Set集合

HashSet （哈希表）唯一，无序；添加、删除、查询速度快
LinkedHashSet （哈希表+链表）唯一，有序（添加顺序）；比哈希表慢点
TreeSet 红黑树（一种二叉平衡树）唯一，有序（自然顺序）；查询速度(按内容查)比List快，没HashSet 哈希表快
List针对Collection增加了一些对索引的操作方法，如add(1),remove(1),get(1),set(1,"A")
Set是无序的，不能增加对索引操纵的方法，Set针对Collection没有增加任何方法
List的遍历方式：for循环，for-each循环，Iterator迭代器，流式编程forEach
Set的遍历方式：for-each循环，Iterator迭代器，流式编程forEach

import java.util.Iterator;
import java.util.Set;
import java.util.TreeSet;

public class SetDemo01 {
    public static void main(String[] args) {
        Set<String> set = new TreeSet();
        set.add("JavaSE");
        set.add("JavaEE");
        set.add("MySql");
        set.add("Html");
        set.add("JavaSE");
        System.out.println(set);//[Html, JavaEE, JavaSE, MySql]  唯一，自然顺序
        System.out.println(set.size());// 4
        System.out.println("=============遍历方法一:for-each=============");
        for (String elem : set) {
            System.out.println(elem);
        }
        System.out.println("=============遍历方法二:迭代=============");
        Iterator<String> iterator = set.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }
        System.out.println("=============遍历方法三:拉姆达表达式+流式编程=============");
        set.forEach(item -> System.out.println(item));
    }
}

执行结果：
[Html, JavaEE, JavaSE, MySql]
4
=============遍历方法一:for-each=============
Html
JavaEE
JavaSE
MySql
=============遍历方法二:迭代=============
Html
JavaEE
JavaSE
MySql
=============遍历方法三:拉姆达表达式+流式编程=============
Html
JavaEE
JavaSE
MySql

Process finished with exit code 0

1.创建一个学生类存到Set中

1.1先创建Student类

package com.zhang.setdemo;

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 +
                '}';
    }
}

1.2.创建测试类

package com.zhang.setdemo;

import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.Set;
import java.util.TreeSet;


public class SetDemo01 {
    public static void main(String[] args) {
        Set<Student> set = new HashSet();
        Student stu1 = new Student("zhangsan", 20);
        Student stu2 = new Student("lisi", 23);
        Student stu3 = new Student("wangwu", 23);
        Student stu4 = new Student("zhangsan", 20);
        set.add(stu1);
        set.add(stu2);
        set.add(stu3);
        set.add(stu4);
        System.out.println(set);
        //[Student{name='wangwu', age=23}, Student{name='lisi', age=23}, Student{name='zhangsan', age=20}, Student{name='zhangsan', age=20}]
        Set<Student> set2 = new LinkedHashSet();
        Student stu11 = new Student("zhangsan", 20);
        Student stu22 = new Student("lisi", 23);
        Student stu33 = new Student("wangwu", 23);
        Student stu44 = new Student("zhangsan", 20);
        set2.add(stu11);
        set2.add(stu22);
        set2.add(stu33);
        set2.add(stu44);
        System.out.println(set2);
        //[Student{name='zhangsan', age=20}, Student{name='lisi', age=23}, Student{name='wangwu', age=23}, Student{name='zhangsan', age=20}]
        Set<Student> set3 = new TreeSet();
        Student stu111 = new Student("zhangsan", 20);
        Student stu222 = new Student("lisi", 23);
        Student stu333 = new Student("wangwu", 23);
        Student stu444 = new Student("zhangsan", 20);
        set3.add(stu111);
        set3.add(stu222);
        set3.add(stu333);
        set3.add(stu444);
        System.out.println(set3);//java.lang.ClassCastException: Student cannot be cast to java.lang.Comparable
    }
}

运行发现

问题1.HashSet和LinkedHashSet存储String是唯一的，但是存储Student不唯一
问题2.TreeSet存储String是有序的，但是存储Student报异常：java.lang.ClassCastException: Student cannot be cast to java.lang.Comparable

解决方法：

问题1解决方法

1.在Student类重写equals和hashCode，缺一不可

 @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);
    }

再次运行发现：HashSet和LinkedHashSet的结果唯一。
HashSet的结果：[Student{name='lisi', age=23}, Student{name='zhangsan', age=20}, Student{name='wangwu', age=23}]
    
