JDK8新特性01 Lambda表达式01_设计的由来

Posted 2021-02-05 guchunchao

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：

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

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);
    }
}