I see in Java8 in UnaryOperator Interface following piece of code which does nothing on parameter and returns same value.
static <T> UnaryOperator<T> identity() {
return t -> t;
}
Is there anything for BinaryOperator which accepts two parameters of samekind and returns one value
static <T> BinaryOperator<T> identity() {
return (t,t) -> t;
}
why I am asking this question is for below requirement,
List<String> list = Arrays.asList("Abcd","Abcd");
Map<String,Integer> map = list.stream().collect(Collectors.toMap(str->str,
str->(Integer)str.length(),(t1,t2)->t1));
System.out.println(map.size());
in above code I don't want to do anything for two values of same key, I just wanted return one value, because in my case for sure values will be same.
As am not using t2 value Sonar throwing error, So I am finding out is there any thing like UnaryOperator.identity() for BinaryOpertor also in java8
Your question doesn't really make sense. If you were to paste your proposed BinaryOperator.identity method into an IDE, you would immediately see that it would complain that the identifier t is declared twice.
To fix this, we need a different identifier for each parameter:
return (t, u) -> t;
Now we can clearly see that this is not an identity function. It's a method which takes two arguments and returns the first one. Therefore the best name for this would be something like getFirst.
To answer your question about whether there's anything like this in the JDK: no. Using an identity function is a common use case, so defining a method for that is useful. Arbitrarily returning the first argument of two is not a common use case, and it's not useful to have a method to do that.
T means they have the same types, not the same values, that is not an identity per-se.
It just means that BinaryOperator will be used for the same types, but providing an identity for different values... this somehow sounds like foldLeft or foldRight or foldLeftIdentity/foldRightIdentity, which java does not have.
Your code seemingly can be improved as
List<String> list = Arrays.asList("Abcd", "Abcd");
Map<String, Integer> map = list.stream()
.collect(Collectors.toMap(Function.identity(), String::length, (a, b) -> a));
System.out.println(map.size());
Or possibly for your use case I don't want to do anything for two values of same key, I just wanted return one value, you may just choose to randomly return any value in using an implementation as following:
private static <T> BinaryOperator<T> any() {
return Math.random() < 0.5 ? ((x, y) -> x) : ((x, y) -> y);
}
and then in your code use it as
Map<String, Integer> map = list.stream()
.collect(Collectors.toMap(Function.identity(), String::length, any()));
Thanks to the suggestions from Holger, Eugene, and Federico, there are other efficient implementations of the any method that can actually involve using :
private static <T> BinaryOperator<T> any() {
// suggested by Holger
return ThreadLocalRandom.current().nextBoolean() ? ((x, y) -> x) : ((x, y) -> y);
// suggested by Eugene
long nt = System.nanoTime();
((nt >>> 32) ^ nt) > 0 ? ((x, y) -> x) : ((x, y) -> y);
}
Related
I have the following pattern repeated throughout my code:
class X<T, V>
{
V doTransform(T t) {
return null; // dummy implementation
}
Map<T, V> transform(List<T> item) {
return item.stream().map(x->new AbstractMap.SimpleEntry<>(x, doTransform(x))).collect(toMap(x->x.getKey(), x->x.getValue()));
}
}
Requiring the use of AbstractMap.SimpleEntry is messy and clunky. Linqs use of anonymous types is more elegant.
Is there a simpler way to achieve this using streams?
Thx in advance.
You can call doTransform in the value mapper:
Map<T, V> transform(List<T> item) {
return item.stream().collect(toMap(x -> x, x -> doTransform(x)));
}
Unfortunately, Java doesn't have an exact equivalent of C#'s anonymous types.
In this specific case, you don't need the intermediate map operation as #Jorn Vernee has suggested. instead, you can perform the key and value extraction in the toMap collector.
However, when it gets to cases where you think you need something as such of C#'s anonymous types you may consider:
anonymous objects (may not always be what you want depending on your use case)
Arrays.asList(...), List.of(...) (may not always be what you want depending on your use case)
an array (may not always be what you want depending on your use case)
Ultimately, If you really need to map to something that can contain two different types of elements then I'd stick with the AbstractMap.SimpleEntry.
That, said your current example can be simplified to:
Map<T, V> transform(List<T> items) {
return items.stream().collect(toMap(Function.identity(),this::doTransform));
}
In this specific example, there is no need to do the intermediate storage at all:
Map<T, V> transform(List<T> item) {
return item.stream().collect(toMap(x -> x, x -> doTransform(x)));
}
But if you need it, Java 9 offers a simpler factory method,
Map<T, V> transform(List<T> item) {
return item.stream()
.map(x -> Map.entry(x, doTransform(x)))
.collect(toMap(x -> x.getKey(), x -> x.getValue()));
}
as long as you don’t have to deal with null.
