Java Set集合

1、Set集合的特点

一个不包含重复元素的collection

2、HashSet是如何保证元素唯一性的？

HashSet 底层数据结构是哈希表.，HashSet 不是线程安全的，集合元素可以是 null，其中哈希表是一个元素加数组的数据结构

基本过程可简述为当向 HashSet 集合中存入一个元素时，HashSet 会调用该对象的 hashCode() 方法来得到该对象的 hashCode 值， 然后根据 hashCode 值决定该对象在 HashSet 中的存储位置。

HashSet 保证元素唯一性是靠元素重写hashCode()和equals()方法来保证的，如果不重写则无法保证。

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    Student student = (Student) o;
    return age == student.age &&
            Objects.equals(name, student.name);
}

@Override
public int hashCode() {

    return Objects.hash(name, age);
}

3、TreeSet是如何保证元素唯一性和排序的？

基本过程可简述为先存入一个树根，分两个叉，存储元素时跟树根比较，小的放在左边，大的放在右边，如果相等就不存储，取的时候按照“左中右”的顺序来取

保证元素的唯一性是靠compareTo方法的返回值来确定，如果返回0，表示两个元素相等，则不重复存储

@Override
public int compareTo(Student o) {
    int num = this.name.length() - o.name.length();
    int num2 = num == 0 ? this.name.compareTo(o.name) : num;
    int num3 = num2 == 0 ? this.age - o.age : num;
    return num3; 
}

4、代码实例

A:请编写程序，存储自定义对象到HashSet集合中，并遍历

package org.zhang15.demo;
import java.util.Objects;
/**
 * @Author: goodboy
 * @CreateTime: 2019-01-12 21:40
 */
public class Student {
    private String name;
    private int age;
    public Student() {
    }
    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return age == student.age &&
                Objects.equals(name, student.name);
    }
    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }
}
package org.zhang15.demo;
import java.util.HashSet;
/**
 * @Author: goodboy
 * @CreateTime: 2019-01-12 21:40
 */
public class Demo1 {
    public static void main(String[] args) {
        HashSet<Student> students = new HashSet<>();
        students.add(new Student("小张",12));
        students.add(new Student("小王", 13));
        students.add(new Student("小李", 14));
        students.add(new Student("小王", 13));
        students.add(new Student("小赵", 15));
        for (Student student : students) {
            System.out.println(student.toString());
        }
    }
}

B:请编写程序，存储自定义对象到TreeSet集合中，并遍历

package org.zhang15.demo;
import java.util.Objects;
/**
 * @Author: goodboy
 * @CreateTime: 2019-01-12 21:40
 */
//采用自然排序方式
public class Student implements Comparable<Student> {
    private String name;
    private int age;
    public Student() {
    }
    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
    @Override
    public int compareTo(Student o) {
        int sum1 = this.age-o.age;
        int sum2 = sum1==0?this.name.compareTo(o.name):sum1;
        return sum2;
    }
}
package org.zhang15.demo;
import java.util.TreeSet;

/**
 * @Author: goodboy
 * @CreateTime: 2019-01-12 21:57
 */
public class Demo2 {
    public static void main(String[] args) {
        TreeSet<Student> students = new TreeSet<>();
        students.add(new Student("小明",12));
        students.add(new Student("小宋", 14));
        students.add(new Student("小张", 11));
        students.add(new Student("小明", 15));
        students.add(new Student("小明", 12));
        for (Student student : students) {
            System.out.println(student.toString());
        }
    }
}