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