HashSet 的唯一性

HashSet 底層

HashSet 的特性是唯一，底層用HashMap的key來達到唯一性，唯一性跟哈希值和equals方法

@EqualsAndHashCode

lombok.EqualsAndHashCode註解@EqualsAndHashCode，重寫了hashcode和equals方法，讓值一樣的對象哈希值一樣，讓equals比較值，而不是地址值

Objects的equals底層是==，比較的是地址值

例子

Person類

@Builder
@EqualsAndHashCode
public class Person {
    private String name;
    private int age;
}

Student類

@Builder
public class Student {
    private String name;
    private int age;
}

運行結果如下

要想得到值唯一的對象集合，可以用@EqualsAndHashCode+HashSet

public class HashSetDemo {
    public static void main(String[] args) {
        Set<Person> personSet = new HashSet<>();
        Set<Student> studentSet = new HashSet<>();

        Person p1 = Person.builder().age(10).name("胡英文").build();
        Person p2 = Person.builder().age(10).name("胡英文").build();
        Person p3 = Person.builder().age(12).name("胡英文").build();

        Student s1 = Student.builder().age(10).name("胡英文").build();
        Student s2 = Student.builder().age(10).name("胡英文").build();

        personSet.add(p1);
        personSet.add(p2);
        studentSet.add(s1);
        studentSet.add(s2);
        System.out.println(s1.hashCode());//931919113
        System.out.println(s2.hashCode());//1607521710

        System.out.println(p1.hashCode());//1933516976
        System.out.println(p2.hashCode());//1933516976
        System.out.println(p3.hashCode());//1933516978

        System.out.println(personSet);//[com.hyg.basedemo2.hashset.Person@733f20b0]
        System.out.println(studentSet);//[com.hyg.basedemo2.hashset.Student@378bf509, com.hyg.basedemo2.hashset.Student@5fd0d5ae]
        System.out.println(p1.equals(p2));//true
        System.out.println(p1==p2);//false
        System.out.println(s1==s2);//false
        System.out.println(s1.equals(s2));//false
    }
}