Java8中Stream API的小練習
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給定一個數字列表，如何返回一個由每個數的平方構成的列表？給定【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); } -
怎麼用map和reduce方法數一數流中有多少個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("張三", 18 ,9999.99), new Employee("李四", 50, 5555.99), new Employee("王五", 50, 6666.66), new Employee("趙六", 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()); } - 根據“交易員類”和“交易類”完成相應的功能
//交易員類
public class Trader {
private String name; //交易員姓名
private String city; //交易員城市
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 + "]";
}
}//交易類
public class Transaction {
private Trader trader; //交易員
private int year; //交易時間
private int value; //交易額
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年發生的所有交易， 並按交易額排序（從低到高）
@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. 交易員都在哪些不同的城市工作過？
@Test
public void test2(){
transactions.stream()
.map((t) -> t.getTrader().getCity())
.distinct()
.forEach(System.out::println);
}
//3. 查找所有來自劍橋的交易員，並按姓名排序
@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. 返回所有交易員的姓名字符串，按字母順序排序
@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. 有沒有交易員是在米蘭工作的？
@Test
public void test5(){
boolean bl = transactions.stream()
.anyMatch((t) -> t.getTrader().getCity().equals("Milan"));
System.out.println(bl);
}
//6. 打印生活在劍橋的交易員的所有交易額
@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，最高的交易額是多少
@Test
public void test7(){
Optional<Integer> max = transactions.stream()
.map((t) -> t.getValue())
.max(Integer::compare);
System.out.println(max.get());
}
//8. 找到交易額最小的交易
@Test
public void test8(){
Optional<Transaction> op = transactions.stream()
.min((t1, t2) -> Integer.compare(t1.getValue(), t2.getValue()));
System.out.println(op.get());
}
}