java-stream

I've created method whih numerating each character of alphabet. I'm learning streams(functional programming) and try to use them as often as possible, but I don't know how to do it in this case: private Map<Character, Integer> numerateAlphabet(List<Character> alphabet) { Map<Character, Integer> m = new HashMap<>(); for (int i = 0; i < alphabet.size(); i++) m.put(alphabet.get(i), i); return m; } So, how to rewrite it using streams of Java 8? Misha Avoid stateful index counters like the AtomicInteger -based solutions presented in other answers. They will fail if the stream were parallel. Instead

Sometimes when processing a Java stream() I find myself in need of a non-terminal forEach() to be used to trigger a side effect but without terminating processing. I suspect I could do this with something like .map(item -> f(item)) where the method f performs the side effect and returns the item to the stream, but it seems a tad hokey. Is there a standard way of handling this? Yes there is. It is called peek() (example from the JavaDoc ): Stream.of("one", "two", "three", "four") .peek(e -> System.out.println("Original value: " + e)) .filter(e -> e.length() > 3) .peek(e -> System.out.println(

To my understanding, the following code should print true , since both Stream and Iterator are pointing to the first element. However, when I run the following code it is printing false : final HashMap<String, String> map = new HashMap<>(); map.put("A", "B"); final Set<Map.Entry<String, String>> set = Collections.unmodifiableMap(map).entrySet(); Map.Entry<String, String> entry1 = set.iterator().next(); Map.Entry<String, String> entry2 = set.stream().findFirst().get(); System.out.println(entry1 == entry2); What is the reason of the different behavior? Both entries are referring to the same

Lets say I have two classes and two methods: class Scratch { private class A{} private class B extends A{} public Optional<A> getItems(List<String> items){ return items.stream() .map(s -> new B()) .findFirst(); } public Optional<A> getItems2(List<String> items){ return Optional.of( items.stream() .map(s -> new B()) .findFirst() .get() ); } } Why does getItems2 compile while getItems gives compiler error incompatible types: java.util.Optional<Scratch.B> cannot be converted to java.util.Optional<Scratch.A> So when I get the value of the Optional returned by findFirst and wrap it again with

Is it possible to convert stream of chars str.chars() to stream with Strings, where each String contains 5 characters, for example? I don't think trying to combine elements from the characters is a good fit for Java streams without using some third party libraries. If you want a stream of 5 character substrings I would split them like this: String s = "1234567890123456789012345678901234567890"; IntStream.range(0, s.length()/5) .mapToObj(i -> s.substring(i*5, (i+1)*5)) .forEach(System.out::println); You can simply split you string into five character sized strings using String[] split = string

I was working with some Java 8 Stream APIs. I am confused to see the performance difference between below two solutions, that are just printing the contents of Stream . Solution 1: int[] array = new int[] { 0, 1, 2, 3, 4, 5 }; start = System.nanoTime(); Arrays.stream(array).forEach(System.out::println); System.out.println((System.nanoTime() - start) / 1000000.0f); Solution 2: int[] array = new int[] { 0, 1, 2, 3, 4, 5 }; start = System.nanoTime(); Arrays.stream(array).forEach(new IntConsumer() { @Override public void accept(int value) { System.out.println(value); } }); System.out.println(

I'm wondering what the most compact and efficient way to multiple 2 double[][] arrays matrices using streams. The approach should follow matrix multiplication rules as illustrated here: http://www.mathwarehouse.com/algebra/matrix/multiply-matrix.php Here's one way to do it using for loops ('this' is the first matrix'): final int nRows = this.getRowDimension(); final int nCols = m.getColumnDimension(); final int nSum = this.getColumnDimension(); final double[][] outData = new double[nRows][nCols]; // Will hold a column of "m". final double[] mCol = new double[nSum]; final double[][] mData = m

I have gone through related questions like How to ensure order of processing in java8 streams? , still ordering of output element is not completely clear to me. Therefore please clarify my following doubt. Integer[] intArray = {1, 2, 3, 4, 5, 6, 7, 8 }; List<Integer> listOfIntegers = new ArrayList<>(Arrays.asList(intArray)); listOfIntegers .parallelStream() .unordered() .forEachOrdered(e -> System.out.print(e + " ")); I think at least theoretically(or according to java specification) it can print in random order than 1, 2, 3, 4, 5, 6, 7, 8. Am I correct? One more related question- the decision