LinkedHashSet的结果：[Student{name='zhangsan', age=20}, Student{name='lisi', age=23}, Student{name='wangwu', age=23}]

问题2解决方法

方法1：Student类实现Comparable接口,重写compareTo方法

public class Student implements Comparable<Student> {
    @Override
    public int compareTo(Student other) {
        return this.name.compareTo(other.name);
    }
}

运行结果：
HashSet的结果：
	[Student{name='lisi', age=23}, Student{name='zhangsan', age=20}, Student{name='wangwu', age=23}]
    
LinkedHashSet的结果：
	[Student{name='zhangsan', age=20}, Student{name='lisi', age=23}, Student{name='wangwu', age=23}]
    
TreeSet的结果：
	[Student{name='lisi', age=23}, Student{name='wangwu', age=23}, Student{name='zhangsan', age=20}]

Process finished with exit code 0

方法2：自定义外部比较器

有自定义的外部比较器以外部比较器为准，没有外部比较器的走内部比较器

按年龄升序，姓名降序排列

方法三：匿名内部类

package com.zhang.setdemo;

import java.util.Comparator;
import java.util.Set;
import java.util.TreeSet;

public class SetDemo03 {
    public static void main(String[] args) {
        //匿名内部类，年龄升序，姓名升序
        Comparator com = new Comparator<Student>() {
            @Override
            public int compare(Student stu1, Student stu2) {
                if (stu1.getAge() < stu2.getAge()) {
                    return -1;
                } else if (stu1.getAge() > stu2.getAge()) {
                    return 1;
                } else {
                    return stu1.getName().compareTo(stu2.getName());
                }
            }
        };
        Set<Student> set3 = new TreeSet(com);
        Student stu111 = new Student("zhangsan", 20);
        Student stu222 = new Student("lisi", 23);
        Student stu333 = new Student("wangwu", 23);
        Student stu444 = new Student("zhangsan", 20);
        set3.add(stu111);
        set3.add(stu222);
        set3.add(stu333);
        set3.add(stu444);
        System.out.println(set3);
    }
}

执行结果：
[Student{name='zhangsan', age=20}, Student{name='lisi', age=23}, Student{name='wangwu', age=23}]

Process finished with exit code 0

方法四：拉姆达表达式

package com.zhang.setdemo;

import java.util.Set;
import java.util.TreeSet;

public class SetDemo04 {
    public static void main(String[] args) {
        //拉姆达表达式
        Set<Student> set3 = new TreeSet<Student>((Student stu1, Student stu2) -> {
            if (stu1.getAge() < stu2.getAge()) {
                return -1;
            } else if (stu1.getAge() > stu2.getAge()) {
                return 1;
            } else {
                return stu1.getName().compareTo(stu2.getName());
            }
        });
        Student stu111 = new Student("zhangsan", 20);
        Student stu222 = new Student("lisi", 23);
        Student stu333 = new Student("wangwu", 23);
        Student stu444 = new Student("zhangsan", 20);
        set3.add(stu111);
        set3.add(stu222);
        set3.add(stu333);
        set3.add(stu444);
        System.out.println(set3);
    }
}

方法五：拉姆达表达式一条语句的用法

package com.zhang.setdemo;

import java.util.Set;
import java.util.TreeSet;

public class SetDemo05 {
    public static void main(String[] args) {
        //拉姆达表达式
        Set<Student> set3 = new TreeSet<Student>((stu1, stu2) -> stu1.getName().compareTo(stu2.getName()));
        Student stu111 = new Student("zhangsan", 20);
        Student stu222 = new Student("lisi", 23);
        Student stu333 = new Student("wangwu", 23);
        Student stu444 = new Student("zhangsan", 20);
        set3.add(stu111);
        set3.add(stu222);
        set3.add(stu333);
        set3.add(stu444);
        System.out.println(set3);
    }
}

执行结果：
[Student{name='lisi', age=23}, Student{name='wangwu', age=23}, Student{name='zhangsan', age=20}]

Process finished with exit code 0

posted @ 2021-07-24 17:47 wlbsm 阅读(43) 评论(0) 编辑收藏举报

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