Java Lambda Stream API 吊打Java面试官之Lambda表达式 Stream API（java设计模式）

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public class LambdaTest {
    @Test
    public void test1(){
        Runnable r1 = new Runnable() {
            @Override
            public void run() {
                System.out.println("我爱北京天安门");
            }
        };
        r1.run();
        System.out.println("******************************************");
        Runnable r2 = () -> System.out.println("我爱北京故宫");
        r2.run();
    }
    @Test
    public void test2(){
        Comparator<Integer> com1 = new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return Integer.compare(o1,o2);
            }
        };
        int compare1 = com1.compare(12,21);
        System.out.println(compare1);
        System.out.println("**********************************");
        //Lambda表达式的写法
        Comparator<Integer> com2 = (o1,o2) -> Integer.compare(o1,o2);
        int compare2 = com2 .compare(32,21);
        System.out.println(compare2);
        System.out.println("************************************");
        //方法引用
        Comparator<Integer> com3 = Integer :: compare;
        int compare3 = com3.compare(32,21);
        System.out.println(compare2);
    }

public class LambdaTest1 {
    //语法格式一：无参，无返回值
    @Test
    public void test1(){
        Runnable r1 = new Runnable() {
            @Override
            public void run() {
                System.out.println("我爱北京天安门");
            }
        };
        r1.run();
        System.out.println("******************************************");
        Runnable r2 = () -> {
            System.out.println("我爱北京故宫");
        };
 
        r2.run();
    }
    //语法格式二：Lambda需要一个参数，但是没有返回值。
    @Test
    public void test2(){
        Consumer<String> con = new Consumer<String>() {
 
            @Override
            public void accept(String s) {
                System.out.println(s);
            }
        };
        con.accept("谎言和誓言的区别什么？");
        System.out.println("**********************************");
        Consumer<String>con1 = (String s) -> {
            System.out.println(s);
        };
        con1.accept("一个是听得人当真了，一个是说的人当真了");
    }
    //语法格式三：数据类型可以省略，因为可由编译器推断得出，称为"类型推断"
    @Test
    public void test3(){
        Consumer<String> con1 = (String s) -> {
            System.out.println(s);
        };
        con1.accept("一个是听的人当真了，一个是说的人当真了");
        System.out.println("****************************");
        Consumer<String> con2 = (s) -> {
            System.out.println(s);
        };
        con2.accept("一个是听的人当真了，一个是说的人当真了");
    }
    @Test
    public void test4(){
        ArrayList<String> objects = new ArrayList<>();//类型推断(泛型)
        int[] arr = {1,2,3};//类型推断(int[] arr = new int[]{1,2,3};)
    }
    //语法格式四：Lambda若只需要一个参数时，参数的小括号可以省略
    @Test
    public void test5(){
        Consumer<String> con1 = (s) -> {
            System.out.println(s);
        };
        con1.accept("一个是听的人当真了，一个是说的人当真了");
        System.out.println("****************************");
        Consumer<String> con2 = s -> {
            System.out.println(s);
        };
        con2.accept("一个是听的人当真了，一个是说的人当真了");
    }
    //语法格式五：Lambda需要两个或两个以上的参数，多条执行语句，并且可以有返回值
    @Test
    public void test6(){
        Comparator<Integer> com1 = new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                System.out.println(o1);
                System.out.println(o2);
                return Integer.compare(o1,o2);
            }
        };
        System.out.println(com1.compare(12,21));
        System.out.println("*******************************");
        Comparator<Integer> com2 = (o1,o2) -> {
            System.out.println(o1);
            System.out.println(o2);
            return o1.compareTo(o2);
        };
        System.out.println(com2.compare(12,6));
    }
    //语法格式六：当Lambda体只有一条语句时，return与大括号若有，都可以省略
    @Test
    public void test7(){
        Comparator<Integer> com1 = (o1,o2) ->{
            return o1.compareTo(o2);
        };
        System.out.println(com1.compare(12,6));
        System.out.println("******************************");
        Comparator<Integer> com2 = (o1,o2) -> o1.compareTo(o2);
        System.out.println(com2.compare(12,21));
    }
    @Test
    public void test8(){
        Consumer<String> con1 = s -> {
            System.out.println(s);
        };
        con1.accept("一个是听的人当真了，一个是说的人当真了");
        System.out.println("*********************************");
 
