Avoiding code duplication for methods with different return types

Question

I have two builder classes that return a Tree and a Set respectively. The execution of the second builder is just the execution of the first builder with an extra step and a different return type. I really don't want to duplicate the same steps in two different classes, because if there is a change in the Tree -building process, I have to change code in two classes. Here are the two builders:

public class TreeBuilder {

    private NodeHelper nodeHelper;

    public TreeBuilder(NodeHelper nodeHelper) {
        this.nodeHelper = nodeHelper;
    }

    public Tree<EquipmentDetails> buildTree() {

        Tree<EquipmentDetails> tree = new Tree<>(nodeHelper.getEquipmentDetails(nodeHelper.getModelName()));

        return buildTree(tree);
    }

    private Tree<EquipmentDetails> buildTree(Tree<EquipmentDetails> parentTree) {

        Map<String, NativeSlotDisplayLabel> childSubTypeMap = nodeHelper.getChildSubTypeMap(parentTree);

        if (CommonUtils.nullOrEmpty(childSubTypeMap))
            return parentTree;

        for (String holderCommonName : childSubTypeMap.keySet()) {

            NativeSlotDisplayLabel nativeSlotDisplayLabel = childSubTypeMap.get(holderCommonName);

            EquipmentDetails equipmentDetails = nodeHelper.constructEqptDetails(holderCommonName, nativeSlotDisplayLabel, parentTree);

            Tree<EquipmentDetails> childTree = new Tree<>(equipmentDetails);

            parentTree.addChild(childTree);

            String equipmentCommonName = equipmentDetails.getModelEquipType();

            if (equipmentCommonName == null)
                continue;

            buildTree(childTree);
        }

        return parentTree;
    }
}

The second builder uses one more dependency and returns a Set instead of a Tree :

public class FreePortsBuilder {

    private NodeHelper nodeHelper;

    private PortLabelGenerator portLabelGenerator;

    public FreePortsBuilder(NodeHelper nodeHelper, PortLabelGenerator portLabelGenerator) {
        this.nodeHelper = nodeHelper;
        this.portLabelGenerator = portLabelGenerator;
    }

    public Set<PortNameAndLabel> collectFreePorts(){

        Tree<EquipmentDetails> tree = new Tree<>(nodeHelper.getEquipmentDetails(nodeHelper.getModelName()));

        Set<PortNameAndLabel> portNameAndLabels = new HashSet<>();

        return collectFreePorts(tree, portNameAndLabels);
    }

    private Set<PortNameAndLabel> collectFreePorts(Tree<EquipmentDetails> parentTree, Set<PortNameAndLabel> portNameAndLabels) {

        Map<String, NativeSlotDisplayLabel> childSubTypeMap = nodeHelper.getChildSubTypeMap(parentTree);

        if (CommonUtils.nullOrEmpty(childSubTypeMap))
            return portNameAndLabels;

        Set<PortNameAndLabel> generatedPortNameAndLabel = portLabelGenerator.getPortNameAndLabel(childSubTypeMap, parentTree.getUserObject());

        portNameAndLabels.addAll(generatedPortNameAndLabel);

        for (String holderCommonName : childSubTypeMap.keySet()) {

            NativeSlotDisplayLabel nativeSlotDisplayLabel = childSubTypeMap.get(holderCommonName);

            EquipmentDetails equipmentDetails = nodeHelper.constructEqptDetails(holderCommonName, nativeSlotDisplayLabel, parentTree);

            Tree<EquipmentDetails> childTree = new Tree<>(equipmentDetails);

            parentTree.addChild(childTree);

            String equipmentCommonName = equipmentDetails.getModelEquipType();

            if (equipmentCommonName == null)
                continue;

            collectFreePorts(childTree, portNameAndLabels);
        }

        return portNameAndLabels;
    }
}

I am looking for a way to avoid the duplication. Any suggestions?

Answer 1

FreePortsBuilder is doing two things at the same time:

1. Building a Tree : as you know, this is identical to what TreeBuilder does.
2. Building a Set<PortNameAndLabel> , where each PortNameAndLabel appears to be associated with a node or child (which is itself a Tree ) in the final Tree .

Instead of doing the above two things simultaneously, do them sequentially. That is, in FreePortsBuilder , first use TreeBuilder to build the Tree , then traverse this Tree to construct the Set<PortNameAndLabel> . One way to do that is the following (I assume that your Tree class has a method like getChildren() that returns a List<Tree<EquipmentDetails>> ).

First, allow a client to obtain a TreeBuilder 's NodeHelper by adding a getter:

public class TreeBuilder {
  private NodeHelper nodeHelper;

  public NodeHelper getNodeHelper() {
    return this.nodeHelper;
  }

  ...
}

Then, inject a TreeBuilder instead of a NodeHelper into FreePortsBuilder ( FreePortsBuilder can use the NodeHelper that is in the TreeBuilder , because of the getter that we just added):

public class FreePortsBuilder {
  private TreeBuilder treeBuilder;
  private PortLabelGenerator portLabelGenerator;

  public FreePortsBuilder(TreeBuilder treeBuilder, PortLabelGenerator portLabelGenerator) {
    this.treeBuilder = treeBuilder;
    this.portLabelGenerator = portLabelGenerator;
  }

  public Set<PortNameAndLabel> collectFreePorts() {
    Tree<EquipmentDetails> tree = treeBuilder.buildTree();
    return collectFreePorts(tree);
  }

  private Set<PortNameAndLabel> collectFreePorts(Tree<EquipmentDetails> tree) {
    Set<PortNameAndLabel> portNameAndLabels = new HashSet<>();

    for (Tree<EquipmentDetails> child : tree.getChildren()) {
      Map<String, NativeSlotDisplayLabel> childSubTypeMap =
          treeBuilder.getNodeHelper().getChildSubTypeMap(child);

      if (CommonUtils.nullOrEmpty(childSubTypeMap))
        continue;

      Set<PortNameAndLabel> generatedPortNameAndLabel =
          portLabelGenerator.getPortNameAndLabel(childSubTypeMap, child.getUserObject());

      portNameAndLabels.addAll(generatedPortNameAndLabel);
    }

    return portNameAndLabels;
  }
}

Now, FreePortsBuilder can truly make use of the TreeBuilder implementation, and the code duplication is removed.

Answer 2

You can use Strategy design pattern or Template Method here. You have an algorithm here where only certain steps change (collectFreePorts & buildTree).

If you cannot use inheritance, you can use a similar solution to this: https://softwareengineering.stackexchange.com/questions/187872/template-method-within-one-class-without-subclasses-or-inheritance