List集合操作

一.将List集合按照指定size分为若干小list

//基本实现
public static void partition(List<Long> list){
        int size = 3;
        List<List<Long>> listArr = new ArrayList<>();
        int arrSize = list.size() % size == 0? list.size()/size : list.size()/size + 1;
        for (int i = 0; i < arrSize; i++) {
            List<Long> sub = new ArrayList<>();
            for (int j = i * size; j <= size * (i + 1); j++) {
                if (j <= list.size() -1) {
                    sub.add(list.get(i));
                }
            }
            listArr.add(sub);
        }
    }

    //java8
    public static void givenList(){
        List<Integer> intList = Lists.newArrayList(1,2,3,4,5,6,7,8,9);
        Map<Integer, List<Integer>> groups = intList.stream().collect(Collectors.groupingBy(s -> (s - 1)/3));
        List<List<Integer>> subSets = new ArrayList<>(groups.values());
    }

//guava实现
public static void partition2(List<Long> list){
    int size = 16;
    List<List<Long>> subSets = Lists.partition(list, size);
}

二、 实现java 中 list集合中有几十万条数据,每100条为一组取出

/**
     * 实现java中 List集合中有几十万数据，每100条为一组取出
     * @param list
     * @return 每一个key中有100条数据的List
     */
    public Map<String, List> groupList(List list){
        int listSize = list.size();
        int toIndex = 100;
        Map<String, List> map = new HashMap<>(5000);
        int keyToken = 0;
        for (int i = 0; i < list.size(); i++) {
            /**作为toIndex最后没有100条数据则剩余几条newList中就装几条**/
            if (i+100 > listSize){
                toIndex = listSize - i;
            }
            List newList = list.subList(i, i + toIndex);
            map.put("keyName"+keyToken, newList);
            keyToken++;
        }
        return map;
    } 

三、List集合取出某一属性

package com.thai.order;

import org.assertj.core.util.Lists;

import java.util.List;
import java.util.stream.Collectors;

/**
 * @author chenmuchao
 * @date 2018-04-19 10:06
 */
public class Order {

    private int id;

    private String name;

    public Order(int id, String name) {
        this.id = id;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

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


    public void convert(){

        List<Order> lists = Lists.newArrayList();
        Order order = new Order(1, "订单1");
        lists.add(order);
        Order order1 = new Order(2, "订单2");
        lists.add(order1);
        Order order2 = new Order(2, "订单3");
        lists.add(order2);

        List ids = lists.stream().map(Order::getId).collect(Collectors.toList());
        System.out.println("ids:" + ids);

        List names = lists.stream().map(Order :: getName).collect(Collectors.toList());
        System.out.println("names:" + names);
    }
}

四、根据条件在List中筛选出符合条件的对象

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Test {
    
    public static void main(String[] args) throws Exception {
        Map<Integer,List<People>> map=new HashMap<Integer,List<People>>();
        People p1=new People(1,"2");
        addPersonToMap(map,p1);
        People p2=new People(3,"4");
        addPersonToMap(map,p2);
        People p3=new People(1,"2");
        addPersonToMap(map,p3);
        System.out.println(map.get(1));
    }
    private static boolean addPersonToMap(Map<Integer,List<People>> map,People p1){
        boolean flag;
        int key=p1.getAge();
        if(map.containsKey(key)){
            List<People> value=map.get(1);
            value.add(p1);
            map.put(key, value);
            flag=true;
        }else{
            List<People> value=new ArrayList<People>();
            value.add(p1);
            map.put(key, value);
            flag=false;
        }
        return flag;
    }
}
class People{
    int age;
    String name;
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public People() {
        super();
    }
    public People(int age, String name) {
        super();
        this.age = age;
        this.name = name;
    }
    
}