体验lambda表达式

@Test
public void lambdaTest01(){
    // 老的写法
   Runnable runnable = new Runnable() {
       @Override
       public void run() {
           System.out.println("Run 方法执行了");
       }
   };
   runnable.run();
   // 新的写法
   Runnable runnable1 = ()->System.out.println("lambda run");
   runnable1.run();
}

lambda 在写法上更加的间接，写的代码行数更少

lambda的语法格式：(形参1,形参2...形参n) -> 重写抽象方法的方法体

无参数，无返回值

@Test
public void lambdaTest02(){
    Runnable runnable = () -> { System.out.println("runnable run"); };
    runnable.run();
    Runnable runnable1 = ()-> System.out.println("runnable run");
    runnable1.run();
}

有参数，没有返回值

@Test
public void lambdaTest03(){
    Consumer consumer = (monry) -> System.out.println("肯德基的价格是："+monry);
    consumer.accept(100);
    Consumer consumer1 = (money) -> { System.out.println("汉堡的价格是：" + money); };
    consumer1.accept(100);
    Consumer consumer2 = (mv) -> { System.out.println("我爱看：" + mv); };
    consumer2.accept("四驱兄弟");
}

有一个参数，小括号可以省略

@Test
public void lambdaTest04(){
    Consumer consumer1 = money ->  System.out.println("汉堡的价格是：" + money);
    consumer1.accept(100);
}

有参数，有返回值

@Test
public void lambdaTest05(){
    Comparator<Integer> comparator = (o1, o2) -> {return o1.compareTo(o2);};
    int compare = comparator.compare(25, 20);
    System.out.println(compare);

    Comparator<Integer> comparator1 = (o1,o2) -> o1.compareTo(o2);
    int compare1 = comparator1.compare(25, 20);
    System.out.println(compare1);
}

认识函数式接口

一个接口里只有一个抽象方法，并且在这个抽象方法上标注FunctionalInterface注解的。

Java内置四大核心函数式接口

演示案例

@Test
public void lambdaTest06(){
    //Consumer
    Consumer consumer = (name -> {
        System.out.println("我叫：" + name);
    });
    consumer.accept("牛某某");
    //Supplier
    Supplier supplier = () -> {return "You are Smart";};
    Object o = supplier.get();
    System.out.println(o);
    //Function
    Function<Integer,Double> function = (num1) -> {return Double.valueOf(num1); };
    Double apply = function.apply(10);
    System.out.println(apply);
    //Predicate
    Predicate<Integer> predicate = (number) -> {return  number >= 20;};
    boolean test = predicate.test(20);
    System.out.println(test);
}

运行结果：

方法引用

实例::对象方法

Consumer 中的 void accept(T t)

PrintStream 中的 void println(T t)

@Test
public void lambdaTest07(){
    Consumer<String> con1 = s -> System.out.println(s);
    con1.accept("上海");
    System.out.println("******************");
    PrintStream ps = System.out;
    Consumer<String> con2 = ps::println;
    con2.accept("北京");
}

@Test
public void lambdaTest08(){
    // Supplier    T get();
    // Employee    int getAge()
    Employee employee = new Employee();
    employee.setAge(18);
    Supplier<Integer> supplier = employee::getAge;
    Integer age = supplier.get();
    System.out.println(age);
    //Consumer  void accept(T t);
    //Employee  void setSalary(double salary)
    Consumer<Double> consumer = employee::setSalary;
    consumer.accept(100.00);
    System.out.println(employee.getSalary());
}

类::静态方法

@Test
public void lambdaTest09(){
    //lambda表达式的写法
    Comparator<Integer> com = (t1,t2) -> Integer.compare(t1,t2);
    System.out.println(com.compare(12, 21));

    //方法引用的写法
    Comparator<Integer> com1 = Integer::compare;
    System.out.println(com1.compare(10, 20));
}

