Java 8中Stream API的这些奇技淫巧！你Get了吗？

public class Data { private static List<PersonModel> list = null;  static {  PersonModel wu = new PersonModel("wu qi", 18, "男");  PersonModel zhang = new PersonModel("zhang san", 19, "男");  PersonModel wang = new PersonModel("wang si", 20, "女");  PersonModel zhao = new PersonModel("zhao wu", 20, "男");  PersonModel chen = new PersonModel("chen liu", 21, "男");  list = Arrays.asList(wu, zhang, wang, zhao, chen); }  public static List<PersonModel> getData() {  return list; }}

 /**  * 过滤所有的男性  */ public static void fiterSex(){  List<PersonModel> data = Data.getData();   //old  List<PersonModel> temp=new ArrayList<>();  for (PersonModel person:data) {   if ("男".equals(person.getSex())){    temp.add(person);   }  }  System.out.println(temp);  //new  List<PersonModel> collect = data    .stream()    .filter(person -> "男".equals(person.getSex()))    .collect(toList());  System.out.println(collect); }  /**  * 过滤所有的男性 并且小于20岁  */ public static void fiterSexAndAge(){  List<PersonModel> data = Data.getData();   //old  List<PersonModel> temp=new ArrayList<>();  for (PersonModel person:data) {   if ("男".equals(person.getSex())&&person.getAge()<20){    temp.add(person);   }  }   //new 1  List<PersonModel> collect = data    .stream()    .filter(person -> {     if ("男".equals(person.getSex())&&person.getAge()<20){      return true;     }     return false;    })    .collect(toList());  //new 2  List<PersonModel> collect1 = data    .stream()    .filter(person -> ("男".equals(person.getSex())&&person.getAge()<20))    .collect(toList());  }

/**  * 取出所有的用户名字  */ public static void getUserNameList(){  List<PersonModel> data = Data.getData();   //old  List<String> list=new ArrayList<>();  for (PersonModel persion:data) {   list.add(persion.getName());  }  System.out.println(list);   //new 1  List<String> collect = data.stream().map(person -> person.getName()).collect(toList());  System.out.println(collect);   //new 2  List<String> collect1 = data.stream().map(PersonModel::getName).collect(toList());  System.out.println(collect1);   //new 3  List<String> collect2 = data.stream().map(person -> {   System.out.println(person.getName());   return person.getName();  }).collect(toList()); }

public static void flatMapString() {  List<PersonModel> data = Data.getData();  //返回类型不一样  List<String> collect = data.stream()    .flatMap(person -> Arrays.stream(person.getName().split(" "))).collect(toList());   List<Stream<String>> collect1 = data.stream()    .map(person -> Arrays.stream(person.getName().split(" "))).collect(toList());   //用map实现  List<String> collect2 = data.stream()    .map(person -> person.getName().split(" "))    .flatMap(Arrays::stream).collect(toList());  //另一种方式  List<String> collect3 = data.stream()    .map(person -> person.getName().split(" "))    .flatMap(str -> Arrays.asList(str).stream()).collect(toList()); }

 public static void reduceTest(){  //累加，初始化值是 10  Integer reduce = Stream.of(1, 2, 3, 4)    .reduce(10, (count, item) ->{   System.out.println("count:"+count);   System.out.println("item:"+item);   return count + item;  } );  System.out.println(reduce);   Integer reduce1 = Stream.of(1, 2, 3, 4)    .reduce(0, (x, y) -> x + y);  System.out.println(reduce1);   String reduce2 = Stream.of("1", "2", "3")    .reduce("0", (x, y) -> (x + "," + y));  System.out.println(reduce2); }

 /**  * toList  */ public static void toListTest(){  List<PersonModel> data = Data.getData();  List<String> collect = data.stream()    .map(PersonModel::getName)    .collect(Collectors.toList()); }  /**  * toSet  */ public static void toSetTest(){  List<PersonModel> data = Data.getData();  Set<String> collect = data.stream()    .map(PersonModel::getName)    .collect(Collectors.toSet()); }  /**  * toMap  */ public static void toMapTest(){  List<PersonModel> data = Data.getData();  Map<String, Integer> collect = data.stream()    .collect(      Collectors.toMap(PersonModel::getName, PersonModel::getAge)    );   data.stream()    .collect(Collectors.toMap(per->per.getName(), value->{   return value+"1";  })); }  /**  * 指定类型  */ public static void toTreeSetTest(){  List<PersonModel> data = Data.getData();  TreeSet<PersonModel> collect = data.stream()    .collect(Collectors.toCollection(TreeSet::new));  System.out.println(collect); }  /**  * 分组  */ public static void toGroupTest(){  List<PersonModel> data = Data.getData();  Map<Boolean, List<PersonModel>> collect = data.stream()    .collect(Collectors.groupingBy(per -> "男".equals(per.getSex())));  System.out.println(collect); }  /**  * 分隔  */ public static void toJoiningTest(){  List<PersonModel> data = Data.getData();  String collect = data.stream()    .map(personModel -> personModel.getName())    .collect(Collectors.joining(",", "{", "}"));  System.out.println(collect); }  /**  * 自定义  */ public static void reduce(){  List<String> collect = Stream.of("1", "2", "3").collect(    Collectors.reducing(new ArrayList<String>(), x -> Arrays.asList(x), (y, z) -> {     y.addAll(z);     return y;    }));  System.out.println(collect); }

