Java8-04-04-笔记

Posted 2022-12-04 寻7

Java8-04-04-笔记

• Stream API练习
• • 1，小试牛刀（@—@）
  • 2，交易员练习

Stream API练习

• 使用之前学到的内容：创建Stream、Stream中间操作、Stream终止操作(终端操作)，进行一个简单的练习

1，小试牛刀（@—@）

package com.sunstone.stream;

import com.sunstone.lambda.Employee;
import org.junit.Test;

import java.util.Arrays;
import java.util.List;
import java.util.Optional;

public class TestStreamAPI 
    //1,给定一个数字列表，输出每个数的平方数列表[1,2,3,4,5]->[1,4,9,16,25]
    @Test
    public void test1()
        Integer[] nums=new Integer[]1,2,3,4,5;
        Arrays.stream(nums).map((x)->x*x).forEach(System.out::println);
    

    //2,使用map和reduce方法数一数流中有多少个元素
    @Test
    public void test2()
        List<Employee> emps= Arrays.asList(
                new Employee("sun",17,9999),
                new Employee("stone",27,99999)
        );
        Optional<Integer> count = emps.stream().map((e)->1).reduce(Integer::sum);
        //Integer count =emps.stream().count();使用该种方式当然是可以的
        System.out.println(count.get());
    

2，交易员练习

package com.sunstone.stream;

//交易员
public class Trader 

    private String name;
    private String city;

    public Trader() 
    

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

    public String getName() 
        return name;
    

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

    public String getCity() 
        return city;
    

    public void setCity(String city) 
        this.city = city;
    

    @Override
    public String toString() 
        return "Trader" +
                "name='" + name + '\\'' +
                ", city='" + city + '\\'' +
                '';
    

package com.sunstone.stream;

//交易事件
public class Transaction 

    private Trader trader;
    private Integer year;
    private Integer value;

    public Transaction() 
    

    public Transaction(Trader trader, Integer year, Integer value) 
        this.trader = trader;
        this.year = year;
        this.value = value;
    

    public Trader getTrader() 
        return trader;
    

    public void setTrader(Trader trader) 
        this.trader = trader;
    

    public Integer getYear() 
        return year;
    

    public void setYear(Integer year) 
        this.year = year;
    

    public Integer getValue() 
        return value;
    

    public void setValue(Integer value) 
        this.value = value;
    

    @Override
    public String toString() 
        return "Transaction" +
                "trader=" + trader +
                ", year=" + year +
                ", value=" + value +
                '';
    

package com.sunstone.stream;

import org.junit.Before;
import org.junit.Test;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Optional;

public class TestTransaction 

    List<Transaction> transactions = new ArrayList<>();

    @Before
    public void before() 
        Trader raoul = new Trader("Raoul", "Cambridge");
        Trader mario = new Trader("Mario", "Milan");
        Trader alan = new Trader("Alan", "Cambridge");
        Trader brian = new Trader("Brian", "Cambridge");
        transactions = Arrays.asList(
                new Transaction(brian, 2009, 777),
                new Transaction(brian, 2013, 10000),
                new Transaction(raoul, 2016, 7777),
                new Transaction(mario, 2017, 789),
                new Transaction(raoul, 2017, 999),
                new Transaction(alan, 2020, 520)
        );
    

    @Test
    public void test1() //1,找出2017年发生的所有交易，并按交易额排序（从低到高）
        transactions.stream().filter((t) -> t.getYear() == 2017)
                .sorted((t1, t2) -> Integer.compare(t1.getValue(), t2.getValue()))
                .forEach(System.out::println);
    

    @Test//2,交易员都在哪些不同的城市工作过
    public void test2() 
        transactions.stream().map((t) -> t.getTrader().getCity())
                .distinct().forEach(System.out::println);
    

    @Test//3,在所有的交易信息中，查找来自剑桥的交易员，并按姓名排序
    public void test3() 
        transactions.stream().filter((t) -> t.getTrader().getCity().equals("Cambridge"))
                .map(Transaction::getTrader)
                .sorted((t1, t2) -> t1.getName().compareTo(t2.getName()))
                .forEach(System.out::println);
    

    @Test//4,返回所有交易员的姓名字符串，按字母顺序排序
    public void test4() 
        transactions.stream().map((t) -> t.getTrader().getName()).sorted().forEach(System.out::println);
        transactions.stream().map((t) -> t.getTrader().getName()).sorted().reduce("", String::concat);
    

    @Test//5,有没有交易员在米兰工作
    public void test5() 
        boolean bl1 = transactions.stream().map((t) -> t.getTrader().getCity()).anyMatch((n) -> n.equals("Milan"));
        boolean bl2 = transactions.stream().anyMatch((t) -> t.getTrader().getCity().equals("Milan"));
        System.out.println("bl1:" + bl1 + "  bl2:" + bl2);
    

    @Test//6,获取在剑桥工作的交易员的交易总额
    public void test6() 
        Optional<Integer> sum = transactions.stream().filter((e) -> e.getTrader().getCity().equals("Cambridge"))
                .map(Transaction::getValue).reduce(Integer::sum);
    

    @Test//7,所有交易中,最高交易额
    public void test7() 
        Optional<Integer> maxValue = transactions.stream().map((t) -> t.getValue())
                .max(Integer::compare);
    

    @Test//8,交易额最小的交易记录
    public void test8() 
        Optional<Transaction> opt = transactions.stream()
                .min((t1, t2) -> Integer.compare(t1.getValue(), t2.getValue()));