类::非静态方法

@Test
public void lambdaTest10(){
    Comparator<String> com1 = (s1,s2) -> s1.compareTo(s2);
    System.out.println(com1.compare("abc", "def"));
    //Comparator   int compare(T o1, T o2);
    //String中的    int o1.compareTo(o2)
    Comparator<String> com2 = String::compareTo;
    System.out.println(com2.compare("abc", "def"));
}

构造器引用

@Test
public void lambda11(){
    //Supplier T get();
    //Employee Employee() 
    System.out.println();
    //lambda表达式写法
    Supplier<Employee> sup = ()->new Employee();
    System.out.println(sup.get());
    //构造器引用的写法
    Supplier<Employee> sup1 = Employee :: new;
    System.out.println(sup1.get());
}

数组引用

@Test
public void lambda11(){
    //lambda 表达式
    Function<Integer,String[]> func1 = length -> new String[length];
    String[] apply = func1.apply(5);
    System.out.println(Arrays.toString(apply));
    System.out.println("******************************************");
    //数组引用的写法
    Function<Integer,String[]> func2 = String[] :: new;
    String[] apply1 = func2.apply(10);
    System.out.println(Arrays.toString(apply1));
}

StreamAPI

Stream 的三个操作步骤

1- 创建 Stream

一个数据源（如：集合、数组），获取一个流

2- 中间操作

一个中间操作链，对数据源的数据进行处理

3- 终止操作(终端操作)

一旦执行终止操作，就执行中间操作链，并产生结果。之后，不会再被使用

Stream 实例化

package com.java.entity;

/**
 * 其他的方法 请自行生成
 */
public class Employee {

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


/**
 * 提供用于测试的数据
 * 
 * @author shkstart 邮箱：shkstart@126.com
 *
 */
public class EmployeeData {
    
    public static List<Employee> getEmployees(){
        List<Employee> list = new ArrayList<>();
        list.add(new Employee(1001, "马化腾", 34, 6000.38));
        list.add(new Employee(1002, "马云", 12, 9876.12));
        list.add(new Employee(1003, "刘强东", 33, 3000.82));
        list.add(new Employee(1004, "雷军", 26, 7657.37));
        list.add(new Employee(1005, "李彦宏", 65, 5555.32));
        list.add(new Employee(1006, "比尔盖茨", 42, 9500.43));
        list.add(new Employee(1007, "任正非", 26, 4333.32));
        list.add(new Employee(1008, "扎克伯格", 35, 2500.32));
        list.add(new Employee(1003, "刘强东", 33, 3000.82));
        return list;
    }    
}

实例化方式一：通过集合

// default Stream<E> stream() : 返回一个顺序流
// default Stream<E> parallelStream() : 返回一个并行流
List<Employee> employees = EmployeeData.getEmployees();
//返回一个顺序流
Stream<Employee> stream = employees.stream();
//返回一个并行流
Stream<Employee> parallelStream = employees.parallelStream();

实例化方式二：通过数组

//static <T> Stream<T> stream(T[] array): 返回一个流    
@Test
    public void streamTest1(){
        int[] arr = new int[]{1,2,3,4,5,6};
        IntStream stream = Arrays.stream(arr);

        Employee e1 = new Employee(1001, "Tom");
        Employee e2 = new Employee(1002, "lucy");

        Employee[] arr1 = {e1, e2};
        Stream<Employee> stream1 = Arrays.stream(arr1);
    }

实例化方式三：通过Stream.of

// public static<T> Stream<T> of(T... values) : 返回一个流
@Test
public void streamTest2(){
  Stream<Integer> stream = Stream.of(1, 2, 3, 4, 5, 6);
}

创建无限流

//迭代
//public static<T> Stream<T> iterate(final T seed, final UnaryOperator<T> f) 
//生成
//public static<T> Stream<T> generate(Supplier<T> s) 
@Test
public void streamTest3(){
//遍历前10个数
Stream.iterate(0,t->t+2).limit(10).forEach(System.out::println);
//生成10个数
Stream.generate(Math::random).limit(10).forEach(System.out::println);
}

