Java 8 中新增Streams 学习笔记 [参考教程](https://www.tutorialspoint.com/java8/java8_streams.htm) ###什么是Stream? * stream是java8中新增的一个抽象层.它代表数据源的一组支持聚合操作(aggregate operations)的对象序列.它具有以下特点: ** 元素的序列----Stream 提供一个特定类型的顺序排列元素的集合.如果需要,Stream获取或者计算这些元素.但是,它不存储任何元素. ** 数据源----Stream把Collection,Array或者IO数据源作为输入源. ** 聚合操作---- Stream 提供像过滤,映射,限制,减少,寻找,匹配,等等聚合操作. ** 管道(方法链操作)----大部分Stream的操作返回一个stream本身,因此操作的输出可以当作下一个操作的输入.这些操做称为中间操作.collect()方法是终端操作,它常常代表着管道操作的结束并终止stream. ** 自动迭代----Stream操作在数据源元素之上在其内部进行迭代,与之相对,Collections需要显示的生成一个迭代器. ### 生成Streams Java8中,Collection 接口新增了两个方法以产生Stream: ** stream()----返回一个以Collection中数据为数据源的Stream ** parallelStream()----与stream()相似,支持并行操作. ``` List strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl"); List filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList()); ``` ### forEach Stream 提供了一个新的方法"forEach",以便迭代流中的所有元素.下面的代码展示了如何用forEach打印10个随机数. ``` Random random = new Random(); random.ints().limit(10).forEach(System.out::println); ``` ### map "map'方法用于把每个元素映射成与它对应的元素.以下代码获取流中元素的不重复的平方. ``` List numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5); //get list of unique squares List squaresList = numbers.stream().map( i -> i*i).distinct().collect(Collectors.toList()); ``` ###filter "filter"方法用于通过特定的规则,排除一些元素.以下代码块展示了如何去除空字符串. ``` Liststrings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl"); //get count of empty string int count = strings.stream().filter(string -> string.isEmpty()).count(); ``` ###limit "limit"方法用于减小流的长度.以下代码打印10个随机数. ``` Random random = new Random(); random.ints().limit(10).forEach(System.out::println); ``` ###sorted "sorted"方法用于给流排序. ``` Random random = new Random(); random.ints().limit(10).sorted().forEach(System.out::println); ``` ###并行操作 parallelStream 用于并行操作.多核机器会提高效率. ``` List strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl"); //get count of empty string int count = strings.parallelStream().filter(string -> string.isEmpty()).count(); ``` ### Collectors Collectors常用于聚合流的操作结果.Collectors用于返回列表或字符串. ``` Liststrings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl"); List filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList()); System.out.println("Filtered List: " + filtered); String mergedString = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.joining(", ")); System.out.println("Merged String: " + mergedString); ``` ### Statistics 流操作完毕后,statics collectors(数据收集器)用来计算所有统计数据. ``` List numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5); IntSummaryStatistics stats = integers.stream().mapToInt((x) -> x).summaryStatistics(); System.out.println("Highest number in List : " + stats.getMax()); System.out.println("Lowest number in List : " + stats.getMin()); System.out.println("Sum of all numbers : " + stats.getSum()); System.out.println("Average of all numbers : " + stats.getAverage()); ``` ###栗子 ``` import java.util.ArrayList; import java.util.Arrays; import java.util.IntSummaryStatistics; import java.util.List; import java.util.Random; import java.util.stream.Collectors; import java.util.Map; public class Java8Tester { public static void main(String args[]) { System.out.println("Using Java 7: "); // Count empty strings List strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl"); System.out.println("List: " +strings); long count = getCountEmptyStringUsingJava7(strings); System.out.println("Empty Strings: " + count); count = getCountLength3UsingJava7(strings); System.out.println("Strings of length 3: " + count); //Eliminate empty string List filtered = deleteEmptyStringsUsingJava7(strings); System.out.println("Filtered List: " + filtered); //Eliminate empty string and join using comma. String mergedString = getMergedStringUsingJava7(strings,", "); System.out.println("Merged String: " + mergedString); List numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5); //get list of square of distinct numbers List squaresList = getSquares(numbers); System.out.println("Squares