You can use an anonymous inner class here,
Map<T, V> transform(List<T> item) {
return item.stream()
.map(x -> new Object(){ T t = x; V v = doTransform(x); })
.collect(toMap(x -> x.t, x -> x.v));
}
but it’s less efficient. It’s an inner class which captures a reference to the surrounding this, also it captures x, so you have two fields, t and the synthetic one for capturing x, for the same thing.
The latter could be circumvented by using a method, e.g.
Map<T, V> transform(List<T> item) {
return item.stream()
.map(x -> new Object(){ T getKey() { return x; } V v = doTransform(x); })
.collect(toMap(x -> x.getKey(), x -> x.v));
}
But it doesn’t add to readability.
The only true anonymous types are the types generated for lambda expressions, which could be used to store information via higher order functions:
Map<T, V> transform(List<T> item) {
return item.stream()
.map(x -> capture(x, doTransform(x)))
.collect(HashMap::new, (m,f) -> f.accept(m::put), HashMap::putAll);
}
public static <A,B> Consumer<BiConsumer<A,B>> capture(A a, B b) {
return f -> f.accept(a, b);
}
but you’d soon hit the limitations of Java’s type system (it still isn’t a functional programming language) if you try this with more complex scenarios.
I have some stream handling code that takes a stream of words and performs some operations on them, then reduces them to a Map containing the words as keys and the number of occurrences of the word as a Long value. For the sake of the brevity of the code, I used the jOOL library's Seq class, which contains a number of useful shortcut methods.
The code compiles just fine if I write it like this:
item.setWordIndex (
getWords (item) // returns a Seq<String>
.map (this::removePunctuation) // String -> String
.map (stemmer::stem) // String -> String
.groupBy(str -> str, Collectors.counting ()));
However, if I attempt to replace the str -> str lambda with the more self-documenting Function::identity, I get the following errors:
The method setWordIndex(Map<String,Long>) in the type MyClass is not applicable for the arguments (Map<Object,Long>)
The type Function does not define identity(String) that is applicable here
Why does Function::identity behave any differently to str -> str, which I (perhaps naively) assumed was directly equivalent, and why can't the compiler handle it when it is used?
(And yes, I'm aware I could remove the identity function by moving the previous map application into the groupBy operation, but I find the code clearer like this, because it follows the application logic more directly)
You want Function.identity() (which returns a Function<T, T>), not Function::identity (which matches the SAM type Supplier<Function<T, T>>).
The following code compiles fine:
static String removePunctuation(String x) { return x; }
static String stem(String x) { return x; }
// ...
final Map<String, Long> yeah = Seq.of("a", "b", "c")
.map(Test::removePunctuation)
.map(Test::stem)
.groupBy(Function.identity(), Collectors.counting());
There is a slight difference between the two types; they are not directly equivalent:
Function.identity() has to return the input type, because its type is Function<T, T>;
str -> str can return a wider type; effectively it is Function<? extends T, T>.
What is the equivalent of of Scala's great foldLeft in Java 8?
I was tempted to think it was reduce, but reduce has to return something of identical type to what it reduces on.
Example:
import java.util.List;
public class Foo {
// this method works pretty well
public int sum(List<Integer> numbers) {
return numbers.stream()
.reduce(0, (acc, n) -> (acc + n));
}
// this method makes the file not compile
public String concatenate(List<Character> chars) {
return chars.stream()
.reduce(new StringBuilder(""), (acc, c) -> acc.append(c)).toString();
}
}
The problem in the code above is the accumulator: new StringBuilder("")
Thus, could anyone point me to the proper equivalent of the foldLeft/fix my code?
There is no equivalent of foldLeft in Java 8's Stream API. As others noted, reduce(identity, accumulator, combiner) comes close, but it's not equivalent with foldLeft because it requires the resulting type B to combine with itself and be associative (in other terms, be monoid-like), a property that not every type has.
There is also an enhancement request for this: add Stream.foldLeft() terminal operation
To see why reduce won't work, consider the following code, where you intend to execute a series of arithmetic operations starting with given number:
val arithOps = List(('+', 1), ('*', 4), ('-', 2), ('/', 5))
val fun: (Int, (Char, Int)) => Int = {
case (x, ('+', y)) => x + y
case (x, ('-', y)) => x - y
case (x, ('*', y)) => x * y
case (x, ('/', y)) => x / y
}
val number = 2
arithOps.foldLeft(number)(fun) // ((2 + 1) * 4 - 2) / 5
If you tried writing reduce(2, fun, combine), what combiner function could you pass that combines two numbers? Adding the two numbers together clearly does not solve it. Also, the value 2 is clearly not an identity element.
