Java Lambda Stream

::方法使用

条件：lambada表达式的主体仅包含一个表达式，且lambada表达式只调用一个已经存在的方法；被引用的方法的参数列表与lambada表达式的输入输出一致

以下是Java 8中方法引用的一些语法：

• 静态方法引用（static method）语法：classname::methodname 例如：Person::getAge
• 对象的实例方法引用语法：instancename::methodname 例如：System.out::println
• 对象的超类方法引用语法： super::methodname
• 类构造器引用语法： classname::new 例如：ArrayList::new
• 数组构造器引用语法： typename[]::new 例如： String[]:new

DataStreamSource<String> kafkaDS = env.addSource(MyKafkaUtil.getKafkaConsumer(sourceTopic, groupId));

kafkaDS.map(line->{return JSON.parseObject(line);});
//Lambda 简写
kafkaDS.map(JSON::parseObject);

@Test
void printTest1() {
    List<String> list = Arrays.asList("a", "b", "c", "d");

    System.out.println("for  => ");
    for (String item : list) {
        AcceptMethod.printValue(item);
    }

    System.out.println("forEach => ");
    list.forEach(item -> AcceptMethod.printValue(item));

    System.out.println("forEach => ：：");
    //list.forEach(new AcceptMethod()::printValue); //非静态方法
    list.forEach(AcceptMethod::printValue);

    System.out.println("forEach list => ：：");
    List<LoginUser> loginUserList = new ArrayList<>();
    LoginUser loginUser = new LoginUser();
    loginUser.setOs("iOS");
    loginUserList.add(loginUser);
    loginUser = new LoginUser();
    loginUser.setOs("Android");
    loginUserList.add(loginUser);
    loginUserList.forEach(AcceptMethod::printList);

}

public static class AcceptMethod {
    public static void printValue(String str) {
        System.out.println("print value : " + str);
    }

    public static void printList(LoginUser map) {
        System.out.println("print value : " + map.getOs());
    }
}

Stream 示例

public class Java8Tester {
   public static void main(String args[]){
      System.out.println("使用 Java 7: ");

      // 计算空字符串
      List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
      System.out.println("列表: " +strings);
      long count = getCountEmptyStringUsingJava7(strings);

      System.out.println("空字符数量为: " + count);
      count = getCountLength3UsingJava7(strings);

      System.out.println("字符串长度为 3 的数量为: " + count);

      // 删除空字符串
      List<String> filtered = deleteEmptyStringsUsingJava7(strings);
      System.out.println("筛选后的列表: " + filtered);

      // 删除空字符串，并使用逗号把它们合并起来
      String mergedString = getMergedStringUsingJava7(strings,", ");
      System.out.println("合并字符串: " + mergedString);
      List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);

      // 获取列表元素平方数
      List<Integer> squaresList = getSquares(numbers);
      System.out.println("平方数列表: " + squaresList);
      List<Integer> integers = Arrays.asList(1,2,13,4,15,6,17,8,19);

      System.out.println("列表: " +integers);
      System.out.println("列表中最大的数 : " + getMax(integers));
      System.out.println("列表中最小的数 : " + getMin(integers));
      System.out.println("所有数之和 : " + getSum(integers));
      System.out.println("平均数 : " + getAverage(integers));
      System.out.println("随机数: ");

      // 输出10个随机数
      Random random = new Random();

      for(int i=0; i < 10; i++){
         System.out.println(random.nextInt());
      }

      System.out.println("使用 Java 8: ");
      System.out.println("列表: " +strings);

      count = strings.stream().filter(string->string.isEmpty()).count();
      System.out.println("空字符串数量为: " + count);

      count = strings.stream().filter(string -> string.length() == 3).count();
      System.out.println("字符串长度为 3 的数量为: " + count);

      filtered = strings.stream().filter(string ->!string.isEmpty()).collect(Collectors.toList());
      System.out.println("筛选后的列表: " + filtered);

      mergedString = strings.stream().filter(string ->!string.isEmpty()).collect(Collectors.joining(", "));
      System.out.println("合并字符串: " + mergedString);

      squaresList = numbers.stream().map( i ->i*i).distinct().collect(Collectors.toList());
      System.out.println("Squares List: " + squaresList);
      System.out.println("列表: " +integers);

      IntSummaryStatistics stats = integers.stream().mapToInt((x) ->x).summaryStatistics();

      System.out.println("列表中最大的数 : " + stats.getMax());
      System.out.println("列表中最小的数 : " + stats.getMin());
      System.out.println("所有数之和 : " + stats.getSum());
      System.out.println("平均数 : " + stats.getAverage());
      System.out.println("随机数: ");

      random.ints().limit(10).sorted().forEach(System.out::println);

      // 并行处理
      count = strings.parallelStream().filter(string -> string.isEmpty()).count();
      System.out.println("空字符串的数量为: " + count);
   }

   private static int getCountEmptyStringUsingJava7(List<String> strings){
      int count = 0;

      for(String string: strings){

         if(string.isEmpty()){
            count++;
         }
      }
      return count;
   }

   private static int getCountLength3UsingJava7(List<String> strings){
      int count = 0;

      for(String string: strings){

         if(string.length() == 3){
            count++;
         }
      }
      return count;
   }

   private static List<String> deleteEmptyStringsUsingJava7(List<String> strings){
      List<String> filteredList = new ArrayList<String>();

      for(String string: strings){

         if(!string.isEmpty()){
             filteredList.add(string);
         }
      }
      return filteredList;
   }

   private static String getMergedStringUsingJava7(List<String> strings, String separator){
      StringBuilder stringBuilder = new StringBuilder();

      for(String string: strings){

         if(!string.isEmpty()){
            stringBuilder.append(string);
            stringBuilder.append(separator);
         }
      }
      String mergedString = stringBuilder.toString();
      return mergedString.substring(0, mergedString.length()-2);
   }

   private static List<Integer> getSquares(List<Integer> numbers){
      List<Integer> squaresList = new ArrayList<Integer>();

      for(Integer number: numbers){
         Integer square = new Integer(number.intValue() * number.intValue());

         if(!squaresList.contains(square)){
            squaresList.add(square);
         }
      }
      return squaresList;
   }

   private static int getMax(List<Integer> numbers){
      int max = numbers.get(0);

      for(int i=1;i < numbers.size();i++){

         Integer number = numbers.get(i);

         if(number.intValue() > max){
            max = number.intValue();
         }
      }
      return max;
   }

   private static int getMin(List<Integer> numbers){
      int min = numbers.get(0);

      for(int i=1;i < numbers.size();i++){
         Integer number = numbers.get(i);

         if(number.intValue() < min){
            min = number.intValue();
         }
      }
      return min;
   }

   private static int getSum(List numbers){
      int sum = (int)(numbers.get(0));

      for(int i=1;i < numbers.size();i++){
         sum += (int)numbers.get(i);
      }
      return sum;
   }

   private static int getAverage(List<Integer> numbers){
      return getSum(numbers) / numbers.size();
   }
}

实例

/**
* java list 中的字符是否包括在另一个 list 中
*/
@Test
void anyMatchTest() {
    List<String> permission = Arrays.asList("A", "B", "D");
    List<String> arr = Arrays.asList("C", "D", "E", "F");
    boolean b = arr.stream().anyMatch(permission::contains);
    System.out.println("" + b);
}

/**
* 数组转Set\List
*/
void anyMatchTest() {
 String[] permsArr = "a,b,c".split(",");
 Set<String> permsSet = Arrays.stream(permsArr).collect(Collectors.toSet());
}