lamdba初体验

我要去巴萨

假设我们有一个场景：需要查询指定条件的人员信息，Person类定义了人员属性，Utils生成我们需要的人员名单，下面的例子我们来探讨下，

Person：

package crelle.test.java.other.beans;

import java.time.LocalDate;

/**
 * @author:crelle
 * @className:Person
 * @version:1.0.0
 * @date:2020/8/15
 * @description:XX
 **/
public class Person {

    public enum Sex {
        MALE, FEMALE
    }
    private String name;

    private LocalDate birthday;

    private Sex gender;

    private String emailAddress;

    private int age;

    public void printPerson() {
      System.out.println(toString());
    }

    public String getName() {
        return name;
    }

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

    public LocalDate getBirthday() {
        return birthday;
    }

    public void setBirthday(LocalDate birthday) {
        this.birthday = birthday;
    }

    public Sex getGender() {
        return gender;
    }

    public void setGender(Sex gender) {
        this.gender = gender;
    }

    public String getEmailAddress() {
        return emailAddress;
    }

    public void setEmailAddress(String emailAddress) {
        this.emailAddress = emailAddress;
    }

    public int getAge() {
        return age;
    }

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

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", birthday=" + birthday +
                ", gender=" + gender +
                ", emailAddress='" + emailAddress + '\'' +
                ", age=" + age +
                '}';
    }
}

Utils：

package crelle.test.java.other;

import crelle.test.java.other.beans.Order;
import crelle.test.java.other.beans.Person;

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;

/**
 * @author:crelle
 * @className:utils
 * @version:1.0.0
 * @date:2020/8/15
 * @description:XX
 **/
public class Utils {
    public static List<Person> getPersonsLists(){
        List<Person> personList = new ArrayList<Person>();
        Person person1 = new Person();
        person1.setAge(15);
        LocalDate localDate1 = LocalDate.of(2020,4,1);
        person1.setBirthday(localDate1);
        person1.setGender(Person.Sex.MALE);
        person1.setEmailAddress("123@qq.com");
        person1.setName("tom");
        personList.add(person1);

        Person person2 = new Person();
        person2.setAge(17);
        LocalDate localDate2 = LocalDate.of(2020,6,1);
        person2.setBirthday(localDate2);
        person2.setGender(Person.Sex.MALE);
        person2.setEmailAddress("456@qq.com");
        person2.setName("john");
        personList.add(person2);

        Person person3 = new Person();
        person3.setAge(19);
        LocalDate localDate3 = LocalDate.of(2020,8,1);
        person3.setBirthday(localDate3);
        person3.setGender(Person.Sex.FEMALE);
        person3.setEmailAddress("789@qq.com");
        person3.setName("lili");
        personList.add(person3);
        return personList;
    }

}

1、获取年龄为15的人员

方法的入参为人员集合，年龄

package crelle.test.java.util.function.lambdatutorials;

import crelle.test.java.other.Utils;
import crelle.test.java.other.beans.Person;

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;

/**
 * @author:crelle
 * @className:Approach1
 * @version:1.0.0
 * @date:2020/8/15
 * @description: Create Methods That Search for Members That Match One Characteristic
 **/
public class Approach1 {
    public static void main(String[] args) {
        printPersonsOlderThan(Utils.getPersonsLists(),15);
    }

    public static void printPersonsOlderThan(List<Person> roster, int age) {
        for (Person p : roster) {
            if (p.getAge() >= age) {
                p.printPerson();
            }
        }
    }
}

现在我们想获取年龄在16-25之间的人员

2、获取年龄在16-25之间的人员

方法的入参为人员集合，年龄范围

package crelle.test.java.util.function.lambdatutorials;

import crelle.test.java.other.Utils;
import crelle.test.java.other.beans.Person;

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;