Note that no operation that requires a sequential execution can be expressed in terms of reduce. foldLeft is actually more generic than reduce: you can implement reduce with foldLeft but you cannot implement foldLeft with reduce.
Update:
Here is initial attempt to get your code fixed:
public static String concatenate(List<Character> chars) {
return chars
.stream()
.reduce(new StringBuilder(),
StringBuilder::append,
StringBuilder::append).toString();
}
It uses the following reduce method:
<U> U reduce(U identity,
BiFunction<U, ? super T, U> accumulator,
BinaryOperator<U> combiner);
It may sound confusing but if you look at the javadocs there is a nice explanation that may help you quickly grasp the details. The reduction is equivalent to the following code:
U result = identity;
for (T element : this stream)
result = accumulator.apply(result, element)
return result;
For a more in-depth explanation please check this source.
This usage is not correct though because it violates the contract of reduce which states that the accumulator should be an associative, non-interfering, stateless function for incorporating an additional element into a result. In other words since the identity is mutable the result will be broken in case of parallel execution.
As pointed in the comments below a correct option is using the reduction as follows:
return chars.stream().collect(
StringBuilder::new,
StringBuilder::append,
StringBuilder::append).toString();
The supplier StringBuilder::new will be used to create reusable containers which will be later combined.
The method you are looking for is java.util.Stream.reduce, particularly the overload with three parameters, identity, accumulator, and binary function. That is the correct equivalent to Scala's foldLeft.
However, you are not allowed to use Java's reduce that way, and also not Scala's foldLeft for that matter. Use collect instead.
It can be done by using Collectors:
public static <A, B> Collector<A, ?, B> foldLeft(final B init, final BiFunction<? super B, ? super A, ? extends B> f) {
return Collectors.collectingAndThen(
Collectors.reducing(Function.<B>identity(), a -> b -> f.apply(b, a), Function::andThen),
endo -> endo.apply(init)
);
}
Usage example:
IntStream.rangeClosed(1, 100).boxed().collect(foldLeft(50, (a, b) -> a - b)); // Output = -5000
For your question, this does what you wanted:
public String concatenate(List<Character> chars) {
return chars.stream()
.collect(foldLeft(new StringBuilder(), StringBuilder::append)).toString();
}
Others are correct there's no equivalent though. Here's a util that comes close-
<U, T> U foldLeft(Collection<T> sequence, U identity, BiFunction<U, ? super T, U> accumulator) {
U result = identity;
for (T element : sequence)
result = accumulator.apply(result, element);
return result;
}
your case using the above method would look like-
public String concatenate(List<Character> chars) {
return foldLeft(chars, new StringBuilder(""), StringBuilder::append).toString();
}
Or without the lambda method ref sugar,
public String concatenate(List<Character> chars) {
return foldLeft(chars, new StringBuilder(""), (stringBuilder, character) -> stringBuilder.append(character)).toString();
}
Using the Java Stream API, is there a way to do additional processing to adjust the value of whatever is passed to a method reference?
I'll give two examples.
Example 1.
In the first example, I start with a Stream<Path>, and I want to return a Map<String, Path> in which the keys in the map are processed version of the filename using another function that takes a String filename (not a Path). Specifically:
public Map<String, Path> createMap(Path sourceFolder, PathMatcher filter) {
return stream.filter(filter::matches)
.collect(Collectors.toMap(FilenameHelper::parseFilename, Function.identity()));
parseFilename(String filename) takes a String filename, but of course the method reference gets a Path. I'd like to say something like, FilenameHelper::parseFilename(((Path)Function.identity()).toFile().getName()) but that doesn't work (Eclipse says: "The left-hand side of an assignment must be a variable"). I can work around it by creating a new method that takes a Path and just does return parseFilename(path.toFile().toName()) but that's not cool.
Example 2.
In the second example, I have rows, a List<List<String>>> that represents a data table (rows, then columns). I have a method that should return a List<String> consisting of a specific column in that table for every nth row. I want to do something like:
public List<String> getDataFromColumn(String columnName, int nth) {
/// Without a clause at ???, this returns a List<List<String>>
return IntStream.range(0, rows.size())
.filter(n -> n % nth == 0) // Get every nth row
.mapToObj(rows::get)
.???
.collect(Collectors.toList());
}
Where "???" should be something like map(ArrayList::get(headers.indexOf(columnName))) (where headers is a List<String> containing the column headers) but if I put that in, I get an AssignmentOperator syntax error in the get part of this clause. Replacing map with forEach doesn't help here. In other words, I don't want rows.get(n), I want rows.get(n).get(headers.indexOf(columnName).
