Consider I have a class which represents a tree element. An element can be changed, inspected children elements can be added to it (say trough the add() method). But also I have a class which contains root of the tree (State). And for convenience's sake I have added static overload of add() method which adds elements to root. And as result element class needs to get current application state through a singleton (which existence is justified) and therefore element class is bound to State class. Is it good? Is it justified? Because otherwise I have to add method add to State (but this seems like SRP violation though).

  • No, it doesn't sound like an SRP violation to me. Is there any particular reason why you're treating the root of the t ree differently than all of the other nodes? Every other node is also the root of a tree. Jan 28, 2020 at 15:22
  • @RobertHarvey you mean why I store root in other class? Or why the static add adds to root?
    – wcobalt
    Jan 28, 2020 at 15:24
  • Most trees don't require special operations for the root, because each node is already a root to another tree. Jan 28, 2020 at 15:31
  • @RobertHarvey probably I spoke incorrectly. Root is also an element, but instance of the element which represents root is contained as field/member/property in different class State (because I have to store root somewhere)
    – wcobalt
    Jan 28, 2020 at 15:32
  • Any reason why it is not State.root.add?
    – Theraot
    Jan 28, 2020 at 15:39

1 Answer 1


The SRP is not about doing one thing, but about reasons to change.

The case that you describe is on one side a Tree class that manages elements, and on the other side a State class that makes use of a specific tree.

At first sight, creating an overload to enrich Tree with an add at root level does not seem to break the SRP: this overload does not create a new reason for change. It's just a convenience.

Unfortunately, it's not fully clear how root relates to a node of the Tree:

  • Is there only one single root for all the Tree elements? In this case, it's not about SRP but it's more about OCP and extending Tree to allow reusability.
  • Are there several trees, each with it's own root? In this case, how do you find the right root? There are two variants:
    • There's no direct relation: the using context has to know its root. In this case, State would have a root. But other classes could have a different root. Then your add() overload would belong to State since it addresses an encapsulated element. But the add() signature would depend on the Tree::add(). In this case you would break SRP.
    • There is a relation. For example, each tree would point to a root node. Or each tree would point to a parent tree, allowing to go back to the root. Or the tree keeps trace of the root, and the tree elements are a nested class. Whatever the design, you could then have an add() for the root level without breaking any SOLID principles.

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