JDK8的新特性(二)

本文已被阅读过 Posted by Liao Can on 2019-03-03

创建流

示例:Collection 提供了两个方法 stream() 与 parallelStream()

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List<String> list = new ArrayList<>();
Stream<String> stream = list.stream(); //获取一个顺序流
Stream<String> parallelStream = list.parallelStream(); //获取一个并行流

示例:通过 Arrays 中的 stream() 获取一个数组流

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Integer[] nums = new Integer[10];
Stream<Integer> stream1 = Arrays.stream(nums);

示例:通过 Stream 类中静态方法 of()

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Stream<Integer> stream2 = Stream.of(1,2,3,4,5,6);

示例:通过迭代创建无限流

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Stream<Integer> stream3 = Stream.iterate(0, (x) -> x + 2).limit(10);
stream3.forEach(System.out::println);

示例:通过生成创建无限流

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Stream<Double> stream4 = Stream.generate(Math::random).limit(2);
stream4.forEach(System.out::println);

中间操作

筛选与切片

筛选与切片

  1. filter:接收 Lambda,从流中排除某些元素
  2. limit:截断流,使其元素不超过给定数量
  3. skip(n) :跳过元素,返回一个扔掉了前 n 个元素的流。若流中元素不足 n 个,则返回一个空流。与 limit(n) 互补
  4. distinct:筛选,通过流所生成元素的 hashCode() 和 equals() 去除重复元素

示例:条件与分页查询

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public class Employee {

private int id;
private String name;
private int age;
private double salary;

public Employee() {
}

public Employee(String name) {
this.name = name;
}

public Employee(String name, int age) {
this.name = name;
this.age = age;
}

public Employee(int id, String name, int age, double salary) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
}

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

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

public int getAge() {
return age;
}

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

public double getSalary() {
return salary;
}

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

public String show() {
return "测试方法引用!";
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + age;
result = prime * result + id;
result = prime * result + ((name == null) ? 0 : name.hashCode());
long temp;
temp = Double.doubleToLongBits(salary);
result = prime * result + (int) (temp ^ (temp >>> 32));
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Employee other = (Employee) obj;
if (age != other.age)
return false;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
return false;
return true;
}

@Override
public String toString() {
return "Employee [id=" + id + ", name=" + name + ", age=" + age + ", salary=" + salary + "]";
}

}

List<Employee> emps = Arrays.asList(
new Employee(102, "李四", 59, 6666.66),
new Employee(101, "张三", 18, 9999.99),
new Employee(103, "王五", 28, 3333.33),
new Employee(104, "赵六", 8, 7777.77),
new Employee(104, "赵六", 8, 7777.77),
new Employee(104, "赵六", 8, 7777.77),
new Employee(105, "田七", 38, 5555.55)
);
@Test
public void streamApiTest(){
emps.parallelStream()
.filter((e) -> e.getSalary() >= 5000)
.skip(2)
.limit(5)
.distinct()
.forEach(System.out::println);
}

映射

  1. map:接收 Lambda,将元素转换成其他形式或提取信息。接收一个函数作为参数,该函数会被应用到每个元素上,并将其映射成一个新的元素
  2. flatMap:接收一个函数作为参数,将流中的每个值都换成另一个流,然后把所有流连接成一个流

示例:提取员工名称列表

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public class Employee {

private int id;
private String name;
private int age;
private double salary;
private Status status;

public Employee() {
}

public Employee(String name) {
this.name = name;
}

public Employee(String name, int age) {
this.name = name;
this.age = age;
}

public Employee(int id, String name, int age, double salary) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
}

public Employee(int id, String name, int age, double salary, Status status) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
this.status = status;
}

public Status getStatus() {
return status;
}

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

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

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

public int getAge() {
return age;
}

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

public double getSalary() {
return salary;
}

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

public String show() {
return "测试方法引用!";
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + age;
result = prime * result + id;
result = prime * result + ((name == null) ? 0 : name.hashCode());
long temp;
temp = Double.doubleToLongBits(salary);
result = prime * result + (int) (temp ^ (temp >>> 32));
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Employee other = (Employee) obj;
if (age != other.age)
return false;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
return false;
return true;
}

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

public enum Status {
FREE, BUSY, VOCATION;
}

}

List<Employee> emps = Arrays.asList(
new Employee(102, "李四", 59, 6666.66),
new Employee(101, "张三", 18, 9999.99),
new Employee(103, "王五", 28, 3333.33),
new Employee(104, "赵六", 8, 7777.77),
new Employee(104, "赵六", 8, 7777.77),
new Employee(104, "赵六", 8, 7777.77),
new Employee(105, "田七", 38, 5555.55)
);

