JDK8新特性01 Lambda表达式01_设计的由来

1.java bean 

public class Employee {

	private int id;
	private String name;
	private int age;
	private double salary;

	public Employee() {
	}

	public Employee(String name) {
		this.name = name;
	}

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

	public Employee(int id, String name, int age, double salary) {
		this.id = id;
		this.name = name;
		this.age = age;
		this.salary = salary;
	}

	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	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;
	}

	public double getSalary() {
		return salary;
	}

	public void setSalary(double salary) {
		this.salary = salary;
	}

	public String show() {
		return "测试方法引用！";
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + age;
		result = prime * result + id;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		long temp;
		temp = Double.doubleToLongBits(salary);
		result = prime * result + (int) (temp ^ (temp >>> 32));
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Employee other = (Employee) obj;
		if (age != other.age)
			return false;
		if (id != other.id)
			return false;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
			return false;
		return true;
	}

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

}

2.早期Java版本的设计策略和8版本的Lambda的接口类：

//1.策略模式的定义接口
@FunctionalInterface
public interface MyPredicate<T> {
	public boolean test(T t);
	
}

//2.策略模式的接口实现类1
public class FilterEmployeeForAge implements MyPredicate<Employee>{
	@Override
	public boolean test(Employee t) {
		return t.getAge() <= 35;
	}

}

//3.策略模式的接口实现类2
public class FilterEmployeeForSalary implements MyPredicate<Employee> {
	@Override
	public boolean test(Employee t) {
		return t.getSalary() >= 5000;
	}

}

//4.lambda表达式定义的接口
@FunctionalInterface
public interface MyFun {
	public Integer getValue(Integer num);
	
}

3.原来的实现以及现在的Lambda的优化

package com.atguigu.java8;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.TreeSet;

import org.junit.Test;

public class TestLambda1 {
	
	//原来的匿名内部类
	@Test
	public void test1(){
		Comparator<String> com = new Comparator<String>(){
			@Override
			public int compare(String o1, String o2) {
				return Integer.compare(o1.length(), o2.length());
			}
		};
		
		TreeSet<String> ts = new TreeSet<>(com);
		
		TreeSet<String> ts2 = new TreeSet<>(new Comparator<String>(){
			@Override
			public int compare(String o1, String o2) {
				return Integer.compare(o1.length(), o2.length());
			}
			
		});
	}
	
	//现在的 Lambda 表达式
	@Test
	public void test2(){
		Comparator<String> com = (x, y) -> Integer.compare(x.length(), y.length());
		TreeSet<String> ts = new TreeSet<>(com);
	}
	
	List<Employee> emps = Arrays.asList(
			new Employee(101, "张三", 18, 9999.99),
			new Employee(102, "李四", 59, 6666.66),
			new Employee(103, "王五", 28, 3333.33),
			new Employee(104, "赵六", 8, 7777.77),
			new Employee(105, "田七", 38, 5555.55)
	);

	//需求：获取公司中年龄小于 35 的员工信息
	public List<Employee> filterEmployeeAge(List<Employee> emps){
		List<Employee> list = new ArrayList<>();
		
		for (Employee emp : emps) {
			if(emp.getAge() <= 35){
				list.add(emp);
			}
		}
		
		return list;
	}
	
	@Test
	public void test3(){
		List<Employee> list = filterEmployeeAge(emps);
		
		for (Employee employee : list) {
			System.out.println(employee);
		}
	}
	
	//需求：获取公司中工资大于 5000 的员工信息
	public List<Employee> filterEmployeeSalary(List<Employee> emps){
		List<Employee> list = new ArrayList<>();
		
		for (Employee emp : emps) {
			if(emp.getSalary() >= 5000){
				list.add(emp);
			}
		}
		
		return list;
	}
	
	//优化方式一：策略设计模式
	public List<Employee> filterEmployee(List<Employee> emps, MyPredicate<Employee> mp){
		List<Employee> list = new ArrayList<>();
		
		for (Employee employee : emps) {
			if(mp.test(employee)){
				list.add(employee);
			}
		}
		
		return list;
	}
	
    //策略模式的调用
	@Test
	public void test4(){
        //策略1
		List<Employee> list = filterEmployee(emps, new FilterEmployeeForAge());
		for (Employee employee : list) {
			System.out.println(employee);
		}
		
		System.out.println("------------------------------------------");
		
        //策略2
		List<Employee> list2 = filterEmployee(emps, new FilterEmployeeForSalary());
		for (Employee employee : list2) {
			System.out.println(employee);
		}
	}
	
	//优化方式二：匿名内部类.（策略模式每次都需要创建新的实现类，麻烦）
	@Test
	public void test5(){
		List<Employee> list = filterEmployee(emps, new MyPredicate<Employee>() {
			@Override
			public boolean test(Employee t) {
				return t.getId() <= 103;
			}
		});
		
		for (Employee employee : list) {
			System.out.println(employee);
		}
	}

	//优化方式三：Lambda 表达式
    //精简匿名内部类的实现，将方法的实现在方法的参数中传递，不再new
	@Test
	public void test6(){
		List<Employee> list = filterEmployee(emps, (e) -> e.getAge() <= 35);
		list.forEach(System.out::println);
		
		System.out.println("------------------------------------------");
		
		List<Employee> list2 = filterEmployee(emps, (e) -> e.getSalary() >= 5000);
		list2.forEach(System.out::println);
	}
	

	//优化方式四：Stream API
	@Test
	public void test7(){
		emps.stream()
			.filter((e) -> e.getAge() <= 35)
			.forEach(System.out::println);
		
		System.out.println("----------------------------------------------");
		
		emps.stream()
			.map(Employee::getName)
			.limit(3)
			.sorted()
			.forEach(System.out::println);
	}
}