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);

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