Java 8 - Stream实战

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Java 8 - Stream流骚操作解读

Java 8 - Stream流骚操作解读2_归约操作

都学了这俩，是不是该出来练一练了？

练习

基础数据

  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)
        );

(1) 找出2011年发生的所有交易，并按交易额排序（从低到高）

 List<Transaction> collect = transactions.stream()
                .filter(t -> t.getYear() == 2011)
                .sorted(Comparator.comparing(Transaction::getValue))
                .collect(toList());

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(2) 交易员都在哪些不同的城市工作过？

        List<String> collect1 = transactions.stream()
                .map(t->t.getTrader().getCity())
                .distinct()
                .collect(toList());

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(3) 查找所有来自于剑桥的交易员，并按姓名排序。

 transactions.stream().map(t->t.getTrader())
                .filter(x -> x.getCity().equals("Cambridge"))
                .distinct()
                .sorted(Comparator.comparing(Trader::getName))
                .forEach(System.out::println);

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(4) 返回所有交易员的姓名字符串，按字母顺序排序。

   String value = transactions.stream().map(t -> t.getTrader().getName())
                .distinct()
                .sorted()
                .reduce("", (name1, name2) -> name1 + name2);
        System.out.println(value);

请注意，此解决方案效率不高（所有字符串都被反复连接，每次迭代的时候都要建立一个新的 String 对象）。

更为高效的解决方案，它像下面这样使用 joining

String traderStr =
	transactions.stream()
		.map(transaction -> transaction.getTrader().getName())
		.distinct()
		.sorted()
		.collect(joining());

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(5) 有没有交易员是在米兰工作的？

    boolean liveInMilan1 = transactions.stream().anyMatch(t -> t.getTrader().getCity().equals("Milan"));
        boolean liveInMilan2 = transactions.stream().map(Transaction::getTrader).anyMatch(t -> t.getCity().equals("Milan"));

        System.out.println(liveInMilan1);
        System.out.println(liveInMilan2);

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(6) 打印生活在剑桥的交易员的所有交易额。

   transactions.stream().filter(t -> t.getTrader().getCity().equals("Cambridge"))
                .map(Transaction::getValue)
                .forEach(System.out::println);

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(7) 所有交易中，最高的交易额是多少？

    Optional<Integer> maxValue = transactions.stream().map(Transaction::getValue).reduce((i, j) -> i > j ? i : j);
        System.out.println(maxValue.get());

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(8) 找到交易额最小的交易

        Optional<Integer> minValue = transactions.stream().map(Transaction::getValue).reduce(Integer::min);
        System.out.println(minValue.get());

还可以做得更好。流支持 min 和 max 方法，它们可以接受一个 Comparator 作为参数，指定计算最小或最大值时要比较哪个键值：

Optional<Transaction> smallestTransaction =
	transactions.stream()
				.min(comparing(Transaction::getValue));

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附 Trader & Transaction

package com.artisan.java8.stream.excise;

public class Trader{
     

    private final String name;
    private final String city;

    public Trader(String n, String c){
     
        this.name = n;
        this.city = c;
    }
    public String getName(){
     
        return this.name;
    }
    public String getCity(){
     
        return this.city;
    }
    public String toString(){
     
        return "Trader:"+this.name + " in " + this.city;
    }
}

package com.artisan.java8.stream.excise;


public class Transaction {
     
    private final Trader trader;
    private final int year;
    private final int value;

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

    public Trader getTrader() {
     
        return this.trader;
    }

    public int getYear() {
     
        return this.year;
    }

    public int getValue() {
     
        return this.value;
    }

    public String toString() {
     
        return "{" + this.trader + ", " +
                "year: " + this.year + ", " +
                "value:" + this.value + "}";
    }
}