When and where to use Visitor Pattern?

Hello,
I am confused about Visiotr Pattern. Observe this example from
nice.sf.net:

package syntax;

abstract class ExpressionVisitor
{
abstract void visitIntExp(IntExp e);
abstract void visitAddExp(AddExp e);
}

abstract class Expression
{
abstract void accept(ExpressionVisitor v);
}

class IntExp extends Expression
{
int value;

void accept(ExpressionVisitor v)
{
v.visitIntExp(this);
}
}

class AddExp extends Expression
{
Expression e1, e2;

void accept(ExpressionVisitor v)
{
v.visitAddExp(this);
}
}

// Behaviour can now be defined on Expressions

package tools;

class PrettyPrint extends ExpressionVisitor
{
void visitIntExp(IntExp e)
{
System.out.print(e.value);
}

void visitAddExp(AddExp e)
{
e.e1.accept(this);
System.out.print(" + ");
e.e2.accept(this);
}
}
Why make it so complicated? Can't we use this one instead?
class IntExp1 extends Expression
{
int value;

void visit()
{
System.out.println(value);
}
}

class AddExp1 extends Expression
{
Expression e1, e2;

void visit()
{
e1.visit();
System.out.print(" + ");
e2.visit();
}
}
I dont know in what cirumstances the pattern is used sensablely. I hope
someone can address my confusion. Thanks!

Regards,
Sam Hwang

.

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