        Consumer<String> con2 = s -> System.out.println(s);
        con2.accept("一个是听的人当真了，一个是说的人当真了");
    }
}

java内置的4大核心函数式接口
 消费型接口Consumer<T>       void accept(T t)
 供给型接口Supplier<T>       T get()
 函数式接口Function<T,R>     R apply(T t)
 断定型接口Predicate<T>      boolean test(T t)
 
public class LambdaTest2 {
    @Test
    public void test1(){
        happyTime(500, new Consumer<Double>() {
            @Override
            public void accept(Double aDouble) {
                System.out.println("学习太累了，去天上人间买了瓶矿泉水，价格为：" + aDouble);
            }
        });
        System.out.println("*************************************");
        happyTime(400,money -> System.out.println("学习太累了，去天上人间喝两口矿泉水，价格为：" + money));
    }
    public void happyTime(double money, Consumer<Double> con){
        con.accept(money);
    }
    @Test
    public void test2(){
        List<String> list = Arrays.asList("北京", "南京", "天津", "东京", "西京", "普京");
        List<String> filterStrs = filterString(list, new Predicate<String>() {
            @Override
            public boolean test(String s) {
                return s.contains("京");
            }
        });
        System.out.println(filterStrs);
        List<String> filterStrs1 = filterString(list, s -> s.contains("京"));
        System.out.println(filterStrs1);
    }
    //根据给定的规则，过滤集合中的字符串，此规则由Predicate的方法决定
    public List<String> filterString(List<String> list,Predicate<String> pre){
        ArrayList<String> filterList = new ArrayList<>();
        for(String s : list){
            if(pre.test(s)){
                filterList.add(s);
            }
        }
        return filterList;
    }

 情况一：对象 :: 实例方法
    Consumer中的void accept(T t)
    PrintStream中的void println(T t)
    @Test
    public void test1(){
        Consumer<String> con1 = str -> System.out.println(str);
        con1.accept("北京");
        System.out.println("**************************");
        PrintStream ps = System.out;
        Consumer<String> con2 = ps :: println;
        con2.accept("beijing");
    }
    //Supplier中的T get()
    //Employee中的String getName()
    @Test
    public void test2(){
        Employee emp = new Employee(1001, "Tom", 23, 5600);
        Supplier<String> sup1 = () -> emp.getName();
        System.out.println(sup1.get());
        System.out.println("****************************");
        Supplier<String> sup2 = emp :: getName;
        System.out.println(sup2.get());
    }
    //情况二：类 :: 静态方法
    //Comparator中的int compare(T t1,T t2)
    //Integer中的int compare(T t1,T t2)
    @Test
    public void test3(){
        Comparator<Integer> com1 = (t1, t2) -> Integer.compare(t1,t2);
        System.out.println(com1.compare(12,21));
        System.out.println("*********************************");
        Comparator<Integer> com2 = Integer :: compare;
        System.out.println(com2.compare(12,3));
    }
    //Function中的R apply(T t)
    //Math中的Long round(Double d)
    @Test
    public void test4(){
        Function<Double, Long> func = new Function<Double, Long>() {
            @Override
            public Long apply(Double d) {
                return Math.round(d);
            }
        };
        System.out.println("************************************");
        Function<Double,Long> func1 = d -> Math.round(d);
        System.out.println(func1.apply(12.3));
 
