get first digit

Author: realDuYuanChao

src/main/java/com/examplehub/maths/GetTheFirstDigit.java

@@ -0,0 +1,29 @@
+package com.examplehub.maths;
+
+public class GetTheFirstDigit {
+
+    /**
+     * Get the first digit of a number.
+     *
+     * @param number the number to be checked.
+     * @return the first digit of given number.
+     */
+    public static int firstDigit(int number) {
+        number = Math.abs(number);
+        while (number >= 10) {
+            number /= 10;
+        }
+        return number;
+    }
+
+    /**
+     * Get the first digit of a number using string.
+     *
+     * @param number the number to be checked.
+     * @return the first digit of given number.
+     */
+    public static int firstDigitByString(int number) {
+        number = Math.abs(number);
+        return Integer.parseInt(("" + number).substring(0, 1));
+    }
+}

src/test/java/com/examplehub/maths/GetTheFirstDigitTest.java

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+package com.examplehub.maths;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class GetTheFirstDigitTest {
+
+    @Test
+    void test() {
+        int[][] testNumbers = {{-123, 1}, {0, 0}, {123, 1}, {123456789, 1}};
+        for (int[] testNumber : testNumbers) {
+            assertEquals(testNumber[1], GetTheFirstDigit.firstDigit(testNumber[0]));
+            assertEquals(testNumber[1], GetTheFirstDigit.firstDigitByString(testNumber[0]));
+        }
+    }
+}