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】

@Testpublic 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;}@Overridepublic String toString() { return "Employee{" + "name='" + name + '\'' + ", age=" + age + ", salary=" + salary + ", status=" + status + '}';}@Overridepublic 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);}@Overridepublic int hashCode() { return Objects.hash(name, age, salary);}public enum Status{ FREE, BUSY, VOCATION;}}

List<Employee> 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));@Testpublic void test2(){ Optional<Integer> 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<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()); }}