/**
 * @author:crelle
 * @className:Approach1
 * @version:1.0.0
 * @date:2020/8/15
 * @description: Create More Generalized Search Methods
 **/
public class Approach2 {
    public static void main(String[] args) {

        printPersonsWithinAgeRange(Utils.getPersonsLists(),16,25);
    }

    public static void printPersonsWithinAgeRange(
            List<Person> roster, int low, int high) {
        for (Person p : roster) {
            if (low <= p.getAge() && p.getAge() < high) {
                p.printPerson();
            }
        }
    }
}

我们做一个改进，把查询条件做一个抽取，我们抽取一个接口CheckPerson：

package crelle.test.java.util.function.lambdatutorials;

import crelle.test.java.other.beans.Person;

/**
 * @author:crelle
 * @className:CheckPerson
 * @version:1.0.0
 * @date:2020/8/15
 * @description:XX
 **/
public interface CheckPerson {
    boolean test(Person p);
}

3、查询16-25的男性

方法的入参为人员集合，CheckPerson实现类

package crelle.test.java.util.function.lambdatutorials;

import crelle.test.java.other.Utils;
import crelle.test.java.other.beans.Person;

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;

/**
 * @author:crelle
 * @className:Approach1
 * @version:1.0.0
 * @date:2020/8/15
 * @description: Specify Search Criteria Code in a Local Class
 **/
public class Approach3 {
    public static void main(String[] args) {
        printPersons(Utils.getPersonsLists(), new CheckPersonEligibleForSelectiveService());
    }


    public static void printPersons(
            List<Person> roster, CheckPerson tester) {
        for (Person p : roster) {
            if (tester.test(p)) {
                p.printPerson();
            }
        }
    }
}


class CheckPersonEligibleForSelectiveService implements CheckPerson {
    public boolean test(Person p) {
        return p.getGender() == Person.Sex.MALE &&
                p.getAge() >= 16 &&
                p.getAge() <= 25;
    }
}

可以看出我们的条件入参由基本类型演变成了一个接口的实现类，下面我们用匿名类实现

4、查询16-25的男性

package crelle.test.java.util.function.lambdatutorials;

import crelle.test.java.other.Utils;
import crelle.test.java.other.beans.Person;

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;

/**
 * @author:crelle
 * @className:Approach1
 * @version:1.0.0
 * @date:2020/8/15
 * @description: Specify Search Criteria Code in an Anonymous Class
 **/
public class Approach4 {
    public static void main(String[] args) {

        printPersons(Utils.getPersonsLists(),new CheckPerson() {
                    public boolean test(Person p) {
                        return p.getGender() == Person.Sex.MALE &&
                                p.getAge() >= 16 &&
                                p.getAge() <= 25;
                    }
                }
        );
    }


    public static void printPersons(
            List<Person> roster, CheckPerson tester) {
        for (Person p : roster) {
            if (tester.test(p)) {
                p.printPerson();
            }
        }
    }
}

条件入参我们还可以做改变，我们替换匿名类用lambda表达式来实现

5、查询16-25的男性

package crelle.test.java.util.function.lambdatutorials;

import crelle.test.java.other.Utils;
import crelle.test.java.other.beans.Person;

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;

/**
 * @author:crelle
 * @className:Approach1
 * @version:1.0.0
 * @date:2020/8/15
 * @description: Specify Search Criteria Code with a Lambda Expression
 **/
public class Approach5 {
    public static void main(String[] args) {
        printPersons(Utils.getPersonsLists(),(Person p) -> p.getGender() == Person.Sex.MALE && p.getAge() >= 16 && p.getAge() <= 25);
    }

    public static void printPersons(
            List<Person> roster, CheckPerson tester) {
        for (Person p : roster) {
            if (tester.test(p)) {
                p.printPerson();
            }
        }
    }
}

我们仔细研究一下CheckPerson接口，返现只有一个抽象方法，输入为对象，输出为boolean类型，我们发现和下面的接口test方法很像：

 我替换CheckPerson来使用它

6、查询16-25的男性

package crelle.test.java.util.function.lambdatutorials;

import crelle.test.java.other.Utils;
import crelle.test.java.other.beans.Person;

