Java programming course: 11.4 Using a Map for zookeeper's responsibilities

In the previous lesson we looked an maps.

• Previous lesson: 11.3 Maps
• Course contents 

Using a Map for zookeeper's responsibilities

At the zoo, each zookeeper is responsible for certain pens, as listed below:

• Alice looks after the monkeys and penguins
• Bob looks after the lions, monkeys and penguins
• Charles only looks after the penguins

This can be modelled using a Map by taking the view that each zookeeper will be the "key" and each will require an associated value of a collection of pens.

In the ZooAdministrator class declare a new instance variable called responsibilities that will be a Map, keyed by ZooKeeper objects and with the associated value being a collection of Pen objects:

private Map<ZooKeeper, Collection<Pen>> responsibilities; 

Define a new private method createExampleResponsibilities() which will instantiate the map as a HashMap and then assign the appropriate pens into the map, for each zookeeper:

private void createExampleResponsibilities() {
    responsibilities = new HashMap<>();

    // Alice
    Collection<Pen> alicePens = new HashSet<>();
    alicePens.add(monkeyPen);
    alicePens.add(penguinPen);
    responsibilities.put(alice, alicePens);
        
    // Bob
    Collection<Pen> bobPens = new HashSet<>();
    bobPens.add(lionPen);
    bobPens.add(penguinPen);
    bobPens.add(monkeyPen);
    responsibilities.put(bob, bobPens);
        
    // Charles
    Collection<Pen> charlesPens = new HashSet<>();
    charlesPens.add(penguinPen);
    responsibilities.put(charles, charlesPens);
}
 

• Note how for each zookeeper, a HashSet is used to group the relevant pens, and this is used as the value argument when being placed into the map, which is keyed on the ZooKeeper object.

You can now invoke the method at the end of the constructor:

public ZooAdministrator() {
    createExampleZooKeepers();
    createExampleVisitors();
    createExamplePens();
    createExampleResponsibilities();
}
 

It will be useful to output each zookeeper's responsibilities, so define a new method in ZooAdministrator called showZooKeeperResponsibilities(). This will contain some nested loops:

• The outer loop will iterate over each zookeeper in turn
• For each zookeeper, a second loop will iterate over each pen the zookeeper is responsible for in turn
• For each pen, a third loop will iterate over each of the animals that exist in that pen in turn, sending the animal's state to output.

Here is the method:

public void showZooKeeperResponsibilities() {
    System.out.println("Zoo keeper responsibilities");
    System.out.println("===========================");

    // For each zoo keeper in turn...
    for (ZooKeeper aZooKeeper : responsibilities.keySet()) {
        System.out.println(aZooKeeper.getName() +" looks after:");
            
        // Get current zoo keeper's pens
        Collection<Pen> pens = responsibilities.get(aZooKeeper);
            
        // For each pen in turn...
        for (Pen aPen : pens) {
                
            // Get the animals in the current pen
            Collection<Animal> animals = aPen.getAnimals();
                
            // For each animal in turn...
            for (Animal anAnimal : animals) {
                System.out.println("-> " + anAnimal);
            }
        }
          
        // That's it for this zoo keeper
        System.out.println("---------------------------");
    }
}
 

• You will see each nested loop indented inside the one outside of it.

In the main() method of Experiments you can now invoke the above method:

ZooAdministrator admin = new ZooAdministrator();
admin.showZooKeeperResponsibilities();
 

In the next series of lesson we will look at multithreading.

Next lesson: 12.1 Multithreading