test(api): add a test for BoxAPI#api

Author: Siroshun09

api/src/test/java/net/okocraft/box/api/BoxAPITest.java

@@ -0,0 +1,28 @@
+package net.okocraft.box.api;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import org.mockito.Mockito;
+
+class BoxAPITest {
+
+    @Test
+    void testObtainingAPI() {
+        try {
+            // First, checking if the BoxAPI is not set.
+            Assertions.assertFalse(BoxAPI.isLoaded());
+            Assertions.assertThrows(IllegalStateException.class, BoxAPI::api);
+
+            // Set dummy BoxAPI
+            BoxProvider.API = Mockito.mock(BoxAPI.class);
+
+            // Checking if the BoxAPI can be obtained.
+            Assertions.assertTrue(BoxAPI.isLoaded());
+            Assertions.assertDoesNotThrow(BoxAPI::api);
+            Assertions.assertSame(BoxProvider.API, BoxAPI.api());
+        } finally {
+            BoxProvider.API = null;
+        }
+    }
+
+}