为什么我的 TreeSet 没有在第一个元素之外添加任何内容?

如果您将对象放在 TreeSet 中，则需要提供 Comparator 的实现构造函数中的接口，或者您需要您的对象属于实现 Comparable 的类.

您说您从 Comparable 接口实现 compareTo ，但在您的评论中您说您没有，所以我假设您只是 return 0; 在 compareTo 方法中?这可以解释你的问题，因为 TreeSet 会根据 compareTo 方法的结果认为你所有的对象都是相同的".

基本上，在 TreeSet 中，您的对象按排序顺序维护，排序由 Comparable/Comparator 方法的结果决定.这用于在 TreeSet 中快速查找重复项，并具有额外的好处，即当您遍历 TreeSet 时，您可以按排序顺序获得结果.

TreeSet 的 Javadoc 说:

请注意，由集合维护的顺序(无论是否显式提供了比较器)必须与equals一致如果它是正确实现Set接口.

实现这一点的最简单方法是让您的 equals 方法调用 compareTo 方法并检查结果是否为 0.

I know my objects are working well because when I try to do the same thing but with ArrayList instead, everything works fine. So I'm guessing I'm using the TreeSet wrong.

If it helps, Patient has a method called getFirstName(), and when I try to do the following:

Iterator<Patient> patItr = patTreeSet.iterator();

while(patItr.hasNext()){
    System.out.println(patItr.next().getFirstName());

}

Then only "John" prints, which obviously shouldn't be the case... So, am I totally misusing the TreeSet?

Thanks in advance for any help!

EDIT below

================PatientImpl Class====================

public class PatientImpl implements Patient, Comparable{

    Calendar cal = new GregorianCalendar();
    private String firstName;
    private String lastName;
    private String SSN;
    private Date dob;
    private int age;
    private int thisID;             
    public static int ID = 0;       



    public PatientImpl(String fullName, String SSN, Date dob){

        String[] name = fullName.split(" ");
        firstName = name[0];
        lastName = name[1];

        this.SSN = SSN;

        this.dob = dob;

        thisID = ID += 1;
    }

@Override
    public boolean equals(Object p) {

        //for some reason casting here and reassigning the value of p doesn't take care of the need to cast in the if statement...
        p = (PatientImpl) p;

        Boolean equal = false;
        //make sure p is a patient before we even compare anything
        if (p instanceof Patient) {

            Patient temp = (Patient) p;

            if (this.firstName.equalsIgnoreCase(temp.getFirstName())) {
                if (this.lastName.equalsIgnoreCase(temp.getLastName())) {
                    if (this.SSN.equalsIgnoreCase(temp.getSSN())) {
                        if(this.dob.toString().equalsIgnoreCase(((PatientImpl) p).getDOB().toString())){
                            if(this.getID() == temp.getID()){
                                equal = true;
                            }
                        }
                    }
                }
            }
         }
        return equal;
    }

and then all the getters are below, as well as the compareTo() method from the Comparable interface

If you put your objects in a TreeSet, you need to either provide an implementation of the Comparator interface in the constructor, or you need your objects to be of a class that implements Comparable.

You said you implement compareTo from the Comparable interface, but in your comment you say that you didn't, so am I correct in assuming that you just return 0; in the compareTo method? That would explain your problem, because TreeSet would then think that all your objects are 'the same' based on the compareTo method result.

Basically, in a TreeSet, your objects are maintained in a sorted order, and the sorting is determined by the outcome of the Comparable/Comparator method. This is used to quickly find duplicates in a TreeSet and has the added benefit that when you iterate over the TreeSet, you get the results in sorted order.

The Javadoc of TreeSet says:

Note that the ordering maintained by a set (whether or not an explicit comparator is provided) must be consistent with equals if it is to correctly implement the Set interface.

The easiest way to achieve that is to let your equals method call the compareTo method and check if the result is 0.

Given your PatientImpl class, I assume that you would want to sort patients first by their last name, then by their first name, and then by the rest of the fields in the class.

You could implement a compareTo method like this:

@Override
public int compareTo(Object o) {
    if (!(o instanceof Patient))
        return -1;
    Patient temp = (Patient) o;
    int r = this.lastName.compareToIgnoreCase(temp.getLastName());
    if (r == 0)
        r = this.firstName.compareToIgnoreCase(temp.getFirstName());
    if (r == 0)
        r = this.SSN.compareToIgnoreCase(temp.getSSN());
    if (r == 0)
        r = this.dob.toString().compareToIgnoreCase(temp.getDOB().toString());
    if (r == 0)
        r = Integer.compare(this.getID(), temp.getID());
    return r;
}

I believe that would solve the problem you described. I would advise you to read up (Javadoc or books) on TreeSet and HashSet and the importance of the equals, compareTo and hashCode methods. If you want to put your objects in a Set or a Map, you need to know about these to implement that correctly.

Note I based this compareTo method on your equals method. You were comparing the date-of-birth by first calling toString. That's not a very good way of doing that - you can use the equals method in java.util.Date directly. In a compareTo method the problem gets worse because dates do not sort correctly when you sort them alphabetically. java.util.Date also implements Comparable so you can replace that comparison in the method with:

    if (r == 0)
        r = this.dob.compareTo(temp.getDOB());

In addition, if any of the fields could be null, you need to check for that as well.