当compareto返回0时了解TreeSet [英] Understanding TreeSet when compareto returns 0

问题描述

我创建了一个像这样的学生班:

I have created a Student class like this:

public class Student implements Comparable<Student> {

    private String firstName;
    private String lastName;

    public Student(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    // Getters & Setters follow here...

    @Override
    public int compareTo(Student student) {
        int hash = this.firstName.compareTo(student.firstName);
        return hash;
    }

    @Override
    public String toString() {
        return "Student [firstName=" + firstName + ", lastName=" + lastName
                + "]";
    }

}

这是我的测试类，我只在其中将元素添加到TreeSet中:

This is my test class where I just add elements to my TreeSet:

public class SortedSetExample1 {
    public static void main(String[] args) {
        SortedSet<Student> set = new TreeSet<Student>();
        set.add(new Student("A1","A2"));
        set.add(new Student("B1","B2"));
        set.add(new Student("A1","B2"));
        set.add(new Student("A2","B2"));
        System.out.println(set);
    }
}

根据我的程序，输出为:

As per my program the output is:

[Student [firstName=A1, lastName=A2], Student [firstName=A2, lastName=B2], Student [firstName=B1, lastName=B2]]

在测试类中，我将Student对象添加到TreeSet，并且我也没有覆盖hashCode& equals方法.因此，我期望TreeSet将容纳所有4个对象，但我也可以看到它包含3个对象.您能解释一下为什么new Student("A1","B2")不属于我的TreeSet吗?

In my test class I am adding Student objects to TreeSet, and also I have not overridden the hashCode & equals methods. So I was expecting that the TreeSet will hold all the 4 objects but I can also see that it contains 3 objects. Can you please explain why new Student("A1","B2") is not part of my TreeSet?

也按照 TreeSet的Java文档在这里说:

将指定的元素添加到该集中(如果尚不存在). 更正式地说，将指定的元素e添加到该集合中(如果该集合存在的话) 不包含任何元素e2使​​得(e == null?e2 == null:e.equals(e2)). 如果此集合已经包含该元素，则调用将离开该集合 不变并返回false.

Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if the set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.

由于我还没有重写equals方法，那么为什么集合中没有全部四个元素呢?

As I have not overridden the equals method then why the collection is not having all the four elements?

推荐答案

作为 java.util.TreeSet 说:

TreeSet实例使用其compareTo(或compare)方法执行所有元素比较，因此从该集合的角度来看，此方法认为相等的两个元素相等.

a TreeSet instance performs all element comparisons using its compareTo (or compare) method, so two elements that are deemed equal by this method are, from the standpoint of the set, equal

对 @Jon Skeet 表示敬意.

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