java当only equals被重写时,为什么列表会将项查找为true?
我知道hashcode和equals都需要重写。我尝试了下面的程序,想知道为什么列表能够搜索学生,而set不能搜索学生,因为只有equals被覆盖:
import java.util.*;
class Student {
private int id;
private String name;
public Student(int id, String name) {
this.name = name;
this.id = id;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public boolean equals(Object obj) {
if (obj == null) return false;
if (!(obj instanceof Student))
return false;
if (obj == this)
return true;
return this.getId() == ((Student) obj).getId();
}
}
public class StudentHashcodeEquals {
public static void main(String[] args) {
Student alex1 = new Student(1, "Alex");
Student alex2 = new Student(1, "Alex");
System.out.println("alex1 hashcode = " + alex1.hashCode());
System.out.println("alex2 hashcode = " + alex2.hashCode());
System.out.println("Checking equality between alex1 and alex2 = " + alex1.equals(alex2));
List<Student> studentsLst = new ArrayList <Student>();
studentsLst.add(alex1);
studentsLst.add(alex2);
System.out.println("Arraylist size = " + studentsLst.size());
System.out.println("Arraylist contains Alex = " + studentsLst.contains(new Student(1, "Alex")));
HashSet <Student> students = new HashSet <Student>();
students.add(alex1);
students.add(alex2);
System.out.println("HashSet size = " + students.size());
System.out.println("HashSet contains Alex = " + students.contains(new Student(1, "Alex")));
}
}
/*
alex1 hashcode = 366712642
alex2 hashcode = 1829164700
Checking equality between alex1 and alex2 = true
Arraylist size = 2
Arraylist contains Alex = true
HashSet size = 2
HashSet contains Alex = false
*/
当student被插入列表时,我假设它会将它插入两个不同的bucket中,因为hashcode实现不是prsent,默认对象hashcode会被插入,它会将它们存储到两个不同的hash Code中,当搜索另一个对象时,它必须首先通过哈希代码定位bucket,但哈希代码实现不存在,然后列表如何仍然能够识别它,而set却不能
谢谢
# 1 楼答案
列表不使用对象的哈希代码。根据其类型,它们存储对对象的引用数组
如果在列表中调用
contains
,它将使用.equals()
在列表中迭代,以检查对象是否在列表中