Java8中StreamAPI的小练习

Java8中Stream API的小练习

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】
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```
@Test
public void test1(){
    Integer[] nums = new Integer[]{1, 2, 3, 4, 5};

    Arrays.stream(nums)
          .map((x) -> x * x)
          .forEach(System.out::println);
}
```

怎么用map和reduce方法数一数流中有多少个Employee？

public class Employee {

private String name;
private Integer age;
private Double salary;
private Status status;

public Employee() {
    super();
}

public  Employee(Integer age){
    this.age = age;
}

public Employee(String name, Integer age, Double salary) {
    super();
    this.name = name;
    this.age = age;
    this.salary = salary;
}

public Employee(String name, Integer age, Double salary, Status status) {
    this.name = name;
    this.age = age;
    this.salary = salary;
    this.status = status;
}

public String getName() {
    return name;
}

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

public int getAge() {
    return age;
}

public void setAge(Integer age) {
    this.age = age;
}

public double getSalary() {
    return salary;
}

public void setSalary(Double salary) {
    this.salary = salary;
}

public Status getStatus() {
    return status;
}

public void setStatus(Status status) {
    this.status = status;
}

@Override
public String toString() {
    return "Employee{" +
            "name='" + name + '\'' +
            ", age=" + age +
            ", salary=" + salary +
            ", status=" + status +
            '}';
}

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    Employee employee = (Employee) o;
    return age == employee.age &&
            Double.compare(employee.salary, salary) == 0 &&
            Objects.equals(name, employee.name);
}

@Override
public int hashCode() {
    return Objects.hash(name, age, salary);
}

public enum Status{
    FREE,
    BUSY,
    VOCATION;
}
}

List employees = Arrays.asList(
        new Employee("张三", 18 ,9999.99),
        new Employee("李四", 50, 5555.99),
        new Employee("王五", 50, 6666.66),
        new Employee("赵六", 16, 3333.33),
        new Employee("田七", 8, 7777.77)
);
@Test
public void test2(){
    Optional count = employees.stream()
            .map((e) -> 1)
            .reduce(Integer::sum);
    System.out.println(count.get());
}

根据“交易员类”和“交易类”完成相应的功能

//交易员类
public class Trader {

    private String name; //交易员姓名
    private String city; //交易员城市

    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;  //交易员
    private int year;   //交易时间
    private int value;  //交易额

    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 + "]";
    }

}

public class TestTransaction {

    List 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 filterCharacter(String str){
        List 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 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 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 op = transactions.stream()
                    .min((t1, t2) -> Integer.compare(t1.getValue(), t2.getValue()));

        System.out.println(op.get());
    }

}

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