List: " + squaresList); List integers = Arrays.asList(1,2,13,4,15,6,17,8,19); System.out.println("List: " +integers); System.out.println("Highest number in List : " + getMax(integers)); System.out.println("Lowest number in List : " + getMin(integers)); System.out.println("Sum of all numbers : " + getSum(integers)); System.out.println("Average of all numbers : " + getAverage(integers)); System.out.println("Random Numbers: "); //print ten random numbers Random random = new Random(); for(int i = 0; i < 10; i++) { System.out.println(random.nextInt()); } System.out.println("Using Java 8: "); System.out.println("List: " +strings); count = strings.stream().filter(string->string.isEmpty()).count(); System.out.println("Empty Strings: " + count); count = strings.stream().filter(string -> string.length() == 3).count(); System.out.println("Strings of length 3: " + count); filtered = strings.stream().filter(string ->!string.isEmpty()).collect(Collectors.toList()); System.out.println("Filtered List: " + filtered); mergedString = strings.stream().filter(string ->!string.isEmpty()).collect(Collectors.joining(", ")); System.out.println("Merged String: " + mergedString); squaresList = numbers.stream().map( i ->i*i).distinct().collect(Collectors.toList()); System.out.println("Squares List: " + squaresList); System.out.println("List: " +integers); IntSummaryStatistics stats = integers.stream().mapToInt((x) ->x).summaryStatistics(); System.out.println("Highest number in List : " + stats.getMax()); System.out.println("Lowest number in List : " + stats.getMin()); System.out.println("Sum of all numbers : " + stats.getSum()); System.out.println("Average of all numbers : " + stats.getAverage()); System.out.println("Random Numbers: "); random.ints().limit(10).sorted().forEach(System.out::println); //parallel processing count = strings.parallelStream().filter(string -> string.isEmpty()).count(); System.out.println("Empty Strings: " + count); } private static int getCountEmptyStringUsingJava7(List strings) { int count = 0; for(String string: strings) { if(string.isEmpty()) { count++; } } return count; } private static int getCountLength3UsingJava7(List strings) { int count = 0; for(String string: strings) { if(string.length() == 3) { count++; } } return count; } private static List deleteEmptyStringsUsingJava7(List strings) { List filteredList = new ArrayList(); for(String string: strings) { if(!string.isEmpty()) { filteredList.add(string); } } return filteredList; } private static String getMergedStringUsingJava7(List 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 getSquares(List numbers) { List squaresList = new ArrayList(); 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 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 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 numbers) { return getSum(numbers) / numbers.size(); } } ``` 运行结果如下: ``` Using Java 7: List: [abc, , bc, efg, abcd, , jkl] Empty Strings: 2 Strings of length 3: 3 Filtered List: [abc, bc, efg, abcd, jkl] Merged String: abc, bc, efg, abcd, jkl Squares List: [9, 4, 49, 25] List: [1, 2, 13, 4, 15, 6, 17, 8, 19] Highest number in List : 19 Lowest number in List : 1 Sum of all numbers : 85 Average of all numbers : 9 Random Numbers: -1279735475 903418352 -1133928044 -1571118911 628530462 18407523 -881538250 -718932165 270259229 421676854 Using Java 8: List: [abc, , bc, efg, abcd, , jkl] Empty Strings: 2 Strings of length 3: 3 Filtered List: [abc, bc, efg, abcd, jkl] Merged String: abc, bc, efg, abcd, jkl Squares List: [9, 4, 49, 25] List: [1, 2, 13, 4, 15, 6, 17, 8, 19] Highest number in List : 19 Lowest number in List : 1 Sum of all numbers : 85 Average of all numbers : 9.444444444444445 Random Numbers: -1009474951 -551240647 -2484714 181614550 933444268 1227850416 1579250773 1627454872 1683033687 1798939493 Empty Strings: 2 ```

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