stream与Lambda结合使用（示列）

创建一个POJO对象

public class Person {
 
private String firstName, lastName, job, gender;
private int salary, age;
 
public Person(String firstName, String lastName, String job,
                String gender, int age, int salary)       {
          this.firstName = firstName;
          this.lastName = lastName;
          this.gender = gender;
          this.age = age;
          this.job = job;
          this.salary = salary;
}
// Getter and Setter 
// . . . . .
}

创建两个list,都用来存放Person对象:

List<Person> javaProgrammers = new ArrayList<Person>() {
  {
    add(new Person("Elsdon", "Jaycob", "Java programmer", "male", 43, 2000));
    add(new Person("Tamsen", "Brittany", "Java programmer", "female", 23, 1500));
    add(new Person("Floyd", "Donny", "Java programmer", "male", 33, 1800));
    add(new Person("Sindy", "Jonie", "Java programmer", "female", 32, 1600));
    add(new Person("Vere", "Hervey", "Java programmer", "male", 22, 1200));
    add(new Person("Maude", "Jaimie", "Java programmer", "female", 27, 1900));
    add(new Person("Shawn", "Randall", "Java programmer", "male", 30, 2300));
    add(new Person("Jayden", "Corrina", "Java programmer", "female", 35, 1700));
    add(new Person("Palmer", "Dene", "Java programmer", "male", 33, 2000));
    add(new Person("Addison", "Pam", "Java programmer", "female", 34, 1300));
  }
};
 
List<Person> phpProgrammers = new ArrayList<Person>() {
  {
    add(new Person("Jarrod", "Pace", "PHP programmer", "male", 34, 1550));
    add(new Person("Clarette", "Cicely", "PHP programmer", "female", 23, 1200));
    add(new Person("Victor", "Channing", "PHP programmer", "male", 32, 1600));
    add(new Person("Tori", "Sheryl", "PHP programmer", "female", 21, 1000));
    add(new Person("Osborne", "Shad", "PHP programmer", "male", 32, 1100));
    add(new Person("Rosalind", "Layla", "PHP programmer", "female", 25, 1300));
    add(new Person("Fraser", "Hewie", "PHP programmer", "male", 36, 1100));
    add(new Person("Quinn", "Tamara", "PHP programmer", "female", 21, 1000));
    add(new Person("Alvin", "Lance", "PHP programmer", "male", 38, 1600));
    add(new Person("Evonne", "Shari", "PHP programmer", "female", 40, 1800));
  }
};

使用forEach方法来迭代输出上述列表:

System.out.println("所有程序员的姓名:");
javaProgrammers.forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));
phpProgrammers.forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));

使用forEach方法,增加程序员的工资5%:

System.out.println("给程序员加薪 5% :");
Consumer<Person> giveRaise = e -> e.setSalary(e.getSalary() / 100 * 5 + e.getSalary());
 
javaProgrammers.forEach(giveRaise);
phpProgrammers.forEach(giveRaise)

滤器filter() ,让我们显示月薪超过1400美元的PHP程序员:

System.out.println("下面是月薪超过 $1,400 的PHP程序员:")
phpProgrammers.stream()
          .filter((p) -> (p.getSalary() > 1400))
          .forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));

我们也可以定义过滤器,然后重用它们来执行其他操作:

// 定义 filters
Predicate<Person> ageFilter = (p) -> (p.getAge() > 25);
Predicate<Person> salaryFilter = (p) -> (p.getSalary() > 1400);
Predicate<Person> genderFilter = (p) -> ("female".equals(p.getGender()));
 
System.out.println("下面是年龄大于 24岁且月薪在$1,400以上的女PHP程序员:");
phpProgrammers.stream()
          .filter(ageFilter)
          .filter(salaryFilter)
          .filter(genderFilter)
          .forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));
 
// 重用filters
System.out.println("年龄大于 24岁的女性 Java programmers:");
javaProgrammers.stream()
          .filter(ageFilter)
          .filter(genderFilter)
          .forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));

