Example Visitor vs Revisitor

src_tree_Main.java

package tree;

import tree.model.Leaf;
import tree.model.Node;
import tree.revisitor.RevisitorMax;
import tree.revisitor.RevisitorSum;
import tree.visitor.VisitorMax;
import tree.visitor.VisitorSum;

public class Main {
    public static void main(String[] args) {
        var tree = new Node(new Node(new Leaf(1), new Leaf(2)), new Leaf(3));
        System.out.println(tree.accept(new VisitorSum()));
        System.out.println(tree.accept(new VisitorMax()));
        System.out.println(new RevisitorSum() {}.$(tree));
        System.out.println(new RevisitorMax() {}.$(tree));
    }
}

src_tree_model_Leaf.java

package tree.model;

import tree.visitor.Visitor;

public class Leaf extends Tree {
    private final long value;


    public Leaf(long value) {
        this.value = value;
    }

    public long getValue() {
        return value;
    }

    @Override
    public <T> T accept(Visitor<T> visitor) {
        return visitor.leaf(this);
    }
}

src_tree_model_Node.java

package tree.model;

import tree.visitor.Visitor;

public class Node extends Tree {
    private final Tree left;
    private final Tree right;

    public Node(Tree left, Tree right) {
        this.left = left;
        this.right = right;
    }

    public Tree getLeft() {
        return left;
    }

    public Tree getRight() {
        return right;
    }

    @Override
    public <T> T accept(Visitor<T> visitor) {
        return visitor.node(this);
    }
}

src_tree_model_Tree.java

package tree.model;

import tree.visitor.Visitor;

public abstract class Tree {
    public abstract <T> T accept(Visitor<T> visitor);
}

src_tree_revisitor_Revisitor.java

package tree.revisitor;

import tree.model.Leaf;
import tree.model.Node;
import tree.model.Tree;

public interface Revisitor<TT, LT extends TT, NT extends TT> {

    LT leaf(Leaf it);

    NT node(Node it);


    default TT $(Tree it) {
        if (it instanceof Leaf) return leaf((Leaf) it);
        if (it instanceof Node) return node((Node) it);
        return null;
    }

    default LT $(Leaf it) {
        return leaf(it);
    }

    default NT $(Node it) {
        return node(it);
    }
}

src_tree_revisitor_RevisitorMax.java

package tree.revisitor;

import tree.model.Leaf;
import tree.model.Node;

public interface RevisitorMax extends Revisitor<Long, Long, Long> {

    @Override
    default Long leaf(Leaf it) {
        return it.getValue();
    }

    @Override
    default Long node(Node it) {
        final var left = $(it.getLeft());
        final var right = $(it.getRight());
        return left > right ? left : right;
    }
}

src_tree_revisitor_RevisitorSum.java

package tree.revisitor;

import tree.model.Leaf;
import tree.model.Node;

public interface RevisitorSum extends Revisitor<Long, Long, Long> {

    @Override
    default Long leaf(Leaf it) {
        return it.getValue();
    }

    @Override
    default Long node(Node it) {
        final var left = $(it.getLeft());
        final var right = $(it.getRight());
        return left + right;
    }
}

src_tree_visitor_Visitor.java

package tree.visitor;

import tree.model.Leaf;
import tree.model.Node;

public interface Visitor<T> {
    T leaf(Leaf it);

    T node(Node it);
}

src_tree_visitor_VisitorMax.java

package tree.visitor;

import tree.model.Leaf;
import tree.model.Node;

public class VisitorMax implements Visitor<Long> {
    @Override
    public Long leaf(Leaf it) {
        return it.getValue();
    }

    @Override
    public Long node(Node it) {
        final var left = it.getLeft().accept(this);
        final var right = it.getRight().accept(this);
        return left > right ? left : right;
    }
}

src_tree_visitor_VisitorSum.java

package tree.visitor;

import tree.model.Leaf;
import tree.model.Node;

public class VisitorSum implements Visitor<Long> {

    @Override
    public Long leaf(Leaf it) {
        return it.getValue();
    }

    @Override
    public Long node(Node it) {
        final var left = it.getLeft();
        final var right = it.getRight();
        return left.accept(this) + right.accept(this);
    }
}