public static void main(String[] args) {    PersonModel personModel=new PersonModel();   //对象为空则打出 -  Optional<Object> o = Optional.of(personModel);  System.out.println(o.isPresent()?o.get():"-");   //名称为空则打出 -  Optional<String> name = Optional.ofNullable(personModel.getName());  System.out.println(name.isPresent()?name.get():"-");   //如果不为空，则打出xxx  Optional.ofNullable("test").ifPresent(na->{   System.out.println(na+"ifPresent");  });   //如果空，则返回指定字符串  System.out.println(Optional.ofNullable(null).orElse("-"));  System.out.println(Optional.ofNullable("1").orElse("-"));   //如果空，则返回 指定方法，或者代码  System.out.println(Optional.ofNullable(null).orElseGet(()->{   return "hahah";  }));  System.out.println(Optional.ofNullable("1").orElseGet(()->{   return "hahah";  }));   //如果空，则可以抛出异常  System.out.println(Optional.ofNullable("1").orElseThrow(()->{   throw new RuntimeException("ss");  }));  //  Objects.requireNonNull(null,"is null");    //利用 Optional 进行多级判断  EarthModel earthModel1 = new EarthModel();  //old  if (earthModel1!=null){   if (earthModel1.getTea()!=null){    //...   }  }  //new  Optional.ofNullable(earthModel1)    .map(EarthModel::getTea)    .map(TeaModel::getType)    .isPresent();  //  Optional<EarthModel> earthModel = Optional.ofNullable(new EarthModel());//  Optional<List<PersonModel>> personModels = earthModel.map(EarthModel::getPersonModels);//  Optional<Stream<String>> stringStream = personModels.map(per -> per.stream().map(PersonModel::getName));    //判断对象中的list  Optional.ofNullable(new EarthModel())    .map(EarthModel::getPersonModels)    .map(pers->pers      .stream()      .map(PersonModel::getName)      .collect(toList()))    .ifPresent(per-> System.out.println(per));    List<PersonModel> models=Data.getData();  Optional.ofNullable(models)    .map(per -> per      .stream()      .map(PersonModel::getName)      .collect(toList()))    .ifPresent(per-> System.out.println(per));  }

 //根据数字的大小，有不同的结果 private static int size=10000000; public static void main(String[] args) {  System.out.println("-----------List-----------");  testList();  System.out.println("-----------Set-----------");  testSet(); }  /**  * 测试list  */ public static void testList(){  List<Integer> list = new ArrayList<>(size);  for (Integer i = 0; i < size; i++) {   list.add(new Integer(i));  }   List<Integer> temp1 = new ArrayList<>(size);  //老的  long start=System.currentTimeMillis();  for (Integer i: list) {   temp1.add(i);  }  System.out.println(+System.currentTimeMillis()-start);   //同步  long start1=System.currentTimeMillis();  list.stream().collect(Collectors.toList());  System.out.println(System.currentTimeMillis()-start1);   //并发  long start2=System.currentTimeMillis();  list.parallelStream().collect(Collectors.toList());  System.out.println(System.currentTimeMillis()-start2); }  /**  * 测试set  */ public static void testSet(){  List<Integer> list = new ArrayList<>(size);  for (Integer i = 0; i < size; i++) {   list.add(new Integer(i));  }   Set<Integer> temp1 = new HashSet<>(size);  //老的  long start=System.currentTimeMillis();  for (Integer i: list) {   temp1.add(i);  }  System.out.println(+System.currentTimeMillis()-start);   //同步  long start1=System.currentTimeMillis();  list.stream().collect(Collectors.toSet());  System.out.println(System.currentTimeMillis()-start1);   //并发  long start2=System.currentTimeMillis();  list.parallelStream().collect(Collectors.toSet());  System.out.println(System.currentTimeMillis()-start2); }

private static void peekTest() {  List<PersonModel> data = Data.getData();   //peek打印出遍历的每个per  data.stream().map(per->per.getName()).peek(p->{   System.out.println(p);  }).collect(toList()); }