第十四章 Lambda 表达式与流处理

14.1.1 Lambda 表达式简介

lambda 表达式可以用非常少的代码实现抽象方法。lambda 表达式不能独立执行，因此必须实现函数式接口，并且会返回一个函数式接口的对象。lambda表达式的语法非常特殊语法如下：

() -> 结果表达式
//无参
参数 -> {代码块}
//一个参数
(参数1,参数2,...,参数n) -> {代码块}
//多个参数

2.Iambda表达式实现无参数抽象方法

例题：

package 十四;

interface SayHi{
	String say();
}

public class NoParamDemo {

	public static void main(String[] args) {
		SayHi sh1 = new SayHi() {
			public String say() {
				return "这是匿名内部类";
			}
		};
		System.out.print(sh1.say());
		
		SayHi sh2 = () ->{return "这是匿名内部类";};
		System.out.print(sh2.say());
	}

}

3.lambda表达式实现有参抽象方法

如果抽象方法中只有一个参数，lambda表达式则可以省略圆括号

package 十四;

interface AddInt{
	int add(int a,int b);
}
public class ParamDemo {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		AddInt ai1 = new AddInt() {
			public int add(int a,int b) {
				return a + b;
			}
		};
		System.out.println("匿名内部类：" + ai1.add(3,5));
		
		AddInt ai2 = (a,b) -> {return a + b;};
		System.out.println("匿名内部类：" + ai2.add(3,5));
		
	}
}

 

4.lambda表达式使用代码块

lambda表达式会自动判断返回值类型是否符合抽象方法的定义

package 十四;
interface CheckGrade{
    String check(int grade);
}

public class GradeDemo {

	public static void main(String[] args) {

	  CheckGrade g = (n) ->{
            if (n >= 90 && n <= 100){
                return "成绩为优";
            } else if (n >= 80 && n < 90) {
                return "成绩为良";
            }else if (n >= 60 && n < 80) {
                return "成绩为中";
            }else if (n >= 00 && n < 60) {
                return "成绩无差";
            }else {
            	return "成绩无效";
            }
        };
        System.out.println(g.check(89));
	}

}

方法的引用 

引用静态方法

语法：

类名：：静态方法名 

新的操作符“：：”，中间无空格，左边表示方法所属的类名，右边是方法名，语法中方法名是没有括号的

例题

package 十四;

interface StaticMethodInteeface{
	int method(int a,int b);
}
public class StaticMethodDemo {
	static int add(int a,int b) {
		return a + b;
	}
	public static void main(String[] args) {
		StaticMethodInteeface sm = StaticMethodDemo::add;
		int result = sm.method(15, 16);
		System.out.println("接口方法结果：" + result);
	}
}

 引用成员方法

引用成员方法的语法：

对象名：：成员方法名

操作符左侧必须是一个对象名，而不是类名

package 十四;

import java.text.SimpleDateFormat;
import java.util.Date;

interface InstanceMethodInterface{
	String method(Date date);
}
public class InstanceMethodDemo {
	public String format(Date date) {
		SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
		return sdf.format(date);
	}
	
	public static void main(String[] args) {
		InstanceMethodDemo demo = new InstanceMethodDemo();
		InstanceMethodInterface im = demo::format;
		Date date = new Date();
		System.out.println("默认格式" + date);
		System.out.println("接口输出模式" + im.method(date));
	}
}

 

 

package 十四;

interface ConstructorsInterface{
	ConstructorsDemo action(int a);
}
public class ConstructorsDemo {
	
	public ConstructorsDemo() {
		System.out.print("无参构造方法");
	}
	public ConstructorsDemo(int a) {
		System.out.print("有参构造方法" + a);
	}
	
	public static void main(String[] args) {
		ConstructorsInterface ci = ConstructorsDemo::new;
		ci.action(8);
	}
}

流处理 

流处理有点类似数据库的 SQL 语句，可以执行非常复杂的过滤、映射、查找和收集功能，并且代码量很少。唯一的缺点是代码可读性不高。

对员工数据进行流处理，常见员工表类

package 十四;

import java.util.ArrayList;
import java.util.List;

public class Employee {
	private String name;
	private int age;
	private double salary;
	private String sex;
	private String dept;
	
	public Employee(String name, int age, double salary, String sex, String depy) {
		super();
		this.name = name;
		this.age = age;
		this.salary = salary;
		this.sex = sex;
		this.dept = depy;
	}

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

	public String getName() {
		return name;
	}
	public int getAge() {
		return age;
	}
	public double getSalary() {
		return salary;
	}
	public String getSex() {
		return sex;
	}
	public String getDept() {
		return dept;
	}
	
	static List getEmployee(){
		List list = new ArrayList();
		list.add(new Employee("老张",40,9000.0,"男","运营部"));
		list.add(new Employee("小刘",24,5000.0,"女","开发部"));
		list.add(new Employee("大纲",32,7500.0,"男","销售部"));
		list.add(new Employee("翠花",28,5500.0,"女","销售部"));
		list.add(new Employee("小马",21,3000.0,"男","开发部"));
		list.add(new Employee("老王",35,6000.0,"女","人事部"));
		list.add(new Employee("小王",21,3000.0,"女","人事部"));
		return list;
	}
}

用流筛选年龄大与30 

package 十四;

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class StreamDemo {
	public static void main(String[] args) {
		List list = Employee.getEmployee();
		Stream stream = list.stream();
		
		stream = stream.filter(people -> people.getAge() > 30) //筛选年龄大于30的员工
				.limit(1);//限制条数
		List result = stream.collect(Collectors.toList());
		for(Employee emp : result) {
			System.out.println(emp);
		}
	}
}

用流筛选

package 十四;

import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class GroupDemo {
	public static void main(String[] args) {
		List list = Employee.getEmployee();
		Stream stream = list.stream();
		Map> map = stream.collect(Collectors.groupingBy(Employee::getDept));
		Set depts = map.keySet();
		for(String dept : depts) {
			System.out.println(dept + "员工列表如下");
			List temp = map.get(dept);
			for(Employee e : temp) {
				System.out.println(e);
			}
			System.out.println();
		}
	}
}