Java工程师面试1000题152-Stream API编程练习

152、Stream API编程练习

①给定一个数字列表，如何返回一个由每个数的平方构成的列表？给定【1,2,3，4,5】返回【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计算流中有多少Employee

    List<Employee> employees = Arrays.asList(
            new Employee("张三",18,9999.99),
            new Employee("李四",58,5555.55),
            new Employee("王五",26,3333.33),
            new Employee("赵柳",36,6666.66),
            new Employee("天琪",12,8888.88),
            new Employee("天琪",12,8888.88));
    @Test
    public void test2(){
        Optional<Integer> reduce = employees.stream().map((e) -> 1).reduce(Integer::sum);
        System.out.println(reduce.get());
    }

③有三个类：

交易员类：

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

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

    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;
    }
}

交易数据类：

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

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

    public Transaction(Trader trader, int year, int value) {

        this.trader = trader;
        this.year = year;
        this.value = value;
    }
}

一些交易数据：

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)
        );
    }
}

要求：

1. 找出2011年发生的所有交易， 并按交易额排序（从低到高）

2. 交易员都在哪些不同的城市工作过？

3. 查找所有来自剑桥的交易员，并按姓名排序

4. 返回所有交易员的姓名字符串，按字母顺序排序

5. 有没有交易员是在米兰工作的？

6. 打印生活在剑桥的交易员的所有交易额

7. 所有交易中，最高的交易额是多少

8. 找到交易额最小的交易

答案：

    //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());
	}