Stream的中间操作

筛选与切片

   @Test
    public void streamTest4(){
        List<Employee> list = EmployeeData.getEmployees();
        //filter
        Stream<Employee> stream = list.stream();
        stream.filter(e -> e.getSalary()>7000).forEach(System.out::println);
        System.out.println();
        //limit
        list.stream().limit(3).forEach(System.out::println);
        System.out.println();
        //skip
        list.stream().skip(4).forEach(System.out::println);
        System.out.println();
        //distinct
        list.stream().distinct().forEach(System.out::println);
    }

映射

    @Test
    public void streamTest5(){
        //map
        List<String> list = Arrays.asList("aa", "bb", "cc", "dd");
        list.stream().map(str -> str.toUpperCase()).forEach(System.out::println);

        //小练习：获取员工姓名长度大于3的员工的姓名
        List<Employee> employees = EmployeeData.getEmployees();
        Stream<String> nameStream = employees.stream().map(Employee::getName);
        nameStream.filter(name -> name.length()>3).forEach(System.out::println);

        Stream<Stream<Character>> streamStream = list.stream().map(jdk8Test2Stream::fromStringStream);
        streamStream.forEach(s -> {
            s.forEach(System.out::println);
        });
        System.out.println("****************************************************************");
        //flagMap
        Stream<Character> characterStream = list.stream().flatMap(jdk8Test2Stream::fromStringStream);
        characterStream.forEach(System.out::println);
    }

    public static Stream<Character> fromStringStream(String str){
        ArrayList<Character> list = new ArrayList<>();
        for (Character c:str.toCharArray()) {
            list.add(c);
        }
        return list.stream();
    }

map(Function f)	接收一个函数作为参数，该函数会被应用到每个元素上，并将其映射成一个新的元素。
mapToDouble(ToDoubleFunction f)	接收一个函数作为参数，该函数会被应用到每个元素上，产生一个新的 DoubleStream。
mapToInt(ToIntFunction f)	接收一个函数作为参数，该函数会被应用到每个元素上，产生一个新的 IntStream。
mapToLong(ToLongFunction f)	接收一个函数作为参数，该函数会被应用到每个元素上，产生一个新的 LongStream。
flatMap(Function f)	接收一个函数作为参数，将流中的每个值都换成另一个流，然后把所有流连接成一个流

方法	描述
filter(Predicate p)	接收 Lambda ，从流中排除某些元素
distinct()	筛选，通过流所生成元素的 hashCode() 和 equals() 去除重复元素
limit(long maxSize)	截断流，使其元素不超过给定数量
skip(long n)	跳过元素，返回一个扔掉了前 n 个元素的流。若流中元素不足 n 个，则返回一个空流。与 limit(n) 互补

排序

    @Test
    public void streamTest6(){
         //sorted 自然排序
        List<Integer> list = Arrays.asList(12, 34, 65, 34, 877, 0, -98, 7);
        list.stream().sorted().forEach(System.out::println);
        //sorted(Comparator com) 定制排序
        List<Employee> employees = EmployeeData.getEmployees();
        employees.stream().sorted((e1,e2) ->                                              Integer.compare(e1.getAge(),e2.getAge())).forEach(System.out::println);
    }

Stream的终止操作

匹配和查找

方法	描述
allMatch(Predicate p)	检查是否匹配所有元素
anyMatch(Predicate p)	检查是否至少匹配一个元素
noneMatch(Predicate p)	检查是否没有匹配所有元素
findFirst()	返回第一个元素
findAny()	返回当前流中的任意元素
count()	返回流中元素总数
max(Comparator c)	返回流中最大值
min(Comparator c)	返回流中最小值
forEach(Consumer c)	内部迭代(使用 Collection 接口需要用户去做迭代，称为外部迭代。相反，Stream API 使用内部迭代——它帮你把迭代做了)

    @Test
    public void streamTest7(){
        List<Employee> employees = EmployeeData.getEmployees();

        //allMatch
        boolean allMatch = employees.stream().allMatch(e -> e.getAge() > 18);
        System.out.println(allMatch);

        //anyMatch
        boolean anyMatch = employees.stream().anyMatch(e -> e.getSalary() > 10000);
        System.out.println(anyMatch);