Question
In both of these examples, I want to do something additional to the value that is being passed to the method pointed to with the method reference operator (::). Is there a "Java Stream-ic" way to do additional processing to the thing being passed to the method reference?
Method references are essentially a convenient substitute for lambdas where the function signature is an exact match to the method signature. In your case you can just use regular lambdas:
public Map<String, Path> createMap(Path sourceFolder, PathMatcher filter) {
return stream.filter(filter::matches)
.collect(Collectors.toMap(path -> FilenameHelper.parseFilename(path.toFile().getName()), Function.identity()));
}
public List<String> getDataFromColumn(String columnName, int nth) {
return IntStream.range(0, rows.size())
.filter(n -> n % nth == 0)
.mapToObj(rows::get)
.map(row -> row.get(headers.indexOf(columnName)))
.collect(Collectors.toList());
}
How about Function.compose? Of course you cannot use FilenameHelper::parseFilename.compose, but you can easily write a static helper method to work around it:
static <T, V, R> Function<T, R> compose(Function<T, V> f, Function<V, R> g) {
return g.compose(f);
}
Now we can compose method references:
return stream.filter(filter::matches)
.collect(Collectors.toMap(
compose(
compose(Path::getFileName, Path::toString),
FilenameHelper::parseFilename),
Function.identity()));
This is actually not very readable but an alternative to writing a full lambda.
No, this functionality is currently not provided.
The usual way would be to just not use a method reference and instead call the method the "usual" way using a lambda expression:
stream.filter(filter::matches)
.collect(Collectors.toMap(p -> FilenameHelper.parseFilename(p.getFileName()), Function.identity()));
No, there is not. There is no syntax to do that.
And if you wanted such a thing then lambda expression is what you want.
Method reference or lambda, under the hood you are still going to get a class that actually implements the Predicate/Function so it does not matter.
And that argument but that's not cool, to me under the conditions that there is no syntax for that, it's the best option you have.
Underneath the actual calls that you there is a MethodHandle (introduced in jdk-7) and MethodHandles do not have a way to achieve what you want. I think the same restriction exists in C++ with method pointers.
This title sounds stupid even to me, but there must be at least somewhat clever way to achieve such effect and I don't know how else to explain it. I need to sort array using sorted in stream API. Here is my stream so far:
Arrays.stream(sequence.split(" "))
.mapToInt(Integer::parseInt)
.boxed()
.sorted((a, b) -> a.compareTo(b))
.forEach(a -> System.out.print(a + " "));
Now I have two different sorts of course - ascending and descending and the sort I need to use is specified in the user input. So what I want to do is having something like switch with 2 cases: "ascending" and "descending" and a variable to store the lambda expression respectively:
switch(command) {
case "ascending": var = a.compareTo(b);
case "descending": var = b.compareTo(a);
}
Then I my sorted looks like:
.sorted((a, b) -> var)
I got the idea in a python course I attended. There it was available to store an object in variable, thus making the variable "executable". I realize that this lambda is not an object, but an expression, but I'm asking is there any clever way that can achieve such result, or should I just have
if(var)
and two diferent streams for each sort order.
The question is not stupid at all. Answering it in a broader sense: Unfortunately, there is no generic solution for that. This is due to the type inference, which determines one particular type for the lambda expression, based on the target type. (The section about type inference may be helpful here, but does not cover all details regarding lambdas).
Particularly, a lambda like x -> y does not have any type. So there is no way of writing
GenericLambdaTypefunction = x -> y;
and later use function as a drop-in replacement for the actual lambda x -> y.
For example, when you have two functions like
static void useF(Function<Integer, Boolean> f) { ... }
static void useP(Predicate<Integer> p) { ... }
you can call them both with the same lambda
useF(x -> true);
useP(x -> true);
but there is no way of "storing" the x -> true lambda in a way so that it later may be passed to both functions - you can only store it in a reference with the type that it will be needed in later:
Function<Integer, Boolean> f = x -> true;
Predicate<Integer> p = x -> true;
useF(f);
useP(p);
For your particular case, the answer by Konstantin Yovkov already showed the solution: You have to store it as a Comparator<Integer> (ignoring the fact that you wouldn't have needed a lambda here in the first place...)
You can switch between using Comparator.reverseOrder() and Comparator.naturalOrder:
Comparator<Integer> comparator = youWantToHaveItReversed ? Comparator.reverseOrder(): Comparator.naturalOrder();
Arrays.stream(sequence.split(" "))
.map(Integer::valueOf)
.sorted(comparator)
.forEach(a -> System.out.print(a + " "));
In Lambdas you can use a functionblock
(a,b) -> { if(anything) return 0; else return -1;}