Java8新特性_stream API 练习

Posted 2021-02-05 超轶绝尘

 

交易员类

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 + "]";
    }

}

 

//交易类

public class Transaction {

    private Trader trader;
    private int year;
    private int value;

    public Transaction() {
    }

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

    public Trader getTrader() {
        return trader;
    }

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

    public int getYear() {
        return year;
    }

    public void setYear(int year) {
        this.year = year;
    }

    public int getValue() {
        return value;
    }

    public void setValue(int value) {
        this.value = value;
    }

    @Override
    public String toString() {
        return "Transaction [trader=" + trader + ", year=" + year + ", value="
                + value + "]";
    }

}

 

 

实例类

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

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

public class TestTransaction {
    
    List<Transaction> transactions = null;
    
    @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, 2011, 300),
                new Transaction(raoul, 2012, 1000),
                new Transaction(raoul, 2011, 400),
                new Transaction(mario, 2012, 710),
                new Transaction(mario, 2012, 700),
                new Transaction(alan, 2012, 950)
        );
    }
    
    //1. 找出2011年发生的所有交易， 并按交易额排序（从低到高）
    @Test
    public void test1(){
        transactions.stream()
                    .filter((t) -> t.getYear() == 2011)
                    .sorted((t1, t2) -> Integer.compare(t1.getValue(), t2.getValue()))
                    .forEach(System.out::println);
    }
    
    //2. 交易员都在哪些不同的城市工作过？
    @Test
    public void test2(){
        transactions.stream()
                    .map((t) -> t.getTrader().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 test3(){
        transactions.stream()
                    .filter((t) -> t.getTrader().getCity().equals("Cambridge"))
                    .map(Transaction::getTrader)
                    .sorted((t1, t2) -> t1.getName().compareTo(t2.getName()))
                    .distinct()
                    .forEach(System.out::println);
    }
    
    //4. 返回所有交易员的姓名字符串，按字母顺序排序
    @Test
    public void test4(){
        transactions.stream()
                    .map((t) -> t.getTrader().getName())
                    .sorted()
                    .forEach(System.out::println);
        
        System.out.println("-----------------------------------");
        
        String str = transactions.stream()
                    .map((t) -> t.getTrader().getName())
                    .sorted()
                    .reduce("", String::concat);
        
        System.out.println(str);
        
        System.out.println("------------------------------------");
        
        transactions.stream()
                    .map((t) -> t.getTrader().getName())
                    .flatMap(TestTransaction::filterCharacter)
                    .sorted((s1, s2) -> s1.compareToIgnoreCase(s2))
                    .forEach(System.out::print);
    }
    
    public static Stream<String> filterCharacter(String str){
        List<String> list = new ArrayList<>();
        
        for (Character ch : str.toCharArray()) {
            list.add(ch.toString());
        }
        
        return list.stream();
    }
    
    //5. 有没有交易员是在米兰工作的？
    @Test
    public void test5(){
        boolean bl = transactions.stream()
                    .anyMatch((t) -> t.getTrader().getCity().equals("Milan"));
        
        System.out.println(bl);
    }
    
    
    //6. 打印生活在剑桥的交易员的所有交易额
    @Test
    public void test6(){
        Optional<Integer> sum = transactions.stream()
                    .filter((e) -> e.getTrader().getCity().equals("Cambridge"))
                    .map(Transaction::getValue)
                    .reduce(Integer::sum);
        
        System.out.println(sum.get());
    }
    
    
    //7. 所有交易中，最高的交易额是多少
    @Test
    public void test7(){
        Optional<Integer> max = transactions.stream()
                    .map((t) -> t.getValue())
                    .max(Integer::compare);
        
        System.out.println(max.get());
    }
    
    //8. 找到交易额最小的交易
    @Test
    public void test8(){
        Optional<Transaction> op = transactions.stream()
                    .min((t1, t2) -> Integer.compare(t1.getValue(), t2.getValue()));
        
        System.out.println(op.get());
    }

}