@Test
public void mapTest(){
Stream<String> str = emps.stream()
.map((e) -> e.getName());
str.forEach(System.out::println);
}

示例:将集合里的字符串拉平

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public class TestStreamAPI {

@Test
public void flatMapTest(){
List<String> strList = Arrays.asList("aaa", "bbb", "ccc", "ddd", "eee");
Stream<Character> stream = strList.stream()
.flatMap(TestStreamAPI::filterCharacter);
stream.forEach(System.out::println);
}

public static Stream<Character> filterCharacter(String str){
List<Character> list = new ArrayList<>();

for (Character ch : str.toCharArray()) {
list.add(ch);
}

return list.stream();
}

}

排序

  1. sorted():自然排序
  2. sorted(Comparator com):定制排序

示例:按年龄和姓名排序

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public class Employee {

private int id;
private String name;
private int age;
private double salary;
private Status status;

public Employee() {
}

public Employee(String name) {
this.name = name;
}

public Employee(String name, int age) {
this.name = name;
this.age = age;
}

public Employee(int id, String name, int age, double salary) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
}

public Employee(int id, String name, int age, double salary, Status status) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
this.status = status;
}

public Status getStatus() {
return status;
}

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

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

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

public int getAge() {
return age;
}

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

public double getSalary() {
return salary;
}

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

public String show() {
return "测试方法引用!";
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + age;
result = prime * result + id;
result = prime * result + ((name == null) ? 0 : name.hashCode());
long temp;
temp = Double.doubleToLongBits(salary);
result = prime * result + (int) (temp ^ (temp >>> 32));
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Employee other = (Employee) obj;
if (age != other.age)
return false;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
return false;
return true;
}

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

public enum Status {
FREE, BUSY, VOCATION;
}

}

public class TestStreamAPI1 {

List<Employee> emps = Arrays.asList(
new Employee(102, "李四", 59, 6666.66),
new Employee(101, "张三", 18, 9999.99),
new Employee(103, "王五", 28, 3333.33),
new Employee(104, "赵六", 8, 7777.77),
new Employee(104, "三八", 8, 7777.77),
new Employee(104, "赵六", 8, 7777.77),
new Employee(105, "田七", 38, 5555.55)
);

@Test
public void sortTest(){
emps.stream()
.sorted((x, y) -> {
if(x.getAge() == y.getAge()){
return x.getName().compareTo(y.getName());
}else{
return Integer.compare(x.getAge(), y.getAge());
}
}).forEach(System.out::println);
}
}

终止操作

查找与匹配

  1. allMatch:检查是否匹配所有元素
  2. anyMatch:检查是否至少匹配一个元素
  3. noneMatch:检查是否没有匹配的元素
  4. findFirst:返回第一个元素
  5. findAny:返回当前流中的任意元素
  6. count:返回流中元素的总个数
  7. max:返回流中最大值
  8. min:返回流中最小值

示例:查找与匹配员工信息

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public class Employee {

private int id;
private String name;
private int age;
private double salary;
private Status status;

public Employee() {
}

public Employee(String name) {
this.name = name;
}

public Employee(String name, int age) {
this.name = name;
this.age = age;
}

public Employee(int id, String name, int age, double salary) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
}

public Employee(int id, String name, int age, double salary, Status status) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
this.status = status;
}

public Status getStatus() {
return status;
}

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

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

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

public int getAge() {
return age;
}

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

public double getSalary() {
return salary;
}

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

public String show() {
return "测试方法引用!";
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + age;
result = prime * result + id;
result = prime * result + ((name == null) ? 0 : name.hashCode());
long temp;
temp = Double.doubleToLongBits(salary);
result = prime * result + (int) (temp ^ (temp >>> 32));
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Employee other = (Employee) obj;
if (age != other.age)
return false;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
return false;
return true;
}

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

public enum Status {
FREE, BUSY, VOCATION;
}

}

List<Employee> emps = Arrays.asList(
new Employee(102, "李四", 59, 6666.66, Status.BUSY),
new Employee(101, "张三", 18, 9999.99, Status.FREE),
new Employee(103, "王五", 28, 3333.33, Status.VOCATION),
new Employee(104, "赵六", 8, 7777.77, Status.BUSY),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(105, "田七", 38, 5555.55, Status.BUSY)
);