使用limit方法,可以限制结果集的个数:

System.out.println("最前面的3个 Java programmers:");
javaProgrammers.stream()
          .limit(3)
          .forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));
 
 
System.out.println("最前面的3个女性 Java programmers:");
javaProgrammers.stream()
          .filter(genderFilter)
          .limit(3)
          .forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));

排序呢? 我们在stream中能处理吗? 答案是肯定的。 在下面的例子中,我们将根据名字和薪水排序Java程序员,放到一个list中,然后显示列表:

import static java.util.stream.Collectors.toList;

 

System.out.println("根据 name 排序,并显示前5个 Java programmers:");
List<Person> sortedJavaProgrammers = javaProgrammers
          .stream()
          .sorted((p, p2) -> (p.getFirstName().compareTo(p2.getFirstName())))
          .limit(5)
          .collect(toList());
 
sortedJavaProgrammers.forEach((p) -> System.out.printf("%s %s; %n", p.getFirstName(), p.getLastName()));
 
System.out.println("根据 salary 排序 Java programmers:");
sortedJavaProgrammers = javaProgrammers
          .stream()
          .sorted( (p, p2) -> (p.getSalary() - p2.getSalary()) )
          .collect( toList() );
 
sortedJavaProgrammers.forEach((p) -> System.out.printf("%s %s; %n", p.getFirstName(), p.getLastName()));

如果我们只对最低和最高的薪水感兴趣,比排序后选择第一个/最后一个 更快的是min和max方法:

ystem.out.println("工资最低的 Java programmer:");
Person pers = javaProgrammers
          .stream()
          .min((p1, p2) -> (p1.getSalary() - p2.getSalary()))
          .get()

System.out.printf("Name: %s %s; Salary: $%,d.", pers.getFirstName(), pers.getLastName(), pers.getSalary())

System.out.println("工资最高的 Java programmer:");
Person person = javaProgrammers
          .stream()
          .max((p, p2) -> (p.getSalary() - p2.getSalary()))
          .get()

System.out.printf("Name: %s %s; Salary: $%,d.", person.getFirstName(), person.getLastName(), person.getSalary())

上面的例子中我们已经看到 collect 方法是如何工作的。 结合 map 方法,我们可以使用 collect 方法来将我们的结果集放到一个字符串,一个 Set 或一个TreeSet中:

System.out.println("将 PHP programmers 的 first name 拼接成字符串:");
String phpDevelopers = phpProgrammers
          .stream()
          .map(Person::getFirstName)
          .collect(joining(" ; ")); // 在进一步的操作中可以作为标记(token)   
 
System.out.println("将 Java programmers 的 first name 存放到 Set:");
Set<String> javaDevFirstName = javaProgrammers
          .stream()
          .map(Person::getFirstName)
          .collect(toSet());
 
System.out.println("将 Java programmers 的 first name 存放到 TreeSet:");
TreeSet<String> javaDevLastName = javaProgrammers
          .stream()
          .map(Person::getLastName)
          .collect(toCollection(TreeSet::new));

Streams 还可以是并行的(parallel)。 示例如下:

System.out.println("计算付给 Java programmers 的所有money:");
int totalSalary = javaProgrammers
          .parallelStream()
          .mapToInt(p -> p.getSalary())
          .sum();

我们可以使用summaryStatistics方法获得stream 中元素的各种汇总数据。 接下来,我们可以访问这些方法,比如getMax, getMin, getSum或getAverage

//计算 count, min, max, sum, and average for numbers
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
IntSummaryStatistics stats = numbers
          .stream()
          .mapToInt((x) -> x)
          .summaryStatistics();
 
System.out.println("List中最大的数字 : " + stats.getMax());
System.out.println("List中最小的数字 : " + stats.getMin());
System.out.println("所有数字的总和   : " + stats.getSum());
System.out.println("所有数字的平均值 : " + stats.getAverage());