Java集合框架之Collections

Posted 2020-12-19 陈彦斌

Collections工具类

• Java里关于聚合的工具类，包含有各种有关集合操作的静态多态方法，不能实例化(把构造函数私有化)

public class Collections {
    // Suppresses default constructor, ensuring non-instantiability.
    private Collections() {
    }
}

和Collection的区别

• Collection是接口，提供了对集合对象进行基本操作的通用接口方法，List、Set等多种具体的实现类
• Collection是工具类，专门从操作Collection接口实现类里面的元素

常见方法

排序

• Sort(List list)

按自然排序的升序排序

    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("aaaa");
        list.add("zzz");
        list.add("gggg");
        System.out.println(list);
        Collections.sort(list);
        System.out.println(list);
    }

自定义排序规则

• Sort(List list,Comparator c)，由Comparator控制排序逻辑

    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
         list.add("aaaa");
         list.add("zzz");
         list.add("gggg");
         System.out.println(list);
         //默认升序
         Collections.sort(list, Comparator.naturalOrder());
         System.out.println(list);
         //降序
         Collections.sort(list, Comparator.reverseOrder());
         System.out.println(list);
    }

随机排序

• shuffle(List list)

    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("1");
        list.add("2");
        list.add("3");
        list.add("4");
        list.add("5");
        list.add("6");
        list.add("7");
        list.add("8");
        list.add("9");
        list.add("10");
        list.add("J");
        list.add("Q");
        list.add("K");
        System.out.println(list);
        Collections.shuffle(list);
        System.out.println(list);
    }

获取最值元素

max(Collection coll)

　　默认比较，不适合对象比较

max(Collection coll,Comparator comparator)

Student.java

package com.cyb.test;

class Student {
    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    private int age;
    private String name;

    public void setAge(int age) {
        this.age = age;
    }

    public int getAge() {
        return age;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "Student{" + "age=" + age + ", name=‘" + name + ‘‘‘ + ‘}‘;
    }
}

Test03.java

package com.cyb.test;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class test03 {
    public static void main(String[] args) {
        List<Student> list = new ArrayList<>();
        list.add(new Student("jack", 26));
        list.add(new Student("tom", 29));
        list.add(new Student("mary", 32));
        list.add(new Student("tony", 19));
        list.add(new Student("smith", 41));
        System.out.println(list);
        Student maxAgeStudent = Collections.max(list, new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o1.getAge() - o2.getAge();
            }
        });
        System.out.println("maxAgeStudent = "+maxAgeStudent.toString());
        Student mixAgeStudent = Collections.min(list, new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o1.getAge() - o2.getAge();
            }
        });
        System.out.println("mixAgeStudent = "+mixAgeStudent.toString());
    }
}

创建不可变集合unmodifiablleXXX()

List<String> list = new ArrayList<>();
 list.add("SpringBoot课程");
 list.add("架构课程");
 list.add("微服务SpringCloud课程"); //设置为只读List集合
 list = Collections.unmodifiableList(list);
 System.out.println(list);
 Set<String> set = new HashSet<>();
 set.add("mysql教程");
 set.add("Linux服务器器教程");
 set.add("Git教程");
 //设置为只读Set集合
 set = Collections.unmodifiableSet(set);
 System.out.println(set);
 Map<String, String> map = new HashMap<>();
 map.put("key1", "课程1");
 map.put("key2", "课程2");
 //设置为只读Map集合
 map = Collections.unmodifiableMap(map);
 System.out.println(map);