@Test
public void matchTest(){
boolean bl = emps.stream()
.allMatch((e) -> e.getStatus().equals(Status.BUSY));

System.out.println(bl);

boolean bl1 = emps.stream()
.anyMatch((e) -> e.getStatus().equals(Status.BUSY));

System.out.println(bl1);

boolean bl2 = emps.stream()
.noneMatch((e) -> e.getStatus().equals(Status.BUSY));

System.out.println(bl2);
}

@Test
public void findTest(){
Optional<Employee> op = emps.stream()
.sorted((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary()))
.findFirst();

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

System.out.println("--------------------------------");

Optional<Employee> op2 = emps.parallelStream()
.filter((e) -> e.getStatus().equals(Status.FREE))
.findAny();

System.out.println(op2.get());
}

@Test
public void calculateTest(){
long count = emps.stream()
.filter((e) -> e.getStatus().equals(Status.FREE))
.count();

System.out.println(count);

Optional<Double> op = emps.stream()
.map(Employee::getSalary)
.max(Double::compare);

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

Optional<Employee> op2 = emps.stream()
.min((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary()));

System.out.println(op2.get());
}

规约

  1. reduce(T identity, BinaryOperator) / reduce(BinaryOperator):可以将流中元素反复结合起来,得到一个值

示例:将数值集合求和

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List<Integer> list = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
Integer sum = list.stream().reduce(0, (x, y) -> x + y);
System.out.println(sum);

示例:员工工资求和

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public class Employee {

private int id;
private String name;
private int age;
private double salary;
private Status status;

public Employee() {
}

public Employee(String name) {
this.name = name;
}

public Employee(String name, int age) {
this.name = name;
this.age = age;
}

public Employee(int id, String name, int age, double salary) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
}

public Employee(int id, String name, int age, double salary, Status status) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
this.status = status;
}

public Status getStatus() {
return status;
}

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

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

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

public int getAge() {
return age;
}

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

public double getSalary() {
return salary;
}

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

public String show() {
return "测试方法引用!";
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + age;
result = prime * result + id;
result = prime * result + ((name == null) ? 0 : name.hashCode());
long temp;
temp = Double.doubleToLongBits(salary);
result = prime * result + (int) (temp ^ (temp >>> 32));
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Employee other = (Employee) obj;
if (age != other.age)
return false;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
return false;
return true;
}

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

public enum Status {
FREE, BUSY, VOCATION;
}

}

List<Employee> emps = Arrays.asList(
new Employee(102, "李四", 79, 6666.66, Status.BUSY),
new Employee(101, "张三", 18, 9999.99, Status.FREE),
new Employee(103, "王五", 28, 3333.33, Status.VOCATION),
new Employee(104, "赵六", 8, 7777.77, Status.BUSY),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(105, "田七", 38, 5555.55, Status.BUSY)
);

@Test
public void reduceTest(){
Optional<Double> op = emps.stream()
.map(Employee::getSalary)
.reduce(Double::sum);

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

收集

  1. collect:将流转换为其他形式。接收一个 Collector接口的实现,用于给Stream中元素做汇总的方法

示例:获取员工名称集合

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public class Employee {

private int id;
private String name;
private int age;
private double salary;
private Status status;

public Employee() {
}

public Employee(String name) {
this.name = name;
}

public Employee(String name, int age) {
this.name = name;
this.age = age;
}

public Employee(int id, String name, int age, double salary) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
}

public Employee(int id, String name, int age, double salary, Status status) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
this.status = status;
}

public Status getStatus() {
return status;
}

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

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

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

public int getAge() {
return age;
}

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

public double getSalary() {
return salary;
}

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

public String show() {
return "测试方法引用!";
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + age;
result = prime * result + id;
result = prime * result + ((name == null) ? 0 : name.hashCode());
long temp;
temp = Double.doubleToLongBits(salary);
result = prime * result + (int) (temp ^ (temp >>> 32));
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Employee other = (Employee) obj;
if (age != other.age)
return false;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
return false;
return true;
}

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

public enum Status {
FREE, BUSY, VOCATION;
}

}

List<Employee> emps = Arrays.asList(
new Employee(102, "李四", 79, 6666.66, Status.BUSY),
new Employee(101, "张三", 18, 9999.99, Status.FREE),
new Employee(103, "王五", 28, 3333.33, Status.VOCATION),
new Employee(104, "赵六", 8, 7777.77, Status.BUSY),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(105, "田七", 38, 5555.55, Status.BUSY)
);

