Objects on a list and a list as part of an object

Next, let's have a look at objects that contain a list. Examples of objects like these include objects that describe sets, for example playlists.

In the following example, we have made a class for the concept of a playlist. The playlist contains songs: songs can be added, songs can be removed, and the songs that the playlist contains can be printed.

// imports

public class Playlist {
    private ArrayList<String> songs;

    public Playlist() {
        this.songs = new ArrayList<>();
    }

    public void addSong(String song) {
        this.songs.add(song);
    }

    public void removeSong(String song) {
        this.songs.remove(song);
    }

    public void printSongs() {
        for (String song: this.songs) {
            System.out.println(song);
        }
    }
}

Creating playlists is easy with the help of the class above.

Playlist list = new Playlist();
list.addSong("Sorateiden kuningas");
list.addSong("Teuvo, maanteiden kuningas");
list.printSongs();

Sample output

Sorateiden kuningas Teuvo, maanteiden kuningas

Objects in an Instance Variable List

A list that is an object's instance variable can contain objects other than strings as long as the type of objects in the list is specified when defining the list.

In the previous section, we created a class called AmusementParkRide, which was used to check whether or not a person was eligible to get on a particular ride. The Amusement park class looks like the following.

public class AmusementParkRide {
    private String name;
    private int minimumHeight;
    private int visitors;

    public AmusementParkRide(String name, int minimumHeight) {
        this.name = name;
        this.minimumHeight = minimumHeight;
        this.visitors = 0;
    }

    public boolean isAllowedOn(Person person) {
        if (person.getHeight() < this.minimumHeight) {
            return false;
        }

        this.visitors++;
        return true;
    }

    public String toString() {
        return this.name + ", minimum height requirement: " + this.minimumHeight +
            ", visitors: " + this.visitors;
    }
}

We'll extend the class so that the amusement park keeps track of the people on the ride. In this version, the ride has, as an instance variable, a list of the people who have been allowed on the ride. The list is created in the constructor.

public class AmusementParkRide {
    private String name;
    private int minimumHeigth;
    private int visitors;
    private ArrayList<Person> riding;

    public AmusementParkRide(String name, int minimumHeigth) {
        this.name = name;
        this.minimumHeigth = minimumHeigth;
        this.visitors = 0;
        this.riding = new ArrayList<>();
    }

    // ..
}

Let's change the method isAllowedOn. The method adds to the list all the persons who meet the height requirements.

public class AmusementParkRide {
    private String name;
    private int minimumHeight;
    private int visitors;
    private ArrayList<Person> riding;

    public AmusementParkRide(String name, int minimumHeight) {
        this.name = name;
        this.minimumHeight = minimumHeight;
        this.visitors = 0;
        this.riding = new ArrayList<>();
    }

    public boolean isAllowedOn(Person person) {
        if (person.getHeight() < this.minimumHeight) {
            return false;
        }

        this.visitors++;
        this.riding.add(person);
        return true;
    }

    public String toString() {
        return this.name + ", minimum height requirement: " + this.minimumHeight +
            ", visitors: " + this.visitors;
    }
}

Printing an Object from a List

Let's now modify the toString method so that the string returned by the method contains the name of each and every person on the ride.

public class AmusementParkRide {
    private String name;
    private int minimumHeight;
    private int visitors;
    private ArrayList<Person> riding;

    // ...

    public String toString() {
        // let's form a string from all the people on the list
        String onTheRide = "";
        for (Person person: riding) {
            onTheRide = onTheRide + person.getName() + "\n";
        }

        // we return a string describing the object
        // including the names of those on the ride
        return this.name + ", minimum height requirement: " + this.minimumHeight +
            ", visitors: " + this.visitors + "\n" +
            "riding:\n" + onTheRide;
    }
}

Let's test out the extended amusement park ride:

Person matti = new Person("Matti");
matti.setWeigth(86);
matti.setHeight(180);

Person juhana = new Person("Juhana");
juhana.setWeigth(34);
juhana.setHeight(132);

AmusementParkRide hurjakuru = new AmusementParkRide("Hurjakuru", 140);
System.out.println(hurjakuru);