        System.out.println("*****************************");
        Function<Double,Long> func2 = Math :: round;
        System.out.println(func2.apply(12.6));
    }
    //情况三：类 :: 实例方法(有难度)
    //String 中的 int t1.compareTo(t2)
    @Test
    public void test5(){
        Comparator<String> com1 = (s1,s2) -> s1.compareTo(s2);
        System.out.println(com1.compare("abc","abd"));
        System.out.println("*****************************");
        Comparator<String> com2 = String :: compareTo;
        System.out.println(com2.compare("abd","abm"));
    }
    //BiPredicate中的boolean test(T t1,T t2);
    //String中的boolean t1.equals(t2)
    @Test
    public void test6(){
        BiPredicate<String,String> pre1 = (s1, s2) -> s1.equals(s2);
        System.out.println(pre1.test("abc","abc"));
        System.out.println("**************************");
        BiPredicate<String,String> pre2 = String :: equals;
        System.out.println(pre1.test("abc","abd"));
    }
    //Function中的R apply(T t)
    //Employee中的String getName();
    @Test
    public void test7(){
        Employee employee = new Employee(1001, "Jerry", 23, 6000);
        Function<Employee,String> func1 = e -> e.getName();
        System.out.println(func1.apply(employee));
        System.out.println("****************************");
        Function<Employee,String> func2 = Employee :: getName;
        System.out.println(func2.apply(employee));
    }

构造器引用
    Supplier中的T get()
    Employee的空参构造器：Employee()
    @Test
    public void test1(){
        Supplier<Employee> sup = new Supplier<Employee>(){
            @Override
            public Employee get(){
                return new Employee();
            }
        };
        System.out.println("****************************");
        Supplier<Employee> sup1 = () -> new Employee();
        System.out.println(sup1.get());
        System.out.println("***********************************");
        Supplier<Employee> sup2 = Employee :: new;
        System.out.println(sup2.get());
    }
    //Function中的R apply(T t)
    @Test
    public void test2(){
        Function<Integer,Employee> func1 = id -> new Employee(id);
        Employee employee = func1.apply(1001);
        System.out.println(employee);
        System.out.println("********************************");
        Function<Integer,Employee> func2 = Employee :: new;
        Employee employee1 = func2.apply(1002);
        System.out.println(employee1);
    }
    //BiFunction中的R apply(T t,U u)
    @Test
    public void test3(){
        BiFunction<Integer,String,Employee> func1 = (id,name) -> new Employee(id,name);
        System.out.println(func1.apply(1001,"Tom"));
        System.out.println("**********************************");
        BiFunction<Integer,String,Employee> func2 = Employee :: new;
        System.out.println(func2.apply(1002,"Tom"));
    }
    //数组引用
    //Function中的R apply(T t)
    @Test
    public void test4(){
        Function<Integer,String[]> func1 = length -> new String[length];
        String[] arr1 = func1.apply(5);
        System.out.println(Arrays.toString(arr1));
        System.out.println("************************");
        Function<Integer,String[]> func2 = String[] :: new;
        String[] arr2 = func2.apply(10);
        System.out.println(Arrays.toString(arr2));
    }

//创建Stream方式一：通过集合
    @Test
    public void test1(){
        List<Employee>employees = EmployeeData.getEmployees();
//        default Stream<E> stream(): 返回一个顺序流
        Stream<Employee> stream = employees.stream();
//        default Stream<E> parallelStream(): 返回一个并行流
        Stream<Employee> parallelStream = employees.parallelStream();
    }
    //创建Stream方式二：通过数组
    @Test
    public void test2(){
        int[] arr = new int[]{1,2,3,4,5,6};
        //调用Arrays类的static <T> stream(T[] array):返回一个流
        IntStream stream = Arrays.stream(arr);
        Employee e1 = new Employee(1001,"Tom");
        Employee e2 = new Employee(1001,"Jerry");
        Employee[] arr1 = new Employee[]{e1,e2};
        Stream<Employee> stream1 = Arrays.stream(arr1);
    }
    //创建Stream方式三：通过Stream的of()
    @Test
    public void test3(){
        Steram<Integer> stream = Stream.of(1,2,3,4,5,6);
    }
    //创建Stream方式四：创建无限流(用的少)
    @Test
    public void test4(){
        //迭代
//        public static<T> Stream<T> iterate(final T seed,final UnaryOperator<T> f)
        //遍历前10个偶数
        Stream.iterate(0,t -> t + 2).limit(10).forEach(System.out :: println);
        //生成
//        public static<T> Stream<T> generate(Supplier<T> s)
        Stream.generate(Math::random).limit(10).forEach(System.out::println);
    }