        //noneMatch
        boolean noneMatch = employees.stream().noneMatch(e -> e.getName().contains("雷"));
        System.out.println(noneMatch);

        //findFirst
        Optional<Employee> findFirst = employees.stream().findFirst();
        System.out.println(findFirst);
        
        //findAny
        Optional<Employee> findAny = employees.stream().findAny();
        System.out.println(findAny);

        //count
        long count = employees.stream().count();
        System.out.println(count);

        //max
        Stream<Double> salaryStream = employees.stream().map(e -> e.getSalary());
        Optional<Double> max = salaryStream.max(Double::compareTo);
        System.out.println(max);

        //min
        Optional<Employee> employee = employees.stream().min((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary()));
        System.out.println(employee);

        //forEach
        employees.stream().forEach(System.out::println);
    }

规约

方法	描述
reduce(T iden, BinaryOperator b)	可以将流中元素反复结合起来，得到一个值。返回 T
reduce(BinaryOperator b)	可以将流中元素反复结合起来，得到一个值。返回 Optional

    @Test
    public void streamTest8(){
        List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
        //reduce
        Integer sum = list.stream().reduce(0, Integer::sum);
        System.out.println(sum);
        //reduce(BinaryOperator b)
        List<Employee> employees = EmployeeData.getEmployees();
        Stream<Double> doubleStream = employees.stream().map(Employee::getSalary);
        Optional<Double> reduce = doubleStream.reduce((d1, d2) -> d1 + d2);
        System.out.println(reduce);
        //Optional<Double> sumMoney = doubleStream.reduce(Double::max);
        //System.out.println(sumMoney);
    }

收集

方法	规约
collect(Collector c)	将流转换为其他形式。接收一个 Collector 接口的实现，用于给Stream中元素做汇总的方法

    @Test
    public void streamTest9(){
        List<Employee> employees = EmployeeData.getEmployees();
        List<Employee> employeeList = employees.stream().filter(e -> e.getSalary() > 6000).collect(Collectors.toList());
        employeeList.forEach(System.out::println);
        System.out.println();
        Set<Employee> employeeSet = employees.stream().filter(e -> e.getSalary() > 6000).collect(Collectors.toSet());
        employeeSet.forEach(System.out::println);
    }

Optional类

Optional类的相关方法

Option类的使用

创建两个新的实体类

public class Boy {
    private Girl girl;
    public Boy() {}
    public Boy(Girl girl) {
        this.girl = girl;
    }
    public Girl getGirl() {
        return girl;
    }
    public void setGirl(Girl girl) {
        this.girl = girl;
    }

    @Override
    public String toString() {
        return "Boy{" +
                "girl=" + girl +
                '}';
    }
}

public class Girl {
    private String name;
    public Girl(String name) {
        this.name = name;
    }
    public Girl() {}
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @Override
    public String toString() {
        return "Girl{" +
                "name='" + name + ''' +
                '}';
    }
}

    @Test
    public void optionTest1(){
        Girl girl = new Girl();
        Optional<Girl> girl1 = Optional.of(girl);
        System.out.println(girl1);
    }

    @Test
    public void optionalTest2(){
        Girl girl = new Girl();
        girl = null;
        Optional<Girl> girl1 = Optional.ofNullable(girl);
        System.out.println(girl1);
    }

    @Test
    public void optionalTest3(){
        Optional<Object> empty = Optional.empty();
        System.out.println(empty);
    }

Optional类的使用举例

//如何有效避免空指针
    @Test
    public void optionalTest4(){
        Girl girl = new Girl();
        girl = null;
        Optional<Girl> optionalGirl = Optional.ofNullable(girl);
        System.out.println(optionalGirl);
        System.out.println("***************************");
        /**
         * orElse(T t)
         *   如果optional内部封装的T是非空的，则返回内部的t
         *   如果optional内部封装的T是空的，则返回orElse()方法中的参数
         */
        Girl otherGirl = optionalGirl.orElse(new Girl("嗯哼"));
        System.out.println(otherGirl);
    }

Lambda的使用方法