Untitled

List<Node> dbNodes = getDBNodes(); //getTerminalNodes(); or getNodesWithoutOutboundEdges();
for (Node dbNode : dbNodes) {
    Double result = traverse(dbNode);
    dbNode.apply(result);
}

public static Double traverse(Node node) {
    if (node == null) {
        return 0;
    }
    List<Node> childs = getChildsInPriorityOrder(node);
    if (childs.isEmpty()) { // or if(node instanceof NumericNode)
        return node.apply();
    }
    List<Double> childResults = new ArrayList<>();
    for (Node child : childs) {
        childResults.add(traverse(child)); 
    }
    return node.apply(childResults);
}

abstract class Node {
   abstract Double apply(List<Double> inputs);
}

class SumNode extends Node {
    Double apply(List<Double> inputs) {
	assertThatHasTwoParameter(inputs);
	return inputs.get(0) + inputs.get(1);
    }
}

class NumericNode extends Node {
    Double value;
    ....
    Double apply(List<Double> inputs) {
	return value;
    }
}

class DivNode extends Node {
    Double apply(List<Double> inputs) {
	assertThatHasTwoParameter(inputs);
        assertThatSecondParameterIsNotNull(inputs);
	return inputs.get(0) / inputs.get(1);
    }
}

class DbNode extends Node {
    DbClient dbClient;
    ....
    Double apply(List<Double> inputs) {
	assertThatHasOneParameter(inputs);
	db.saveToDB(inputs.get(0));
    }
}