うらがみさんからの宿題 http://backpaper0.github.io/2014/10/04/stream_collect.html

ScanCollectorTest.java

package stream;

import org.hamcrest.CoreMatchers;
import org.junit.Assert;
import org.junit.Test;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.UnaryOperator;
import java.util.stream.Collector;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;

import static java.util.stream.Collectors.joining;
import static org.hamcrest.CoreMatchers.*;
import static org.junit.Assert.*;

public class ScanCollectorTest {

  public <T> List<T> scan(Stream<T> stream, T initialValue, BinaryOperator<T> binaryOperator) {
    Collector<T, List<T>, List<T>> collector = Collector.of(() -> {
      List<T> list = new ArrayList<>();
      list.add(initialValue);
      return list;
    }, (list, value) -> {
      list.add(binaryOperator.apply(list.get(list.size() - 1), value));
    }, (list1, list2) -> {
      list1.addAll(list2);
      return list1;
    });
    return stream.collect(collector);
  }

  @Test
  public void testScan() {
    List<Integer> result = scan(
        IntStream.rangeClosed(1, 5).boxed(),
        0,
        (n, m) -> n + m);
    assertThat(result.toString(), is("[0, 1, 3, 6, 10, 15]"));
  }

}