stream（案例）

package IO.stream.work;

import org.junit.Test;

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

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

/**
 * User: 彭家琪
 * Date: 2019/9/16   17:22
 */
public class DemoStream {

    Trader raoul = new Trader("Raoul", "Cambridge");
    Trader mario = new Trader("Mario", "Milan");
    Trader alan = new Trader("Alan", "Cambridge");
    Trader brian = new Trader("Brian", "Cambridge");

    List<Transaction> 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)
    );

    @Test
    public void test1() {
        //1.找出2011年發生的所有交易，並按交易額排序（從低到高）。
        transactions.stream().filter(t -> t.getYear() == 2011).sorted(Comparator.comparingInt(Transaction::getValue)).forEach(System.out::println);
    }

    @Test
    public void test2() {
//2. 交易員都在哪些不同的城市工作過？
        transactions.stream().map(t -> t.getTrader().getCity()).distinct().forEach(System.out::println);
    }

    @Test
    public void test3() {
//3. 查找所有來自於劍橋的交易員，並按姓名排序。

        // .sorted((t1,t2)->t1.getName().compareTo(t2.getName()))
        transactions.stream().map(t -> t.getTrader()).filter(t -> "Cambridge".equals(t.getCity()))
                .sorted(Comparator.comparing(Trader::getName)).forEach(System.out::println);
    }

    @Test
    public void test4() {
        //4. 返回所有交易員的姓名字符串，按字母順序排序。
        transactions.stream().map(t -> t.getTrader().getName()).sorted(String::compareTo).forEach(System.out::println);
    }

    @Test
    public void test5() {
        //5. 有沒有交易員是在米蘭工作的？
        boolean b = transactions.stream().anyMatch(t -> "Milan".equals(t.getTrader().getCity()));
        System.out.println(b);
    }

    @Test
    public void test6() {
        //6. 打印生活在劍橋的交易員的所有交易額。
        Integer sum = transactions.stream().filter(t -> "Cambridge".equals(t.getTrader().getCity()))
                .map(Transaction::getValue).reduce(0, (t1, t2) -> t1 + t2);
        System.out.println("sum = " + sum);
    }

    @Test
    public void test7() {
        //7. 所有交易中，最高的交易額是多少？
        Integer max = transactions.stream().map(Transaction::getValue).reduce(Integer::max).get();
        System.out.println("最高" + max);

    }

    @Test
    public void test8() {
        // 8. 找到交易額最小的交易。
        // Transaction transaction = transactions.stream().reduce((t1, t2) -> t1.getValue() < t2.getValue() ? t1 : t2).get();
        Integer min = transactions.stream().map(Transaction::getValue).reduce(Integer::min).get();
        System.out.println("min = " + min);


    }

    @Test
    public void test9() {
        System.out.println(transactions.stream().collect(Collectors.counting()));//計算數目

        int max= transactions.stream().collect(maxBy(Comparator.comparingInt(t -> t.getValue()))).get().getValue();
        System.out.println(max); //最大值

        Double avg = transactions.stream().collect(averagingInt(Transaction::getValue));
        System.out.println("avg = " + avg);//平均值

        Integer sum = transactions.stream().collect(summingInt(Transaction::getValue));
        System.out.println("sum = " + sum);

        System.out.println(transactions.stream().collect(summarizingInt(Transaction::getValue)));//彙總


        Double aver = transactions.stream().collect(averagingInt(Transaction::getValue));
        System.out.println("aver = " + aver);



    }

}

package IO.stream.work;




/**
 * User: 彭家琪
 * Date: 2019/9/16   17:18
 */

public class Trader {
    private final String name;
    private final String city;

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

    public String getName() {
        return name;
    }

    public String getCity() {
        return city;
    }

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

package IO.stream.work;


/**
 * User: 彭家琪
 * Date: 2019/9/16   17:18
 */
public class Transaction {

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

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

    public Trader getTrader() {
        return trader;
    }

    public int getYear() {
        return year;
    }

    public int getValue() {
        return value;
    }

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