概述

java.utils.Collections:是集合工具类

作用：Collections并不属于集合，是用来操作集合的工具类。

Collections常用的API 

Collections排序相关API

 使用范围：只能对于List集合的排序。

排序方式1：

 注意：本方式不可以直接对自定义类型的List集合排序，除非自定义类型实现了比较规则Comparable接口。

排序方式2：

public class CollectionsDemo01 {public static void main(String[] args) {List<String> names = new ArrayList<>();//names.add("楚留香");//names.add("胡铁花");//names.add("张无忌");//names.add("陆小凤");Collections.addAll(names, "楚留香","胡铁花", "张无忌","陆小凤");System.out.println(names);// 2、public static void shuffle(List<?> list) :打乱集合顺序。Collections.shuffle(names);System.out.println(names);// 3、 public static <T> void sort(List<T> list):将集合中元素按照默认规则排序。 （排值特性的元素）List<Integer> list = new ArrayList<>();Collections.addAll(list, 12, 23, 2, 4);System.out.println(list);Collections.sort(list);System.out.println(list);}
}

 

public class CollectionsDemo02 {public static void main(String[] args) {List<Apple> apples = new ArrayList<>(); // 可以重复！apples.add(new Apple("红富士", "红色", 9.9, 500));apples.add(new Apple("青苹果", "绿色", 15.9, 300));apples.add(new Apple("绿苹果", "青色", 29.9, 400));apples.add(new Apple("黄苹果", "黄色", 9.8, 500));//        Collections.sort(apples); // 方法一：可以的，Apple类已经重写了比较规则
//        System.out.println(apples);// 方式二：sort方法自带比较器对象
//        Collections.sort(apples, new Comparator<Apple>() {
//            @Override
//            public int compare(Apple o1, Apple o2) {
//                return Double.compare(o1.getPrice() , o2.getPrice()); // 按照价格排序！！
//            }
//        });Collections.sort(apples, ( o1,  o2) ->  Double.compare(o1.getPrice() , o2.getPrice()) );System.out.println(apples);}
}

public class Apple implements Comparable<Apple>{private String name;private String color;private double price;private int weight;public Apple() {}public Apple(String name, String color, double price, int weight) {this.name = name;this.color = color;this.price = price;this.weight = weight;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getColor() {return color;}public void setColor(String color) {this.color = color;}public double getPrice() {return price;}public void setPrice(double price) {this.price = price;}public int getWeight() {return weight;}public void setWeight(int weight) {this.weight = weight;}@Overridepublic String toString() {return "Apple{" +"name='" + name + '\'' +", color='" + color + '\'' +", price=" + price +", weight=" + weight +'}';}/**方式一：类自定义比较规则o1.compareTo(o2)* @param o* @return*/@Overridepublic int compareTo(Apple o) {// 按照重量进行比较的return this.weight - o.weight ; // List集存储相同大小的元素 会保留！}
}