System.out.println();

if (hurjakuru.isAllowedOn(matti)) {
    System.out.println(matti.getNimi() + " is allowed on the ride");
} else {
    System.out.println(matti.getNimi() + " is not allowed on the ride");
}

if (hurjakuru.isAllowedOn(juhana)) {
    System.out.println(juhana.getNimi() + " is allowed on the ride");
} else {
    System.out.println(juhana.getNimi() + " is not allowed on the ride");
}

System.out.println(hurjakuru);

The program's output is:

Sample output

Hurjakuru, minimum height requirement: 140, visitors: 0 riding:

Matti is allowed on the ride Juhana is not allowed on the ride Hurjakuru, minimum height requirement: 140, visitors: 1 riding: Matti

Even though there is no one on the ride, the string riding: is on the print output. Let's modify the toString method so that if there is no one on the ride, the string returned by the method informs of it.

public class AmusementParkRide {
    private String name;
    private int minimumHeight;
    private int visitors;
    private ArrayList<Person> riding;

    public AmusementParkRide(String name, int minimumHeight) {
        this.name = name;
        this.minimumHeight = minimumHeight;
        this.visitors = 0;
        this.riding = new ArrayList<>();
    }

    // ...

    public String toString() {

        String printOutput = this.name + ", minimum height requirement: " + this.minimumHeight +
            ", visitors: " + this.visitors + "\n";

        if (riding.isEmpty()) {
            return printOutput + "no one is on the ride.";
        }

        // we form a string from the people on the list
        String peopleOnRide = "";

        for (Person person: riding) {
            peopleOnRide = peopleOnRide + person.getName() + "\n";
        }

        return printOutput + "\n" +
            "on the ride:\n" + peopleOnRide;
    }
}

The print output has now been improved.

Person matti = new Person("Matti");
matti.setWeight(86);
matti.setHeight(180);

Person juhana = new Person("Juhana");
juhana.setWeight(34);
juhana.setHeight(132);

AmusementParkRide hurjakuru = new AmusementParkRide("Hurjakuru", 140);
System.out.println(hurjakuru);

System.out.println();

if (hurjakuru.isAllowedOn(matti)) {
    System.out.println(matti.getName() + " is allowed on the ride");
} else {
    System.out.println(matti.getName() + " is not allowed on the ride");
}

if (hurjakuru.isAllowedOn(juhana)) {
    System.out.println(juhana.getName() + " is allowed on the ride");
} else {
    System.out.println(juhana.getName() + " is not allowed on the ride");
}

System.out.println(hurjakuru);

The program's output is:

Sample output

Hurjakuru, minimum height requirement: 140, visitors: 0 no one is on the ride.

Matti is allowed on the ride Juhana is not allowed on the ride Hurjakuru, minimum height requirement: 140, visitors: 1 on the ride: Matti

Clearing an Object's List

We'll next add a removeEveryoneOnRide method to the amusement park ride, which removes each and every person currently on the ride.The list method clear is very handy here.

public class AmusementParkRIde {
    // ..

    public void removeEveryoneOnRide() {
        this.riding.clear();
    }

    // ..
}

Person matti = new Person("Matti");
matti.setWeight(86);
matti.setHeight(180);

Person juhana = new Person("Juhana");
juhana.setWeight(34);
juhana.setHeight(132);

AmusementParkRide hurjakuru = new AmusementParkRide("Hurjakuru", 140);
System.out.println(hurjakuru);

System.out.println();

if (hurjakuru.isAllowedOn(matti)) {
    System.out.println(matti.getName() + " is allowed on the ride");
} else {
    System.out.println(matti.getName() + " is not allowed on the ride");
}

if (hurjakuru.isAllowedOn(juhana)) {
    System.out.println(juhana.getName() + " is allowed on the ride");
} else {
    System.out.println(juhana.getName() + " is not allowed on the ride");
}

System.out.println(hurjakuru);

hurjakuru.removeEveryoneOnRide();

System.out.println();
System.out.println(hurjakuru);

The program's output is:

Sample output

Hurjakuru, minimum height requirement: 140, visitors: 0 no one is on the ride.

