How to compare two Streams in Java 8

问题:

What would be a good way to compare two Stream instances in Java 8 and find out whether they have the same elements, specifically for purposes of unit testing?

What I've got now is:

@Test
void testSomething() {
  Stream<Integer> expected;
  Stream<Integer> thingUnderTest;
  // (...)
  Assert.assertArrayEquals(expected.toArray(), thingUnderTest.toArray());
}

or alternatively:

Assert.assertEquals(
    expected.collect(Collectors.toList()),
    thingUnderTest.collect(Collectors.toList()));

But that means I'm constructing two collections and discarding them. It's not a performance issue, given the size of my test streams, but I'm wondering whether there's a canonical way to compare two streams.

回答1:

static void assertStreamEquals(Stream<?> s1, Stream<?> s2)
{
    Iterator<?> iter1 = s1.iterator(), iter2 = s2.iterator();
    while(iter1.hasNext() && iter2.hasNext())
        assertEquals(iter1.next(), iter2.next());
    assert !iter1.hasNext() && !iter2.hasNext();
}

回答2:

Collecting the stream under test (as you show) is a straightforward and effective way of performing the test. You may create the list of expected results in the easiest way available, which might not be collecting a stream.

Alternatively, with most libraries for creating mock collaborators, one could mock a Consumer that "expects" a series of accept() calls with particular elements. Consume the Stream with it, and then "verify" that its configured expectations were met.

回答3:

public static boolean equalStreams(Stream<?>...streams) {
    List<Iterator<?>>is = Arrays.stream(streams).map(Stream::iterator).collect(Collectors.toList());
    while(is.stream().allMatch(Iterator::hasNext))
        if(is.stream().map(Iterator::next).distinct().limit(2).count()>1) return false;
    return is.stream().noneMatch(Iterator::hasNext);
}

回答4:

You can assert the stream's content without creating a Stream<> expected.

assertj has fluent and readable solutions for this.

import static org.assertj.core.api.Assertions.assertThat;
import java.util.stream.Stream;
import org.junit.jupiter.api.Test;

class MyTests {
    @Test
    void test() {
        Stream<Integer> actual = Stream.of(0, 8, 15); // your thingUnderTest

        assertThat(actual).containsExactly(0, 8, 15);
    }
}

回答5:

Using the elementsEqual method in the Guava library:

Iterators.elementsEqual(s1.iterator(), s2.iterator())

回答6:

How to Compare Two Streams in java 8 and above: with the example of Comparing IntStream

package com.techsqually.java.language.generics.basics;

import java.util.Iterator;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class TwoStreamComparision {

    public static void main(String[] args) {

        String a = "Arpan";
        String b = "Arpen";

        IntStream s1 =  a.chars();
        IntStream s2  = b.chars();

        Iterator<Integer> s1It = s1.iterator();
        Iterator<Integer> s2It = s2.iterator();

        //Code to check how many characters are not equal in both the string at their Respective Position
        int count = 0;
        while (s2It.hasNext()){
            if (!s1It.next().equals(s2It.next())){
                count++;
            }
        }
        System.out.println(count);
    }
}