    @Test
    public void test1(){
        List<Employee> list = EmployeeData.getEmployees();
//        filter(Predicate p)--接收Lambda,从流中排除某些元素
        Stream<Employee> stream = list.stream();
        //练习：查询员工表中薪资大于7000的员工信息
        stream.filter(e -> e.getSalary() > 7000).forEach(System.out::println);
        System.out.println();
//      limit(n)--截断流，使其3元素不超过给定数量
        list.stream().limit(3).forEach(System.out::println);
        System.out.println();
//        skip(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
        list.stream().skip(3).forEach(System.out::println);
//        distinct()--筛选，通过流所生成元素的hashCode()和equals()去除重复元素
        list.add(new Employee(1010,"刘强东",40,8000));
        list.add(new Employee(1010,"刘强东",40,8000));
        list.add(new Employee(1010,"刘强东",40,8000));
        list.add(new Employee(1010,"刘强东",40,8000));
        list.add(new Employee(1010,"刘强东",40,8000));
        list.stream().distinct().forEach(System.out::println);
    }
    //映射
    @Test
    public void test2(){
//        map(Function f)--接收一个函数作为参数，将元素转换成其他形式或提取信息，该函数
        List<String> list = Arrays.asList("aa","bb","cc","dd");
        list.stream().map(str -> str.toUpperCase()).forEach(System.out::println);
//        练习1：获取员工姓名长度大于3的员工的姓名。
        List<Employee> employees = EmployeeData.getEmployees();
        Stream<String> namesStream = employees.stream().map(employee::getName);
        namesStream.filter(name -> name.length() > 3).forEach(System.out::println);
//        练习2
        Stream<Stream<Character>> streamStream = list.stream().map(StreamAPITest1::fromStringToStream);
        streamStream.forEach(s ->{
            s.forEach(System.out::println);
        });
        System.out.println();
//        flatMap(Function f)--接受一个函数作为参数，将流中的每个值都换成另一个流，然后
        Stream<Character> characterStream = list.stream().flatMap(StreamAPITest1::fromStringToStream);
        characterStream.forEach(System.out::println);
    }
    //将多个字符串中的多个字符构成的集合转换为对应的Stream的实例
    public static Stream<Character> fromStringToStream(String str){
        ArrayList<Character> list = new ArrayList<>();
        for(Character c : str.toCharArray()){
            list.add(c);
        }
        return list.stream();
    }
    //帮助理解Stream映射的集合例子
    @Test
    public void test3(){
        ArrayList list1 = new ArrayList();
        list1.add(1);
        list1.add(2);
        list1.add(3);
        ArrayList list2 = new ArrayList();
        list2.add(4);
        list2.add(5);
        list2.add(6);
//        list1.add(list2);
        list1.addAll(list2);
        System.out.println(list1);
    }
    @Test
    public void test4(){
//        sorted()--自然排序
        List<Integer> list = Arrays.asList(12,43,65,34,87,0,-98,7);
        list.stream().sorted().forEach(System.out::println);
        //抛异常，原因：Employee没有实现Comparable接口
        List<Employee> employees = EmployeeData.getEmployees();
        employees.stream().sorted((e1,e2) ->{
            int ageValue = Integer.compare(e1.getAge(),e2.getAge());
            if(ageValue != 0){
                return ageValue;
            }else{
                return Double.compare(e1.getSalary(),e2.getSalary());
            }
        }).forEach(System.out::println);
    }

    @Test
    public void test1(){
        List<Employee> employees = EmployeeData.getEmployees();
//        allMatch(Predicate p)--检查是否匹配所有元素
//        练习：是否所有的员工的年龄都大于18
        boolean allMatch = employees.stream().allMatch(e -> e.getAge() > 18);
        System.out.println(allMatch);
//        anyMatch(Predicate p)--检查是否至少匹配一个元素
//        练习：是否存在员工的工资大于10000
        boolean anyMatch = employees.stream().anyMatch(e -> e.getSalary() > 10000);
        System.out.println(anyMatch);
//        noneMatch(Predicate p)--检查是否没有匹配的元素
//        练习：是否存在员工姓"雷"
        boolean noneMatch = employees.stream().noneMatch(e -> e.getName().startWith("雷"));
        System.out.println(noneMatch);
//        findFirst--返回第一个元素
        Optional<Employee> employee = employees.stream().findFist();
        System.out.println(employee);
//        findAny--返回当前流中的任意元素
        Optional<Employee> employees = employees.parallelStream().findAny();
        System.out.println(employee1);
 
