ClassicVisitor.java

// Visitor pattern - classic double dispatch visitor; example will PRINT a tree of expressions (ie: 1 + 2)


package behavioral.visitor.classic;

interface ExpressionVisitor
{
    // We overload on the argument!
    void visit(DoubleExpression e);
    void visit(AdditionExpression e);
}

abstract class Expression
{
    // NOW, every single element in the hierarchy has to accept visitor
    public abstract void accept(ExpressionVisitor visitor);
}

class DoubleExpression extends Expression
{
    public double value;

    public DoubleExpression(double value) {
        this.value = value;
    }

    @Override
    public void accept(ExpressionVisitor visitor)
    {
        visitor.visit(this);
    }
}

class AdditionExpression extends Expression
{
    public Expression left, right;

    public AdditionExpression(Expression left, Expression right) {
        this.left = left;
        this.right = right;
    }

    @Override
    public void accept(ExpressionVisitor visitor)
    {
        visitor.visit(this);
    }
}

class ExpressionPrinter implements ExpressionVisitor
{
    private StringBuilder sb = new StringBuilder();

    @Override
    public void visit(DoubleExpression e)
    {
        sb.append(e.value);
    }

    @Override
    public void visit(AdditionExpression e)
    {
        sb.append("(");
        e.left.accept(this); // this allows you to use 'overloading'
        sb.append("+");
        e.right.accept(this);  // this allows you to use 'overloading'
        sb.append(")");
    }

    @Override
    public String toString()
    {
        return sb.toString();
    }
}

class ExpressionCalculator implements ExpressionVisitor
{
    public double result;

    @Override
    public void visit(DoubleExpression e)
    {
        result = e.value;
    }

    @Override
    public void visit(AdditionExpression e)
    {
        e.left.accept(this);
        double cache = result;
        e.right.accept(this);
        double cache2 = result;
        result = cache + cache2;
    }
}

class ClassicDemo
{
    public static void main(String[] args)
    {
        // 1 + (2+3) = 6
        AdditionExpression e = new AdditionExpression(
            new DoubleExpression(1),
            new AdditionExpression(
                new DoubleExpression(2),
                    new DoubleExpression(3)
            )
        );
        ExpressionPrinter ep = new ExpressionPrinter();
        ep.visit(e);
        System.out.println(ep);

        ExpressionCalculator ec = new ExpressionCalculator();
        ec.visit(e);
        System.out.println(ep + " = " + ec.result);
    }
}