# Java8中Stream API的小练习

Java8中Stream API的小练习

### Java8中Stream API的小练习

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. 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<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)
);
@Test
public void test2(){
Optional<Integer> count = employees.stream()
.map((e) -> 1)
.reduce(Integer::sum);
System.out.println(count.get());
}``````
3. 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());
}

}``````