2.1. Понятие редукции. Произвольный Collector для подсчета

src/main/java/ru/job4j/stream/CollectorArithmetic.java

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+package ru.job4j.stream;
+
+import java.util.LinkedList;
+import java.util.List;
+import java.util.function.BiConsumer;
+import java.util.function.BinaryOperator;
+import java.util.function.Function;
+import java.util.function.Supplier;
+import java.util.stream.Collector;
+
+public class CollectorArithmetic {
+    public static Integer collect(List<Integer> list) {
+        Supplier<List<Integer>> supplier = LinkedList::new;
+        BiConsumer<List<Integer>, Integer> consumer = List::add;
+        BinaryOperator<List<Integer>> merger = (xs, ys) -> {
+            xs.addAll(ys);
+            return xs;
+        };
+        Function<List<Integer>, Integer> function = (ns) -> {
+            int result = 1;
+            for (int i : ns) {
+                result *= i;
+            }
+            return result;
+        };
+        return list.stream()
+                .collect(Collector.of(
+                        supplier,
+                        consumer,
+                        merger,
+                        function)
+                );
+    }
+}

src/test/java/ru/job4j/stream/CollectorArithmeticTest.java

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+package ru.job4j.stream;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class CollectorArithmeticTest {
+
+    @Test
+    public void whenWithout0() {
+        int result = CollectorArithmetic.collect(List.of(1, 2, 3));
+        int expected = 6;
+        assertThat(result).isEqualTo(expected);
+    }
+
+    @Test
+    public void whenWith0() {
+        int result = CollectorArithmetic.collect(List.of(0, 2, 3));
+        int expected = 0;
+        assertThat(result).isEqualTo(expected);
+    }
+}