Matti is allowed on the ride. Juhana is not allowed on the ride Hurjakuru, minimum height requirement: 140, visitors: 1 on the ride: Matti

Hurjakuru, minimum height requirement: 140, visitors: 1 no one is on the ride.

Calculating a Sum from Objects on a List

Let's now create a method for the amusement park ride that calculates the average height of the people currently on it. Average height can obtained by calculating the average from the persons on the ride — the average is calculated by adding up the individual values and dividing that sum by the number of values.

The implementation underneath returns -1 if not a single person is on the ride. The result of -1 is impossible in a program that calculates averages. Based on that, we can determine that the average could not have been calculated.

public class AmusementParkRide {
    private String name;
    private int minimumHeight;
    private int visitors;
    private ArrayList<Person> riding;

    // ..

    public double averageHeightOfPeopleOnRide() {
        if (riding.isEmpty()) {
            return -1;
        }

        int sumOfHeights = 0;
        for (Person per: riding) {
            sumOfHeights += per.getHeight();
        }

        return 1.0 * sumOfHeights / riding.size();
    }

    // ..
}

Person matti = new Person("Matti");
matti.setHeight(180);

Person juhana = new Person("Juhana");
juhana.setHeight(132);

Person awak = new Henkilo("Awak");
awak.setHeight(194);

AmusementParkRide hurjakuru = new AmusementParkRide("Hurjakuru", 140);

hurjakuru.isAllowedOn(matti);
hurjakuru.isAllowedOn(juhana);
hurjakuru.isAllowedOn(awak);

System.out.println(hurjakuru);
System.out.println(hurjakuru.averageHeightOfPeopleOnRide());

The program's output is:

Sample output

Hurjakuru, minimum height requirement: 140, visitors: 2 on the ride: Matti Awak 187.0

Retrieving a Specific Object from a List

We'll now create a method for the amusement park ride that returns the tallest person on the ride. As such, the method should both retrieve the tallest person from the list and return it.

Methods that retrieve objects from a list should be implemented in the following way. First off, we'll check whether or not the list is empty - if it is, we return a null reference or some other value indicating that the list had no values. After that, we create an object reference variable that describes the object to be returned. We set the first object on the list as its value. We then go through the values on the list by comparing each list object with the object variable representing the object to be returned. If the comparison finds a better matching object, its assigned to the object reference variable to be returned. Finally, we return the object variable describing the object that we want to return.

public Person getTallest() {
    // return a null reference if there's no one on the ride
    if (this.riding.isEmpty()) {
        return null;
    }

    // create an object reference for the object to be returned
    // its first value is the first object on the list
    Person returnObject = this.riding.get(0);

    // go through the list
    for (Person prs: this.riding) {
        // compare each object on the list
        // to the returnObject -- we compare heights
        // since we're searching for the tallest,

        if (returnObject.getHeight() < prs.getHeight()) {
            // if we find a taller person in the comparison,
            // we assign it as the value of the returnObject
            returnObject = prs;
        }
    }

    // finally, the object reference describing the
    // return object is returned
    return returnObject;
}

Finding the tallest person is now easy.

Person matti = new Person("Matti");
matti.setHeight(180);

Person juhana = new Person("Juhana");
juhana.setHeight(132);

Person awak = new Person("Awak");
awak.setHeight(194);

AmusementParkRide hurjakuru = new AmusementParkRide("Hurjakuru", 140);

hurjakuru.isAllowedOn(matti);
hurjakuru.isAllowedOn(juhana);
hurjakuru.isAllowedOn(awak);

System.out.println(hurjakuru);
System.out.println(hurjakuru.averageHeightOfPeopleOnRide());

System.out.println();
System.out.println(hurjakuru.getTallest().getName());
Person tallest = hurjakuru.getTallest();
System.out.println(tallest.getName());

Sample output

Hurjakuru, minimum height requirement: 140, visitors: 2 on the ride: Matti Awak 187.0

Awak Awak

You have reached the end of this section! Continue to the next section:

2. Separating the user interface from program logic

Remember to check your points from the ball on the bottom-right corner of the material!