add Factorial

Author: realDuYuanChao

build.gradle

@@ -10,5 +10,10 @@ repositories {
 }
 
 dependencies {
+    implementation 'org.junit.jupiter:junit-jupiter:5.4.2'
     testCompile group: 'junit', name: 'junit', version: '4.12'
 }
+
+test{
+    useJUnitPlatform()
+}

src/main/java/com/examplehub/basics/HelloWorld.java

@@ -0,0 +1,7 @@
+package com.examplehub.basics;
+
+public class HelloWorld {
+    public static void main(String[] args) {
+        System.out.println("Hello World from ExampleHub");
+    }
+}

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

@@ -0,0 +1,14 @@
+package com.examplehub.maths;
+
+public class Factorial {
+    public static long factorial(int number) {
+        if (number < 0) {
+            throw new ArithmeticException(number + " is negative");
+        }
+        long fact = 1;
+        for (int i = 1; i <= number; ++i) {
+            fact *= i;
+        }
+        return fact;
+    }
+}

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

@@ -0,0 +1,23 @@
+package com.examplehub.maths;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class FactorialTest {
+
+    @Test
+    void testFactorial() {
+        try {
+            long fact = Factorial.factorial(-1);
+            fail(); /* will not happen */
+        } catch (ArithmeticException e) {
+            assertTrue(true); /* will happen */
+        }
+
+        int[][] testNumbers = {{0, 1}, {1, 1}, {2, 2}, {3, 6}, {4, 24}, {5, 120}};
+        for (int[] testNumber : testNumbers) {
+            assertEquals(Factorial.factorial(testNumber[0]), testNumber[1]);
+        }
+    }
+}