java8-stream常用操作

摘要：交易员练习年的所有交易并按照金额由小到大排序交易员都在哪些不同的城市生活查找所有来自剑桥的交易员，并按姓名排序剑桥查询所有交易员的姓名字符串，并按字母排序有没有交易员在米兰米兰打印在剑桥生活的交易员的所有交易金额剑桥所有交易中，

import org.junit.Test;

import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;

import static java.util.Comparator.comparingInt;
import static java.util.stream.Collectors.*;

/**
 * Created by ibm on 2017/4/12.
 * java8交易员练习
 */
public class TraderExercise {

    //【1.2011年的所有交易并按照金额由小到大排序
    @Test
    public void exercise1(){
        transactions.stream()
                .filter(t -> t.getYear() == 2011)
                .sorted(Comparator.comparing(Transaction::getValue))
                .forEach(System.out :: println);
    }
    //【2.交易员都在哪些不同的城市生活
    @Test
    public void exercise2(){
        traders.stream()
                .map(Trader :: getCity)
                .distinct()
                .forEach(System.out :: println);
    }
    //【3.查找所有来自剑桥的交易员，并按姓名排序
    @Test
    public void exercise3(){
        traders.stream()
                .filter( t -> "剑桥".equals(t.getCity()))
                .sorted(Comparator.comparing(Trader :: getName))
                .forEach(System.out :: println);
    }
    //【4.查询所有交易员的姓名字符串，并按字母排序
    @Test
    public void exercise4(){
        traders.stream()
                .sorted(Comparator.comparing(Trader :: getName).reversed())
                .forEach(t -> System.out.println(t.getName()));
    }
    //【5.有没有交易员在米兰
    @Test
    public void exercise5(){
        traders.stream()
                .filter(t -> "米兰".equals(t.getCity()))
                .findAny()
                .ifPresent(System.out :: println);
    }
    //【6.打印在剑桥生活的交易员的所有交易金额
    @Test
    public void exercise6(){
        int sumValue = transactions.stream()
                .filter(tran -> "剑桥".equals(tran.getTrader().getCity()))
                .map(Transaction::getValue)
                .reduce(0,(value1 , value2) -> value1 + value2);
        System.out.println(sumValue);
    }
    //【7.所有交易中，最高的交易额是多少
    @Test
    public void exercise7(){
         transactions.stream()
                .sorted(Comparator.comparing(Transaction :: getValue).reversed())
                .findFirst()
                .ifPresent(System.out :: println);

         transactions.stream()
                .map(Transaction :: getValue)
                .reduce(Integer :: max)
                .ifPresent(System.out :: println);
    }
    //【8.找到交易额中最小的金额
    @Test
    public void exercise8(){
        transactions.stream()
                .map(Transaction :: getValue)
                .reduce(Integer :: min)
                .ifPresent(System.out :: println);

        transactions.stream()
                .min(Comparator.comparing(Transaction :: getValue))
                .ifPresent(System.out :: println);

        transactions.stream()
                .min(Comparator.comparing((Transaction t1) -> t1.getValue()))
                .ifPresent(System.out :: println);
    }
    //【9.统计每个交易员的记录
    @Test
    public void exercise9(){
        transactions.stream()
                .collect(groupingBy(Transaction :: getTrader))
                .entrySet().stream()
                .forEach(System.out :: println);
    }
    //【10.找到单笔交易最高的交易员
    @Test
    public void exercise(){
        transactions.stream()
                .max(Comparator.comparing(Transaction :: getValue))
                .ifPresent( tran -> {
                    System.out.println(tran.getTrader());
                } );
    }
    //【11.统计交易总额最高的交易员(排序)
    @Test
    public void exercise11(){
        transactions.stream()
                .collect(groupingBy(Transaction :: getTrader))
                .entrySet().stream()
                .map( t -> {
                    Map result = new HashMap<>();
                    int sum = t.getValue().stream().mapToInt(Transaction :: getValue).sum();
                    result.put("sum",sum);
                    result.put("trader",t.getKey());
                    return result;
                })
                .sorted(comparingInt((Map m) -> (int)m.get("sum")).reversed())
                .findFirst().ifPresent(System.out::println);
    }
    //【12.使用方法引用对transaction排序
    @Test
    public void exercise12(){
        transactions.stream()
                .sorted(Comparator.comparing(Transaction :: getValue))
                .forEach(System.out :: println);

