一、简要描述

• Lambda初体验
• Lambda表达式的语法格式
• Lambda表达式应用举例
• Lambda表达式底层实现

二、什么是Lambda表达式

Java8新特性，来源于数学中的λ[læ:mdə]演算

是一套关于函数(f(x))定义、输入量、输出量的计算方案

• Lambda表达式 -> 函数
  • 使代码变得简洁、紧凑
  • 函数式编程：
    • 函数是“第一等公民”
    • 可以作为(其他函数的)返回值
    • 可以作为(其他函数的)参数进行传递
    • 可以赋值给变量

三、代码实现

1、定义User类

package tech.flygo.lambda.demo1;

/**
 * @description: 用户类
 * @author: flygo
 * @time: 2023/5/7 22:59
 */
public class User {

  private String name;

  private int age;

  public User() {}

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

  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;
  }

  @Override
  public String toString() {
    return "User{" + "name='" + name + '\'' + ", age=" + age + '}';
  }
}

2、定义Factory接口

package tech.flygo.lambda.demo1;

/**
 * @description: 工程接口
 * @author: flygo
 * @time: 2023/5/7 22:58
 */
public interface Factory {
  Object getObject();
}

3、定义Factory接口实现类SubClass

package tech.flygo.lambda.demo1;

/**
 * @description: 接口实现类
 * @author: flygo
 * @time: 2023/5/7 22:59
 */
public class SubClass implements Factory {

  @Override
  public Object getObject() {
    return new User();
  }
}

4、定义LambdaTest测试类

package tech.flygo.lambda.demo1;

/**
 * @description: Lambda测试类
 * @author: flygo
 * @time: 2023/5/7 23:01
 */
public class LambdaTest {

  public static void main(String[] args) {
    // 1、子类实现接口
    Factory factory = new SubClass(); // 多态
    User user1 = (User) factory.getObject();
    System.out.println(user1);

    // 2、匿名内部类
    factory =
        new Factory() {
          @Override
          public Object getObject() {
            return new User("张三", 18);
          }
        };

    User user2 = (User) factory.getObject();
    System.out.println(user2);

    // 3、Lambda表达式
    factory =
        () -> {
          return new User("李四", 20);
        };
    User user3 = (User) factory.getObject();
    System.out.println(user3);

    // 4、Lambda作为参数进行传递
    User user4 =
        getUserFromFactory(
            () -> {
              return new User("王五", 22);
            },
            User.class.getSimpleName());
    System.out.println(user4);

    // 5、Lambda作为函数返回值
    factory = getFactory();
    System.out.println(factory.getObject());
  }

  public static User getUserFromFactory(Factory factory, String beanName) {
    Object object = factory.getObject();
    if (object != null && object.getClass().getSimpleName().equals(beanName)) {
      return (User) object;
    }
    return null;
  }

  public static Factory getFactory() {
    return () -> {
      return new User("赵六", 24);
    };
  }
}

5、核心代码块

5.1、核心代码块-Lambda表达式

// 3、Lambda表达式
factory =
    () -> {
        return new User("李四", 20);
    };
User user3 = (User) factory.getObject();
System.out.println(user3);

5.2、核心代码块-Lambda作为参数进行传递

// 4、Lambda作为参数进行传递
User user4 =
    getUserFromFactory(
        () -> {
            return new User("王五", 22);
        },
        User.class.getSimpleName());
System.out.println(user4);

......

public static User getUserFromFactory(Factory factory, String beanName) {
    Object object = factory.getObject();
    if (object != null && object.getClass().getSimpleName().equals(beanName)) {
        return (User) object;
    }
    return null;
}

5.3、核心代码块-Lambda作为函数返回值

// 5、Lambda作为函数返回值
factory = getFactory();
System.out.println(factory.getObject());

......

public static Factory getFactory() {
    return () -> {
        return new User("赵六", 24);
    };
}