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;
import java.util.function.Predicate;

/**
 * @author:crelle
 * @className:Approach1
 * @version:1.0.0
 * @date:2020/8/15
 * @description: Use Standard Functional Interfaces with Lambda Expressions
 **/
public class Approach6 {
    public static void main(String[] args) {
        printPersonsWithPredicate(Utils.getPersonsLists(),(Person p) -> p.getGender() == Person.Sex.MALE && p.getAge() >= 16 && p.getAge() <= 25);
    }

    public static void printPersonsWithPredicate(
            List<Person> roster, Predicate<Person> tester) {
            for (Person p : roster) {
                if (tester.test(p)) {
                    p.printPerson();
                }
            }
    }
}

我们可以把打印人员明细单独抽取出来，作为一个入参

7、查询16-25的男性

package crelle.test.java.util.function.lambdatutorials;

import crelle.test.java.other.Utils;
import crelle.test.java.other.beans.Person;

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;
import java.util.function.Predicate;

/**
 * @author:crelle
 * @className:Approach1
 * @version:1.0.0
 * @date:2020/8/15
 * @description: Use Lambda Expressions Throughout Your Application
 **/
public class Approach7 {
    public static void main(String[] args) {
        processPersons(Utils.getPersonsLists(), p -> p.getGender() == Person.Sex.MALE && p.getAge() >= 16 && p.getAge() <= 25, p -> p.printPerson()
        );
    }

    public static void processPersons(
            List<Person> roster,
            Predicate<Person> tester,
            Consumer<Person> block) {
        for (Person p : roster) {
            if (tester.test(p)) {
                block.accept(p);
            }
        }
    }
}

8、查询16-25的男性，并且打印他们的电子邮箱

package crelle.test.java.util.function.lambdatutorials;

import crelle.test.java.other.Utils;
import crelle.test.java.other.beans.Person;

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;

/**
 * @author:crelle
 * @className:Approach1
 * @version:1.0.0
 * @date:2020/8/15
 * @description: Specify Search Criteria Code with a Lambda Expression
 **/
public class Approach8 {
    public static void main(String[] args) {
        processPersonsWithFunction(
                Utils.getPersonsLists(),
                p -> p.getGender() == Person.Sex.MALE
                        && p.getAge() >= 16
                        && p.getAge() <= 25,
                p -> p.getEmailAddress(),
                email -> System.out.println(email)
        );
    }

    public static void processPersonsWithFunction(
            List<Person> roster,
            Predicate<Person> tester,
            Function<Person, String> mapper,
            Consumer<String> block) {
        for (Person p : roster) {
            if (tester.test(p)) {
                String data = mapper.apply(p);
                block.accept(data);
            }
        }
    }
}

我们用到了下面的函数式接口

   T – the type of the input to the function  函数输入

 R – the type of the result of the function 函数输出

   我们使用强大的stream配合lambda来实现查询16-25的男性，并且打印他们的电子邮箱

9、查询16-25的男性，并且打印他们的电子邮箱

package crelle.test.java.util.function.lambdatutorials;

import crelle.test.java.other.Utils;
import crelle.test.java.other.beans.Person;

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;

/**
 * @author:crelle
 * @className:Approach1
 * @version:1.0.0
 * @date:2020/8/15
 * @description: Use Aggregate Operations That Accept Lambda Expressions as Parameters
 **/
public class Approach9 {
    public static void main(String[] args) {
        Utils.getPersonsLists()
                .stream()
                .filter(
                        p -> p.getGender() == Person.Sex.MALE
                                && p.getAge() >= 12
                                && p.getAge() <= 25)
                .map(p -> p.getEmailAddress())
                .forEach(email -> System.out.println(email));
    }


}

实现同一个功能代码少了多少，自己体会。

完毕！

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