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ClassicVisitor.java
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ClassicVisitor.java
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// 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);
}
}