        System.out.println("------------------------------------------");
        //声明接口的的实现方式
        Function function = Transaction :: getValue;
        Transaction[] transactionsArray = new Transaction[transactions.size()];
        Arrays.sort(transactions.toArray(transactionsArray),Comparator.comparing(function));
        Arrays.asList(transactionsArray).stream().forEach(System.out :: println);
    }
    //【13.根据trader(对象)将transaction分组
    @Test
    public void exercise13(){
        transactions.stream()
                .collect(groupingBy(Transaction :: getTrader))
                .entrySet().stream()
                .forEach(System.out :: println);
    }
    //【14.根据货币（字符串）类型分组
    @Test
    public void exercise14(){
        transactions.stream()
                .collect(groupingBy(Transaction :: getCurrency))
                .entrySet().stream()
                .forEach(System.out :: println);
    }
    //【15.获取交易总金额
    @Test
    public void exercise15(){
        int sum1 = transactions.stream().mapToInt(Transaction::getValue).sum();
        System.out.println("通过map转换求和sum1 = " + sum1);
        int sum2 = transactions.stream().collect(Collectors.summingInt(Transaction::getValue));
        System.out.println("通过collect汇总求和sum2 = " + sum2);
        //规约操作都需要使用map将对象映射为返回值的类型
        int sum3 = transactions.stream().map(Transaction::getValue).reduce(0,(t1,t2) -> t1 + t2);
        System.out.println("通过reduce规约求和sum3 = " + sum3);
    }
    //【16.二级分类，先按照交易员分类，然后交易金额大于800归为high，低于800归为low
    @Test
    public void exercise16(){
        transactions.stream()
                .collect(groupingBy(Transaction :: getTrader,groupingBy(t -> {
                    if(t.getValue()< 800 ){
                        return "low";
                    }else {
                        return "heigh";
                    }
                })))
                .entrySet().stream()
                .forEach(System.out :: println);
    }
    //【17.获取每个交易员，交易最大的一笔
    @Test
    public void exercise17(){
        transactions.stream()
                .collect(groupingBy(Transaction::getTrader,maxBy(Comparator.comparingInt(Transaction::getValue))))
                .entrySet().stream()
                .distinct()
                .sorted(Comparator.comparing((Map.Entry m) -> {
                    Trader t = (Trader) m.getKey();
                    return t.getName();
                }))
                .forEach(System.out :: println);
    }

    //===================================初始化数据================================
    List traders = new LinkedList<>();
    List transactions = new LinkedList<>();

    public TraderExercise(){
        Trader t1 = new Trader("trader1","剑桥");
        Trader t2 = new Trader("trader2","米兰");
        Trader t3 = new Trader("trader3","剑桥");
        Trader t4 = new Trader("trader4","剑桥");
        traders.addAll(Arrays.asList(t1,t2,t3,t4));

        Transaction tran1 = new Transaction(t4,2011,300,"$");
        Transaction tran2 = new Transaction(t1,2012,1000,"$");
        Transaction tran3 = new Transaction(t1,2011,400,"￥");
        Transaction tran4 = new Transaction(t2,2012,710,"￥");
        Transaction tran5 = new Transaction(t2,2012,700,"$");
        Transaction tran6 = new Transaction(t3,2012,950,"￥");
        transactions.addAll(Arrays.asList(tran1,tran2,tran3,tran4,tran5,tran6));
    }

    //交易员对象
    class Trader{

        private String name;
        private String city;

        public Trader(String name,String city){
            this.name = name;
            this.city = city;
        }

        public String getName(){
            return name;
        }
        public String getCity(){
            return city;
        }

        @Override
        public String toString(){
           return "i am trader : " + name + " ; i live in : " + city;
        }
    }
    //交易对象
    class Transaction{

        private Trader trader;
        private int year;
        private int value;
        private String currency;

        public Transaction(Trader trader,int year,int value,String currency){
            this.trader = trader;
            this.year = year;
            this.value = value;
            this.currency = currency;
        }

        public Trader getTrader(){
            return trader;
        }
        public int getYear(){
            return year;
        }
        public int getValue(){
            return value;
        }
        public String getCurrency(){
            return currency;
        }

        @Override
        public String toString(){
            return "i am transaction : my trader is : "
                    + trader.getName()
                    + " ; my year is : "
                    + year
                    + " ; my value is : "
                    + value
                    + " ; my currency is : "
                    + currency;
        }
    }

}