    }
    @Test
    public void test2(){
//        count--返回流中元素的总个数
        long count = employees.stream().filter(e -> e.getSalary() > 5000).count();
        System.out.println(count);
//        max(Comparator c)--返回流中最大值
//        练习:返回最高的工资：
        Stream<Double> salaryStream = employees.stream().map(e -> e.getSalary());
        Optional<Double> maxSalary = salaryStream.max(Double::compare);
        System.out.println(maxSalary);
//        min(Comparator c)--返回流中最小值
//        练习：返回最低工资的员工
        Optional<Employee> employee = employees.stream().min((e1,e2) -> Double.compare(e1.getSalary(),e2.getSalary()));
        System.out.println(employee);
//        forEach(Consumer c)--内部迭代
        employees.stream().forEach(System.out::println);
        employees.forEach(System.out::println);
    }
    //归约
    @Test
    public void test3(){
//        reduce(T identity,BinaryOperator)--可以将流中元素反复结合起来，得到一个值，返回T
//        练习1：计算1-10的自然数的和
        List<Integer> list = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
        Integer sum = list.stream().reduce(0,Itneger::sum);
        System.out.println(sum);
//        reduce(BinaryOperator)--可以将流中元素反复结合起来，得到一个值。返回Optional(T)
//        练习2：计算公司所有员工工资的总和
        List<Employee> employees = EmployeeData.getEmployees();
        Stream<Double> salaryStream = employees.stream().map(Employee::getSalary);
        Optional<Double> sumMoney = salaryStream.reduce((d1, d2) -> d1 + d2);
        System.out.println(sumMoney);
    }
    //3-收集
    @Test
    public void test4(){
//        collect(Collector c)--将流转换为其他形式。接受一个Collector接口的实现，用于给Stream中元素做汇总的方法
//        练习1：查找工资大于6000的员工，结果返回一个List或Set
        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.of(T t):创建一个Optional 实例，t必须非空
    Optional.empty():创建一个空的Optional实例
    Optional.ofNullable(T t):t可以为null
     */
    @Test
    public void test1(){
        Girl girl = new Girl();
        girl = null;
        //of(T t):保证t是非空的
        Optional<Girl> optionalGirl = Optional.of(girl);
    }
    @Test
    public void test2(){
        Girl girl = new Girl();
//        //ofNullable(T t):t可以为null
        Optional<Girl> optionalGirl = Optional.ofNullable(girl);
        System.out.println(optionalGirl);
        //orElse(T t1):如果当前的Optional内部封装的t是非空的，则返回内部的t.
        //入股偶内部的t是空的，则返回orElse()方法中的参数t1.
        Girl girl1 = optionalGirl.orElse(new Girl("憨憨"));
        System.out.println(girl1);
    }
    public String getGirlName(Boy boy){
        return boy.getGirl().getName();
    }
    @Test
    public void test3(){
        Boy boy = new Boy();
        boy = null;
        String girlName = getGirlName(boy);
        System.out.println(girlName);
    }
    //优化以后的getGirlName():
    public String getGirlName1(Boy boy){
        if(boy != null){
            Girl girl = boy.getGirl();
            if(girl != null){
                return girl.getName();
            }
        }
        return null;
    }
    @Test
    public void test4(){
        Boy boy = new Boy();
        boy = null;
        String girlName = getGirlName1(boy);
        System.out.println(girlName);//null
    }
    //使用Optional类的getGirlName()
    public String getGirlName2(Boy boy){
        Optional<Boy> boyOptional = Optional.ofNullable(boy);
        //此时的boy1一定非空
        Boy boy1 = boyOptional.orElse(new Boy(new Girl("漂亮妹妹")));
        Girl girl = boy1.getGirl();
        Optional<Girl> girlOptional = Optional.ofNullable(girl);
        //girl1 一定非空
        Girl girl1 = girlOptional.orElse(new Girl("古力娜扎"));
        return girl1.getName();
    }
    @Test
    public void test5(){
        Boy boy = null;
        String girlName = getGirlName2(boy);
        System.out.println(girlName);
    }