变量的作用域

int num = 1;  
Converter<Integer, String> s = (param) -> System.out.println(String.valueOf(param + num));
s.convert(2);
num = 5;  
//报错信息：Local variable num defined in an enclosing scope must be final or effectively final
//在 Lambda 表达式当中不允许声明一个与局部变量同名的参数或者局部变量。
String first = "";  
Comparator<String> comparator = (first, second) -> Integer.compare(first.length(), second.length());  //编译会出错

//格式：Classname :: staticMethodName  和静态方法调用相比，只是把 . 换为 ::
String::valueOf   等价于lambda表达式 (s) -> String.valueOf(s)
Math::pow       等价于lambda表达式  (x, y) -> Math.pow(x, y);

//格式：instanceReference::methodName
class ComparisonProvider{
    public int compareByName(Person a, Person b){
        return a.getName().compareTo(b.getName());
    }
    public int compareByAge(Person a, Person b){
        return a.getBirthday().compareTo(b.getBirthday());
    }
}
ComparisonProvider myComparisonProvider = new ComparisonProvider();
Arrays.sort(rosterAsArray, myComparisonProvider::compareByName);

public interface MyFunc<T> {
    int func(T[] als, T v);
}
public class MyArrayOps {
     public static <T> int countMatching(T[] vals, T v) {
         int count = 0;
         for (int i = 0; i < vals.length; i++) {
             if (vals[i] == v) count++;
         }
         return count;
     }
}
public class GenericMethodRefDemo {    
    public static <T> int myOp(MyFunc<T> f, T[] vals, T v) {
        return f.func(vals, v);
    }    
    public static void main(String[] args){
        Integer[] vals = {1, 2, 3, 4, 2, 3, 4, 4, 5};
        String[] strs = {"One", "Two", "Three", "Two"};
        int count;
        count=myOp(MyArrayOps::<Integer>countMatching, vals, 4);
        System.out.println("vals contains "+count+" 4s");
        count=myOp(MyArrayOps::<String>countMatching, strs, "Two");
        System.out.println("strs contains "+count+" Twos");
    }
}

//格式：ClassName :: new，调用默认构造器。
//lambda方式
Supplier<Passenger> supplier1 = () -> new Passenger();
//构造器引用:通过类型推断，引用无参构造器
Supplier<Passenger> supplier2 = Passenger::new;
//lambda方式
BiFunction<String, String, Passenger> function1 = (x, y) -> new Passenger(x, y);
//构造器引用:通过类型推断，引用有两个String参数的构造器
BiFunction<String, String, Passenger> function2 = Passenger::new;

//lambda方式
Function<Integer, String[]> fun1 = (x) -> new String[x];
String[] strs1 = fun1.apply(10);
//数组引用
Function<Integer, String[]> fun2 = String[]::new;
String[] strs2 = fun2.apply(10);

@Test
public void createStream() throws FileNotFoundException {
    List<String> nameList = Arrays.asList("Darcy", "Chris", "Linda", "Sid", "Kim", "Jack", "Poul", "Peter");
    String[] nameArr = {"Darcy", "Chris", "Linda", "Sid", "Kim", "Jack", "Poul", "Peter"};
    // 集合获取 Stream 流
    Stream<String> nameListStream = nameList.stream();
    // 集合获取并行 Stream 流
    Stream<String> nameListStream2 = nameList.parallelStream();
    // 数组获取 Stream 流
    Stream<String> nameArrStream = Stream.of(nameArr);
    // 数组获取 Stream 流
    Stream<String> nameArrStream1 = Arrays.stream(nameArr);
    // 文件流获取 Stream 流
    BufferedReader bufferedReader = new BufferedReader(new FileReader("README.md"));
    Stream<String> linesStream = bufferedReader.lines();
    // 从静态方法获取流操作
    IntStream rangeStream = IntStream.range(1, 10);
    rangeStream.limit(10).forEach(num -> System.out.print(num+","));
    System.out.println();
    IntStream intStream = IntStream.of(1, 2, 3, 3, 4);
    intStream.forEach(num -> System.out.print(num+","));
}

@Test
public void mapTest() {
    List<Integer> numberList = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9);
    // 映射成 2倍数字
    List<Integer> collect = numberList.stream()
            .map(number -> number * 2)
            .collect(Collectors.toList());
    collect.forEach(number -> System.out.print(number + ","));
    System.out.println();
    numberList.stream()
            .map(number -> "数字 " + number + ",")
            .forEach(number -> System.out.println(number));
}
@Test
public void flatMapTest() {
    Stream<List<Integer>> inputStream = Stream.of(
            Arrays.asList(1),
            Arrays.asList(2, 3),
            Arrays.asList(4, 5, 6)
    );
    List<Integer> collect = inputStream
            .flatMap((childList) -> childList.stream())
            .collect(Collectors.toList());
    collect.forEach(number -> System.out.print(number + ","));
}
// 输出结果
// 1,2,3,4,5,6,
filter：数据筛选，相当于if判断
@Test
public void filterTest() {
    List<Integer> numberList = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9);
    List<Integer> collect = numberList.stream()
            .filter(number -> number % 2 == 0)
            .collect(Collectors.toList());
    collect.forEach(number -> System.out.print(number + ","));
}
distinct：去重
public void distinctTest() {
    List<String> list = Arrays.asList("AA", "BB", "CC", "BB", "CC", "AA", "AA");
    long l = list.stream().distinct().count();
    System.out.println("count:"+l);
    String output = list.stream().distinct().collect(Collectors.joining(","));
    System.out.println(output);
}
sorted
peek
limit：获取前n个元素
skip：丢弃前n个元素
@Test
public void limitOrSkipTest() {
    List<Integer> ageList = Arrays.asList(11, 22, 13, 14, 25, 26);
    ageList.stream()
            .limit(3)
            .forEach(age -> System.out.print(age+","));、//11，22,13
    System.out.println();
    ageList.stream()
            .skip(3)
            .forEach(age -> System.out.print(age+","));//14,25,26
}
parallel：并行流
public void parallelTest(){
    Long resourse = LongStream.rangeClosed(0,1000000000L)
        .parallel().reduce(0,Long::sum);
    System.out.println(resourse);
}
sequential
unordered