@Test
public void collectTest(){
List<String> list = emps.stream()
.map(Employee::getName)
.collect(Collectors.toList());

list.forEach(System.out::println);

System.out.println("----------------------------------");

HashSet<String> hs = emps.stream()
.map(Employee::getName)
.collect(Collectors.toCollection(HashSet::new));

hs.forEach(System.out::println);
}

示例:获取员工的平均工资

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public class Employee {

private int id;
private String name;
private int age;
private double salary;
private Status status;

public Employee() {
}

public Employee(String name) {
this.name = name;
}

public Employee(String name, int age) {
this.name = name;
this.age = age;
}

public Employee(int id, String name, int age, double salary) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
}

public Employee(int id, String name, int age, double salary, Status status) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
this.status = status;
}

public Status getStatus() {
return status;
}

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

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

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

public int getAge() {
return age;
}

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

public double getSalary() {
return salary;
}

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

public String show() {
return "测试方法引用!";
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + age;
result = prime * result + id;
result = prime * result + ((name == null) ? 0 : name.hashCode());
long temp;
temp = Double.doubleToLongBits(salary);
result = prime * result + (int) (temp ^ (temp >>> 32));
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Employee other = (Employee) obj;
if (age != other.age)
return false;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
return false;
return true;
}

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

public enum Status {
FREE, BUSY, VOCATION;
}

}

List<Employee> emps = Arrays.asList(
new Employee(102, "李四", 79, 6666.66, Status.BUSY),
new Employee(101, "张三", 18, 9999.99, Status.FREE),
new Employee(103, "王五", 28, 3333.33, Status.VOCATION),
new Employee(104, "赵六", 8, 7777.77, Status.BUSY),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(105, "田七", 38, 5555.55, Status.BUSY)
);

@Test
public void collectTest() {
Double avg = emps.stream()
.collect(Collectors.averagingDouble(Employee::getSalary));

System.out.println(avg);
}

示例:员工按状态分组

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public class Employee {

private int id;
private String name;
private int age;
private double salary;
private Status status;

public Employee() {
}

public Employee(String name) {
this.name = name;
}

public Employee(String name, int age) {
this.name = name;
this.age = age;
}

public Employee(int id, String name, int age, double salary) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
}

public Employee(int id, String name, int age, double salary, Status status) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
this.status = status;
}

public Status getStatus() {
return status;
}

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

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

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

public int getAge() {
return age;
}

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

public double getSalary() {
return salary;
}

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

public String show() {
return "测试方法引用!";
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + age;
result = prime * result + id;
result = prime * result + ((name == null) ? 0 : name.hashCode());
long temp;
temp = Double.doubleToLongBits(salary);
result = prime * result + (int) (temp ^ (temp >>> 32));
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Employee other = (Employee) obj;
if (age != other.age)
return false;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
return false;
return true;
}

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

public enum Status {
FREE, BUSY, VOCATION;
}

}

List<Employee> emps = Arrays.asList(
new Employee(102, "李四", 79, 6666.66, Status.BUSY),
new Employee(101, "张三", 18, 9999.99, Status.FREE),
new Employee(103, "王五", 28, 3333.33, Status.VOCATION),
new Employee(104, "赵六", 8, 7777.77, Status.BUSY),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(105, "田七", 38, 5555.55, Status.BUSY)
);

@Test
public void collectTest() {
Map<Status, List<Employee>> map = emps.stream()
.collect(Collectors.groupingBy(Employee::getStatus));

System.out.println(map);
}

示例:员工按状态和年龄多级分组

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public class Employee {

private int id;
private String name;
private int age;
private double salary;
private Status status;

public Employee() {
}

public Employee(String name) {
this.name = name;
}

public Employee(String name, int age) {
this.name = name;
this.age = age;
}

public Employee(int id, String name, int age, double salary) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
}

public Employee(int id, String name, int age, double salary, Status status) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
this.status = status;
}

public Status getStatus() {
return status;
}

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

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

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

public int getAge() {
return age;
}

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

public double getSalary() {
return salary;
}

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

public String show() {
return "测试方法引用!";
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + age;
result = prime * result + id;
result = prime * result + ((name == null) ? 0 : name.hashCode());
long temp;
temp = Double.doubleToLongBits(salary);
result = prime * result + (int) (temp ^ (temp >>> 32));
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Employee other = (Employee) obj;
if (age != other.age)
return false;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
return false;
return true;
}

