("first element is null"));
This is not much different to your updated question. You just have to replace "no such element" with "StringWhenListIsEmpty" and "first element is null" with null. But if you don’t like conditionals, you can achieve it also like:
String firstString = strings.stream()
.map(Optional::ofNullable).findFirst()
.orElseGet(()->Optional.of("StringWhenListIsEmpty"))
.orElse(null);
Now, firstString will be null if an element exists but is null and it will be "StringWhenListIsEmpty" when no element exists.
You can use java.util.Objects.nonNull to filter the list before find
something like
list.stream().filter(Objects::nonNull).findFirst();
The following code replaces findFirst() with limit(1) and replaces orElse() with reduce():
String firstString = strings.
stream().
limit(1).
reduce("StringWhenListIsEmpty", (first, second) -> second);
limit() allows only 1 element to reach reduce. The BinaryOperator passed to reduce returns that 1 element or else "StringWhenListIsEmpty" if no elements reach the reduce.
The beauty of this solution is that Optional isn't allocated and the BinaryOperator lambda isn't going to allocate anything.
Optional is supposed to be a "value" type. (read the fine print in javadoc:) JVM could even replace all Optional<Foo> with just Foo, removing all boxing and unboxing costs. A null Foo means an empty Optional<Foo>.
It is a possible design to allow Optional with null value, without adding a boolean flag - just add a sentinel object. (could even use this as sentinel; see Throwable.cause)
The decision that Optional cannot wrap null is not based on runtime cost. This was a hugely contended issue and you need to dig the mailing lists. The decision is not convincing to everybody.
In any case, since Optional cannot wrap null value, it pushes us in a corner in cases like findFirst. They must have reasoned that null values are very rare (it was even considered that Stream should bar null values), therefore it is more convenient to throw exception on null values instead of on empty streams.
A workaround is to box null, e.g.
class Box<T>
static Box<T> of(T value){ .. }
Optional<Box<String>> first = stream.map(Box::of).findFirst();
(They say the solution to every OOP problem is to introduce another type :)
As I experimented Optional<T> does not handle null elements, so in the following example it throws a NullPointerException in the last statement:
List<String> data = Arrays.asList("Foo", null, "Bar");
data.stream().findFirst().ifPresent(System.out::println);
data.stream().skip(1).findFirst().ifPresent(System.out::println);
So, I still have to explicitly deal with null and filter non-null elements, such as:
data.stream()
.filter(item -> item != null)
.skip(1)
.findFirst()
.ifPresent(System.out::println);
Is there any alternative that avoids dealing explicitly with null as: item != null
You can use .filter(Objects::nonNull) using the method Objects.nonNull(…) added for this purpose.
There is no way to avoid explicit filtering, unless you avoid having nulls in your source list in the first place.
Note that it would be strange if the Optional handled the null in this case as it would yield an empty optional which had the semantic of “there is no first element” which implies “the stream was empty” which is just wrong.
Dealing with nulls explicitly is the cleanest solution here as it also allows you to explicitly tell whether you want .filter(Objects::nonNull).skip(1), .skip(1).filter(Objects::nonNull)…
…or .map(s->s==null? "null-replacement": s).findFirst()
It really depends on what you want to do. If you want to treat null as a valid value, the answer is different than if you want to skip nulls.
If you want to keep "nulls" in your stream:
List<String> data = Arrays.asList("Foo", null, "Bar");
data.stream().map(Optional::ofNullable).findFirst().flatMap(Function.identity()).ifPresent(System.out::println); ;
data.stream().map(Optional::ofNullable).skip(1).findFirst().flatMap(Function.identity()).ifPresent(System.out::println);
If you want to remove nulls from your stream, use data.stream().filter(Objects::nonNull) to filter it out (or as you stated o -> o != null, whatever you prefer.
How is the method SingleOrDefault() evaluated in LINQ? Does it use a Binary Search behind the scenes?
Better than attempting to explain in words, I thought I'd just post the exact code of implementation in the .NET Framework, retrieved using the Reflector program (and reformatted ever so slightly).
public static TSource SingleOrDefault<TSource>(this IEnumerable<TSource> source)
{
if (source == null)
throw Error.ArgumentNull("source");
IList<TSource> list = source as IList<TSource>;
if (list != null)
{
switch (list.Count)
{
case 0:
return default(TSource);
case 1:
return list[0];
}
}
else
{
using (IEnumerator<TSource> enumerator = source.GetEnumerator())
{
if (!enumerator.MoveNext())
return default(TSource);
TSource current = enumerator.Current;
if (!enumerator.MoveNext())
return current;
}
}
throw Error.MoreThanOneElement();
}
It's quite interesting to oberserve that an optimisation is made if the object is of type IList<T>, which seems quite sensible. It simply falls back to enumerating over the object otherwise if the object implements nothing more specific than IEnumerable<T>, and does so just how you'd expect.
Note that it can't use a binary search because the object doesn't necessarily represent a sorted collection. (In fact, in almost all usage cases, it won't.)
I would assume that it simply performs the query and if the result count is zero, it returns the default instance of the class. If the result count is one, it returns that instance, and if the result count is greater than one, it throws an exception.
I don't think it does any searching, it's all about getting the first element of the source [list, result set, etc].
My best guess is that it just pulls the first element. If there is no first it returns the default (null, 0, false, etc). If there is a first, it attempts to pull the second result. If there is a second result it throws an exception. Otherwise it returns the first result.