In this thesis, we present mixsets, an approach to combine annotative and compositional fragments for specifying code variants to form software product lines (SPLs). There are three key contributions of our research: introducing mixsets to represent software variability, extending mixsets to construct feature models, and improving software composition to achieve fine-grained variability. The concept of mixsets is introduced in Umple as a conditional unit and a first-class entity to allow smoothly transitioning software to compositional SPLs. A mixset is a named set of mixins; each mixin belonging to the mixset is called a fragment. A mixset fragment can be a top-level entity that contains nested entities or can be embedded as a conditional fragment in other entities such as methods. Mixset content normally includes code blocks or statements of any type, and may include require statements, which describe explicit dependencies among mixsets. Mixsets can be used to specify product lines using code composition, code annotation or both. A strength of mixsets lies on the straightforward mechanism to transform annotated segments into compositional segments when used in a combined approach. Therefore, preplanning effort and time to transform annotative SPLs to compositional SPLs can be reduced. Mixsets can provide a backbone structure to realize product line features in a feature-based SPL. Hence, a feature model can be formed using a subset of specific dependent mixsets. Feature interactions within an SPL can be identified and separated in specific modules by mixset nesting. Furthermore, product configuration, feature modeling analysis and generation of feature diagrams can be accomplished based on mixsets. The thesis also demonstrates a method to enable the granularity of compositional approaches to be expressed at the statement level. This is achieved by allowing aspects to inject code into labelled places, or points of variation, within method code. Such injected code has a full access to the context in which its placed, such as access to local variables. Mixsets are implemented in Umple, a model-driven development (MDD) technology that allows combining abstract models, such as associations and state machines, with pure code, and generating code in multiple programming languages. Mixsets can thus be used to describe variations in models, not just code. The thesis uses a design science approach as its research methodology. The mixset concept is evaluated through three case studies. The first applies mixsets to the Oracle Berkeley Database SPL, which is used in other literature to evaluate SPL concepts. This study shows that the mixset implementation improves on the state of the art in several respects, such as elimination of complex workarounds that are required by other approaches. In the second study, mixsets help to refactor Umple into feature-oriented SPL. The case study shows how annotative fragments can be easily transformed into compositional counterparts. In the third study, mixsets help to present two alternative solutions the to the Rover Control Challenge Problem of the MDETools 2018 workshop.