Java集合框架Collections原理及用法实例

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);

来源：https://www.cnblogs.com/chenyanbin/p/13401255.html