java流式查询 Java 流查找

Java流 - Java流查找

Streams API支持对流元素的不同类型的查找操作。

Stream接口中的以下方法用于执行查找操作：

Optional<T>   findAny()
Optional<T>   findFirst()

诸如IntStream，LongStream和DoubleStream的原始类型流也包含与谓词一起使用的相同方法和用于原语类型的可选方法。

所有查找操作都是终端操作。它们也是短路操作。短路操作可以不必处理整个流以返回结果。

以下代码显示如何对流执行查找操作。

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.Optional;

public class Main {
  public static void main(String[] args) {
    List<Employee> persons = Employee.persons();
    // Find any male
    Optional<Employee> anyMale = persons.stream().filter(Employee::isMale).findAny();
    if (anyMale.isPresent()) {
      System.out.println("Any male:   " + anyMale.get());
    } else {
      System.out.println("No male  found.");
    }
    // Find the first male
    Optional<Employee> firstMale = persons.stream().filter(Employee::isMale).findFirst();
    if (firstMale.isPresent()) {
      System.out.println("First male:   " + anyMale.get());
    } else {
      System.out.println("No male  found.");
    }
  }
}
class Employee {
  public static enum Gender {
    MALE, FEMALE
  }

  private long id;
  private String name;
  private Gender gender;
  private LocalDate dob;
  private double income;

  public Employee(long id, String name, Gender gender, LocalDate dob,
      double income) {
    this.id = id;
    this.name = name;
    this.gender = gender;
    this.dob = dob;
    this.income = income;
  }
  public boolean isMale() {
    return this.gender == Gender.MALE;
  }
  public static List<Employee> persons() {
    Employee p1 = new Employee(1, "Jake", Gender.MALE, LocalDate.of(1971,
        Month.JANUARY, 1), 2343.0);
    Employee p2 = new Employee(2, "Jack", Gender.MALE, LocalDate.of(1972,
        Month.JULY, 21), 7100.0);
    Employee p3 = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1973,
        Month.MAY, 29), 5455.0);
    Employee p4 = new Employee(4, "Jode", Gender.MALE, LocalDate.of(1974,
        Month.OCTOBER, 16), 1800.0);
    Employee p5 = new Employee(5, "Jeny", Gender.FEMALE, LocalDate.of(1975,
        Month.DECEMBER, 13), 1234.0);
    Employee p6 = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1976,
        Month.JUNE, 9), 3211.0);

    List<Employee> persons = Arrays.asList(p1, p2, p3, p4, p5, p6);

    return persons;
  }
}

上面的代码生成以下结果。