IntrusiveVisitor.java

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

package behavioral.visitor.intrusive;

abstract class Expression
{
    public abstract void print(StringBuilder sb); // visitor here
}

class DoubleExpression extends Expression
{
    private double value;

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

    // Visitor is here.
    @Override
    public void print(StringBuilder sb)
    {
        sb.append(value);
    }
}

class AdditionExpression extends Expression
{
    private Expression left, right;

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

    @Override
    public void print(StringBuilder sb) {
        sb.append("(");
        left.print(sb);
        sb.append("+");
        right.print(sb);
        sb.append(")");
    }
}

class IntrusiveDemo
{
    public static void main(String[] args)
    {
        // 1 + (2+3)
        AdditionExpression e = new AdditionExpression(
            new DoubleExpression(1),
            new AdditionExpression(
                new DoubleExpression(2),
                new DoubleExpression(3)
            )
        );
        StringBuilder sb = new StringBuilder();
        e.print(sb);
        System.out.println(sb);
    }
}