Java中的Collections

1、Collections类及常用API

(1)Collections类集工具类，定义了若干用于类集和映射(Map)的算法，这些算法被定义为静态方法。

(2)常用方法：public static void sort(List list);public static int binarySearch(List list,T key);public static void reverse(List<?> list);public static void shuffle(List<?> list);public static vodi swap(List<?> list,int i,int j);public static void fill(List<? super T> list,T obj);

(3)举例

package com.iotech.set;

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

public class CollectionsDemo1 {

    public static void main(String[] args) {
        List<String> nList=new ArrayList<String>();
        nList.add("chenhao");
        nList.add("zhangsan");
        nList.add("lisi");
        nList.add("wangwu");
        nList.add("rose");
        nList.add("mary");
        System.out.println("操作前");
        for(String s:nList){
            System.out.print(s+" ");
        }

        System.out.println();
        System.out.println("交换顺序后");
        Collections.swap(nList, 1, 2);
        for(String s:nList){
            System.out.print(s+" ");
        }

        System.out.println();
        System.out.println("自然排序后");
        Collections.sort(nList);
        for(String s:nList){
            System.out.print(s+" ");
        }

        System.out.println();
        System.out.println("打乱顺序后");
        Collections.shuffle(nList);
        for(String s:nList){
            System.out.print(s+" ");
        }

        System.out.println();
        System.out.println("填充后");
        Collections.fill(nList, "chenhao");
        for(String s:nList){
            System.out.print(s+" ");
        }
    }
}

2、对ArrayList容器中的内容进行排序

(1)案例介绍

ArrayList中存储了多个person对象，(此对象包含，名字，年龄，ID)要求按年龄从小到大排序，年龄相等的话再按名字的大小来排序输出。

(2)案例设计

使用ArrayList来存储Person对象，使用Collections类所提供的静态sort方法来按要求对ArrayList进行排序，最终输出排好序的结果信息。(这里需要重写Comparator方法。)

(3)代码实现

package com.iotech.set;

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

public class CollectionsDemo2 {

    public static void main(String[] args) {
        ArrayList<Person> nList=new ArrayList<Person>();
        nList.add(new Person("zhangsan", 17, 0));
        nList.add(new Person("lisi", 18, 1));
        nList.add(new Person("xiaohong", 19, 2));
        nList.add(new Person("chenhao", 20, 3));
        nList.add(new Person("mary", 17, 4));

        Collections.sort(nList,new Comparator<Person>(){//匿名内部类

            @Override
            public int compare(Person o1, Person o2) {
                int x=o1.getAge()-o2.getAge();
                if(x>0) return 1;
                else if(x<0) return -1;
                else{
                    return o1.getNameString().compareTo(o2.getNameString());
                }
            }
        });

        for(Person person:nList){
            System.out.println(person.toString());
        }
    }
}

class Person{
     private String nameString;
     private int age;
     private int id;

    public Person(String nameString, int age, int id) {
        super();
        this.nameString = nameString;
        this.age = age;
        this.id = id;
    }
    public String getNameString() {
        return nameString;
    }
    public void setNameString(String nameString) {
        this.nameString = nameString;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    @Override
    public String toString() {
        return "Person [nameString=" + nameString + ", age=" + age + ", id=" + id + "]";
    }
}