Armstrong Number in Java (#12637)

core-java-modules/core-java-numbers-5/src/main/java/com/baeldung/armstrong/ArmstrongNumberUtil.java

@@ -0,0 +1,44 @@
+package com.baeldung.armstrong;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+public class ArmstrongNumberUtil {
+
+    public static boolean isArmstrong(int n) {
+        if (n < 0) {
+            return false;
+        }
+        List<Integer> digitsList = digitsInList(n);
+        int len = digitsList.size();
+        int sum = digitsList.stream()
+            .mapToInt(d -> (int) Math.pow(d, len))
+            .sum();
+        // alternatively, we can use the reduce() method:
+        // int sum = digits.stream()
+        //     .reduce(0, (subtotal, digit) -> subtotal + (int) Math.pow(digit, len));
+        return n == sum;
+    }
+
+    private static List<Integer> digitsInList(int n) {
+        List<Integer> list = new ArrayList<>();
+        while (n > 0) {
+            list.add(n % 10);
+            n = n / 10;
+        }
+        return list;
+    }
+
+    public static List<Integer> getA005188Sequence(int limit) {
+        if (limit < 0) {
+            throw new IllegalArgumentException("The limit cannot be a negative number.");
+        }
+        return IntStream.range(0, limit)
+            .boxed()
+            .filter(ArmstrongNumberUtil::isArmstrong)
+            .collect(Collectors.toList());
+
+    }
+}

core-java-modules/core-java-numbers-5/src/test/java/com/baeldung/armstrong/ArmstrongNumberUtilUnitTest.java

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+package com.baeldung.armstrong;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.List;
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+
+class ArmstrongNumberUtilUnitTest {
+    // @formatter:off
+private static final Map<Integer, Boolean> ARMSTRONG_MAP = ImmutableMap.of(
+    0, true,
+    1, true,
+    2, true,
+    153, true,
+    370, true,
+    407, true,
+    42, false,
+    777, false,
+    12345, false);
+    // @formatter:on
+
+    private static final List<Integer> A005188_SEQ_1K = ImmutableList.of(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 153, 370, 371, 407);
+    private static final List<Integer> A005188_SEQ_10K = ImmutableList.of(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 153, 370, 371, 407, 1634, 8208, 9474);
+
+    @Test
+    void givenIntegers_whenCheckArmstrong_shouldReturnExpectedResult() {
+        ARMSTRONG_MAP.forEach((number, result) -> assertEquals(result, ArmstrongNumberUtil.isArmstrong(number)));
+    }
+
+    @Test
+    void givenALimit_whenFindArmstrong_shouldReturnExpectedResult() {
+        assertEquals(A005188_SEQ_1K, ArmstrongNumberUtil.getA005188Sequence(1000));
+        assertEquals(A005188_SEQ_10K, ArmstrongNumberUtil.getA005188Sequence(10000));
+    }
+}