return optional ofNullable, as the boundary condition may be undefine…

…d during initialization.

src/main/mesh/Boundary.java

@@ -12,8 +12,8 @@ public class Boundary {
     private BoundaryCondition bc;
 
     public Boundary(String name, List<Face> faces, BoundaryCondition bc) {
-        this.name = Objects.requireNonNull(name);
-        this.faces = Objects.requireNonNull(faces);
+        this.name = Objects.requireNonNull(name, "Boundary 'name' cannot be null.");
+        this.faces = Objects.requireNonNull(faces, "Boundary 'faces' cannot be null.");
         this.bc = bc;
     }
 
@@ -22,6 +22,6 @@ public void setBC(BoundaryCondition bc) {
     }
 
     public Optional<BoundaryCondition> bc() {
-        return Optional.of(bc);
+        return Optional.ofNullable(bc);
     }
 }

test/main/mesh/BoundaryTest.java

@@ -0,0 +1,31 @@
+package main.mesh;
+
+import org.junit.Test;
+
+import java.util.List;
+import java.util.Optional;
+
+import static org.junit.Assert.*;
+
+public class BoundaryTest {
+
+    @Test
+    public void constructor_nullability_test() {
+        // null name
+        assertThrows(NullPointerException.class, () -> new Boundary(null, List.of(), null));
+        // null faces
+        assertThrows(NullPointerException.class, () -> new Boundary("name", null, null));
+    }
+
+    @Test
+    public void bc_optionality_test() {
+        Boundary bnd = new Boundary("bnd name", List.of(), null);
+        assertEquals(Optional.empty(), bnd.bc());
+    }
+
+    @Test
+    public void trying_to_set_null_bc() {
+        Boundary bnd = new Boundary("bnd name", List.of(), null);
+        assertThrows(NullPointerException.class, () -> bnd.setBC(null));
+    }
+}