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

public enum Status {
FREE, BUSY, VOCATION;
}

}

List<Employee> emps = Arrays.asList(
new Employee(102, "李四", 79, 6666.66, Status.BUSY),
new Employee(101, "张三", 18, 9999.99, Status.FREE),
new Employee(103, "王五", 28, 3333.33, Status.VOCATION),
new Employee(104, "赵六", 8, 7777.77, Status.BUSY),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(105, "田七", 38, 5555.55, Status.BUSY)
);

@Test
public void collectTest() {
Map<Status, Map<String, List<Employee>>> map = emps.stream()
.collect(Collectors.groupingBy(Employee::getStatus, Collectors.groupingBy((e) -> {
if(e.getAge() >= 60)
return "老年";
else if(e.getAge() >= 35)
return "中年";
else
return "成年";
})));

System.out.println(map);
}

示例:员工按薪资水平分区

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public class Employee {

private int id;
private String name;
private int age;
private double salary;
private Status status;

public Employee() {
}

public Employee(String name) {
this.name = name;
}

public Employee(String name, int age) {
this.name = name;
this.age = age;
}

public Employee(int id, String name, int age, double salary) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
}

public Employee(int id, String name, int age, double salary, Status status) {
this.id = id;
this.name = name;
this.age = age;
this.salary = salary;
this.status = status;
}

public Status getStatus() {
return status;
}

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

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

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

public int getAge() {
return age;
}

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

public double getSalary() {
return salary;
}

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

public String show() {
return "测试方法引用!";
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + age;
result = prime * result + id;
result = prime * result + ((name == null) ? 0 : name.hashCode());
long temp;
temp = Double.doubleToLongBits(salary);
result = prime * result + (int) (temp ^ (temp >>> 32));
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Employee other = (Employee) obj;
if (age != other.age)
return false;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
return false;
return true;
}

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

public enum Status {
FREE, BUSY, VOCATION;
}

}

List<Employee> emps = Arrays.asList(
new Employee(102, "李四", 79, 6666.66, Status.BUSY),
new Employee(101, "张三", 18, 9999.99, Status.FREE),
new Employee(103, "王五", 28, 3333.33, Status.VOCATION),
new Employee(104, "赵六", 8, 7777.77, Status.BUSY),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(104, "赵六", 8, 7777.77, Status.FREE),
new Employee(105, "田七", 38, 5555.55, Status.BUSY)
);

@Test
public void collectTest() {
Map<Boolean, List<Employee>> map = emps.stream()
.collect(Collectors.partitioningBy((e) -> e.getSalary() >= 5000));

System.out.println(map);
}

示例:切割字符串

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@Test
public void collectTest() {
List<String> strList = Arrays.asList("aaa", "bbb", "ccc", "ddd", "eee");

String str = strList.stream()
.collect(Collectors.joining("," , "----", "----"));

System.out.println(str);
}

并行流

示例:使用Fork/Join框架计算0到大数的总和

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import java.util.concurrent.RecursiveTask;

public class ForkJoinCalculate extends RecursiveTask<Long>{

/**
*
*/
private static final long serialVersionUID = 13475679780L;

private long start;
private long end;

private static final long THRESHOLD = 10000L; //临界值

public ForkJoinCalculate(long start, long end) {
this.start = start;
this.end = end;
}

@Override
protected Long compute() {
long length = end - start;

if(length <= THRESHOLD){
long sum = 0;

for (long i = start; i <= end; i++) {
sum += i;
}

return sum;
}else{
long middle = (start + end) / 2;

ForkJoinCalculate left = new ForkJoinCalculate(start, middle);
left.fork(); //拆分,并将该子任务压入线程队列

ForkJoinCalculate right = new ForkJoinCalculate(middle+1, end);
right.fork();

return left.join() + right.join();
}

}

}

@Test
public void forkJoinTest(){
long start = System.currentTimeMillis();

ForkJoinPool pool = new ForkJoinPool();
ForkJoinTask<Long> task = new ForkJoinCalculate(0L, 100000000L);

long sum = pool.invoke(task);
System.out.println(sum);

long end = System.currentTimeMillis();

System.out.println("耗费的时间为: " + (end - start));
}

示例:使用并行流计算0到大数的总和
@Test
public void streamTest(){
long start = System.currentTimeMillis();

Long sum = LongStream.rangeClosed(0L, 1000000000L)
.parallel()
.sum();

System.out.println(sum);

long end = System.currentTimeMillis();

System.out.println("耗费的时间为: " + (end - start));
}

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