TreeSet示例

Question

为什么第三个对象没有被添加到树集中,虽然它是一个不同的？

import java.util.*;

class Student implements Comparable<Student>{
public String fn,ln;
public Student(String fn,String ln){
    this.fn=fn;
    this.ln=ln;
}

//overiding equals

public boolean equals(Object o) {
    if (!(o instanceof Student))
        return false;
    Student s=(Student) o;
    if(this==s)
        return true;
    if(this.fn.equals(s.fn) && this.ln.equals(s.ln))
        return true;
    return false;
}

//overiding hashcode

public int hashCode() {
    return fn.hashCode()+ln.hashCode();
}


//overiding compareTo

public int compareTo(Student o) {

    return this.fn.compareTo(o.fn);
}
  }

public class Practice {


public static void main(String[] args) {
    Student st1=new Student("Girish","J");
    Student st2=new Student("Master","M");
    Student st3=new Student("Girish","Jay");
    Set S=new TreeSet();

           //adding 3 different student objects

    System.out.println(S.add(st1));
    System.out.println(S.add(st2));
    System.out.println(S.add(st3));
    Iterator sitr=S.iterator();
    while(sitr.hasNext())
    {
        Student stu=(Student) sitr.next();
        System.out.println(stu.fn+" "+stu.ln);
    }


}

 }

输出:

true
true
false
Girish J
Master M

Answer 1

您的比较器功能仅使用fn:

public int compareTo(Student o) {
    return this.fn.compareTo(o.fn);
}

TreeSet 仅使用排序比较 - 它不使用hashCode()和equals().

通过这种比较,st1并且st3相等(s1.compareTo(s3)将返回0)因此st3不会被添加到集合中.

如果你想保持区别,你应该比较fn然后使用,ln如果fn值是相同的:

public int compareTo(Student o) {
    int fnResult = this.fn.compareTo(o.fn);
    return fnResult == 0 ? ln.compareTo(o.ln) : fnResult;
}