Explain the Visitor design pattern and provide an example of its implementation in a software system.

The Visitor design pattern is a behavioral design pattern that allows adding new operations to an existing class hierarchy without modifying the classes themselves. It separates the algorithm from the objects on which it operates, enabling the addition of new operations without modifying the classes.

The pattern consists of two main components: the Visitor interface and the ConcreteVisitor classes. The Visitor interface declares a visit method for each class in the object hierarchy. Each ConcreteVisitor class implements the visit method for a specific class or group of classes.

To implement the Visitor pattern, we need to define the object hierarchy and the operations that can be performed on the objects. Let's consider an example of a software system for a zoo, where we have different types of animals such as lions, tigers, and bears.

First, we define the object hierarchy by creating an Animal interface or abstract class that declares an accept method. Each concrete animal class implements the accept method and calls the visit method of the visitor object.

```java
interface Animal {
void accept(Visitor visitor);
}

class Lion implements Animal {
@Override
public void accept(Visitor visitor) {
visitor.visit(this);
}
}

class Tiger implements Animal {
@Override
public void accept(Visitor visitor) {
visitor.visit(this);
}
}

class Bear implements Animal {
@Override
public void accept(Visitor visitor) {
visitor.visit(this);
}
}
```

Next, we define the Visitor interface that declares the visit method for each animal class.

```java
interface Visitor {
void visit(Lion lion);
void visit(Tiger tiger);
void visit(Bear bear);
}
```

Now, we can implement the ConcreteVisitor classes that provide the actual operations on the animals.

```java
class FeedingVisitor implements Visitor {
@Override
public void visit(Lion lion) {
System.out.println("Feeding lion");
}

@Override
public void visit(Tiger tiger) {
System.out.println("Feeding tiger");
}

@Override
public void visit(Bear bear) {
System.out.println("Feeding bear");
}
}

class CleaningVisitor implements Visitor {
@Override
public void visit(Lion lion) {
System.out.println("Cleaning lion's enclosure");
}

@Override
public void visit(Tiger tiger) {
System.out.println("Cleaning tiger's enclosure");
}

@Override
public void visit(Bear bear) {
System.out.println("Cleaning bear's enclosure");
}
}
```

Finally, we can create a software system that utilizes the Visitor pattern to perform operations on the animals.

```java
public class Zoo {
private List animals;

public Zoo() {
animals = new ArrayList<>();
}

public void addAnimal(Animal animal) {
animals.add(animal);
}

public void performOperation(Visitor visitor) {
for (Animal animal : animals) {
animal.accept(visitor);
}
}

public static void main(String[] args) {
Zoo zoo = new Zoo();
zoo.addAnimal(new Lion());
zoo.addAnimal(new Tiger());
zoo.addAnimal(new Bear());

zoo.performOperation(new FeedingVisitor());
zoo.performOperation(new CleaningVisitor());
}
}
```

In this example, the Visitor pattern allows us to add new operations to the zoo animals without modifying the animal classes themselves. The FeedingVisitor and CleaningVisitor classes implement the Visitor interface and provide the feeding and cleaning operations, respectively. The Zoo class acts as the object structure and uses the Visitor pattern to perform operations on the animals.

By using the Visitor pattern, we can easily add new operations to the animal hierarchy by creating new ConcreteVisitor classes without modifying the existing animal classes. This promotes extensibility and separation of concerns in the software system.