public final boolean isEmpty() {
        return (this.firstNode == null);
    }
@Test
    public void testIsEmpty() {
        assertThat(this.list.isEmpty()).as("Empty list should return true on isEmpty()").isTrue();
        
        this.list.add( this.node1 );
        assertThat(this.list.isEmpty()).as("Not empty list should return false on isEmpty()").isFalse();
    }