[Java知识库]Java8新特性（一）：初识Lamda表达式

Java8相较于之前的版本有哪些优点：

1、运行速度更快（内存管理和垃圾回收机制发生了改变）

2、代码更简洁（因为使用了Lamda表达式）

3、强大的Stream API

4、便于并行

5、使用Optional可减少空指针异常

最为核心的的两大特性：Lamda表达式和Stream API

?如何理解Lamda表达式？

Lambda将功能实现（即模型中的 {代码块}）作为参数进行传递。

Lambda式子中,"->“前面的是需要实现的抽象方法的形参,没有形参就”()"为空处理,后面的是实现抽象方法的方法体。即（可省略类型，也可省略参数的参数列表）->{ 代码块 }

如下是Lamda表达式的代码转换模型图：

?最简单的Lamda演示实例：

    // 原来的匿名内部类
    public void test01() {
        Comparator<Integer> com = new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return Integer.compare(o1, o2);
            }
        };
    }

    // Lamda 表达式
    public void test02() {
        Comparator<Integer> com1 = (x,y) -> {return Integer.compare(x, y);};
        Comparator<Integer> com2 = (x,y) -> Integer.compare(x, y);
    }

使用Java8的语法特性优化代码

    List<Employee> employeeList = Arrays.asList(
            new Employee("张三",18,3000),
            new Employee("李四",28,8000),
            new Employee("王五",38,9000),
            new Employee("陈六",48,7000),
            new Employee("李七",58,2000)
    );

    private List<Employee> filterEmployee(List<Employee> list, MyPredicate predicate){
        List<Employee> result = new ArrayList<>();
        for(Employee emp : list){
            if(predicate.filter(emp)){
                result.add(emp);
            }
        }
        return result;
    }

    // 传统的写法
    @Test
    public void test01(){
        // 过滤出年龄于50的
        List<Employee> list01 = new ArrayList<>();
        for(Employee employee : employeeList){
            if(employee.getAge() > 50){
                list01.add(employee);
            }
        }
        for(Employee employee : list01){
            System.out.println(employee);
        }

        // 过滤出工资大于4000的
        List<Employee> list02 = new ArrayList<>();
        for(Employee employee : employeeList){
            if(employee.getSalary() > 5000){
                list02.add(employee);
            }
        }
        for(Employee employee : list02){
            System.out.println(employee);
        }
    }

    // 优化方式一：使用设计模式之：策略模式
    @Test
    public void test02(){
        List<Employee> list01 = filterEmployee(employeeList, new EmployeeFilterByAge());
        for (Employee emp : list01) {
            System.out.println(emp);
        }
        System.out.println("--------------------------------");

        List<Employee> list02 = filterEmployee(employeeList, new EmployeeFilterBySalary());
        for (Employee emp : list02) {
            System.out.println(emp);
        }

    }

    // 优化方式二：使用匿名内部类
    @Test
    public void test03(){
        List<Employee> list = filterEmployee(employeeList, new MyPredicate<Employee>() {
            @Override
            public boolean filter(Employee emp) {
                return emp.getAge() > 50;
            }
        });
        for (Employee emp : list) {
            System.out.println(emp);
        }
    }

    // 优化方式三：使用Lambda表达式
    @Test
    public void test04(){
        List<Employee> list = filterEmployee(employeeList, (MyPredicate<Employee>) emp -> emp.getAge() > 50);
        list.forEach(System.out::println);
    }

    // 优化方式四：使用Stream API
    @Test
    public void test05(){
        employeeList.stream().filter((e) -> e.getAge() >50).forEach(System.out::println);
    }

相关类的定义

public class Employee {

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

    public String name;

    public int age;

    public int salary;

    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 int getSalary() {
        return salary;
    }

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

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

public interface MyPredicate<T> {

    public boolean filter(T t);
}

public class EmployeeFilterByAge implements MyPredicate<Employee> {

    @Override
    public boolean filter(Employee employee) {
        return false;
    }
}

public class EmployeeFilterBySalary implements MyPredicate<Employee> {
    @Override
    public boolean filter(Employee emp) {
        return emp.salary > 5000;
    }
}