Java集获取重复条目 [英] Java Set gets duplicate entry
问题描述
JavaDoc将集合定义为:
JavaDoc defines set as :
一个不包含重复元素的集合。更正式地说,集合
不包含元素对e1和e2,这样e1.equals(e2)
A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2)
同样,我创建了一个非常简单的程序:
To verify the same, i created a very simple program:
import java.util.HashSet;
public class CheckHashSet {
public static void main(String[] args) {
HashSet<Employee> set = new HashSet<Employee>();
set.add(new Employee(10));
set.add(new Employee(10));
System.out.println(set.size());
System.out.println(new Employee(10).equals(new Employee(10)));
}
private static class Employee implements Comparable<Employee> {
private final int id;
public Employee(int id) {
this.id = id;
}
@Override
public int compareTo(Employee o) {
return this.id - o.id;
}
@Override
public boolean equals(Object obj) {
if(obj instanceof Employee) {
return compareTo((Employee)obj)==0;
}
return false;
}
}
}
程序的输出为
2
true
这意味着新员工(10).equals(新员工(10))
返回true,而 set.add (新员工(10)); set.add(new Employee(10));
将对象添加两次。
This means new Employee(10).equals(new Employee(10))
returns true whereas set.add(new Employee(10)); set.add(new Employee(10));
adds the object twice.
我的代码有什么问题?
推荐答案
您的 Employee
类不会覆盖 hashCode
-它需要这样做才能使任何基于哈希的集合正常工作。
Your Employee
class doesn't override hashCode
- it needs to do so in order for any hash-based collection to work.
例如:
@Override
public int hashCode() {
return id;
}
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