public class Contact implements Comparable<Contact> {

    private String firstName;
    private String lastName;
    private String phone;
    private String address;
    private int age;
    private String organization;
    private String group;

    public int compareTo(Contact other) {
        return firstName.compareTo(other.firstName);
    }
}

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// The ArrayList can be declared like this:

List<Contact> contacts = new ArrayList<Contact>();

// ... At the beginning of execution.