For example, let suppose we have binary trees whose nodes are the “plus”
operator and leaves are a “1”.

The code of a sum function for those trees would actually be split
in many classes when written in Java :

interface PlusOrJustInteger {}

class JustInteger implements PlusOrJustInteger {
  private int value = 1;
  …
  public int treeSum() {
    return value;
  }
}

class Plus implements PlusOrJustInteger {
  private PlusOrJustInteger left;
  private PlusOrJustInteger right;
  …
  public int treeSum() {
     return left.treeSum() + right.treeSum();
  }
}

This may, or may not, be the organisation that the user wants.
The Object-Oriented answer to this problem is the visitor pattern. But in Java, this requires writing boilerplate code which most of the time is not type-safe. This project allows you to make the design pattern explicit, supressing the need for this tedious code. And it does that without relying on an external pre-processor, using Java annotations instead.