流/Stream是在JAVA8中引入的一个抽象，可以处理类似SQL语句声明数据。 例如，考虑下面的SQL语句。
SELECT max(salary),employee_id,employee_name FROM Employee
上面的SQL表达式会自动返回最大薪水员工的细节，没有对开发者的最终做任何计算。在Java中使用集合框架，开发人员必须使用循环，使检查反复。另一个值得关注的是效率，多核处理器可放心，Java开发人员必须编写的并行代码处理，但是非常容易出错。
为了解决这样的问题，JAVA8引入了流的概念，它允许开发者通过声明处理数据，并可以leverate多核架构，而不需要编写任何特定的代码。
什么是数据流？
流代表从支持聚合操作源的序列的对象。以下是数据流的特点。
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元素序列%uA0- 流提供了一组特定类型的以顺序方式元素。流获取/计算需求的元素。它不存储元素。
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源- 流使用集合，数组或I/O资源为输入源。
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聚合操作%uA0- 数据流支持如filter, map, limit, reduced, find, match等聚合操作。
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管道传输%uA0- 大多数流操作的返回流本身使他们的结果可以被管道传输。这些操作被称为中间操作以及它们的功能是利用输入，处理输入和输出返回到目标。collect()方法是终端操作，这是通常出现在管道传输操作结束标记流的结束。
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自动迭代%uA0- 流操作内部做了反复对比，其中明确迭代需要集合提供源元素。
生成数据流
使用Java8，Collection 接口有两个方法来生成流。
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stream()%uA0-返回顺序流考虑集合作为其源。
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parallelStream()%uA0- 返回并行数据流考虑集合作为其源。
List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl")
List<String> filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList())
ForEach
数据流提供了新的forEach方法遍历该流中的每个元素。考虑下面的例子打印10个随机数字。
Random random = new Random() random.ints().limit(10).forEach(System.out::println)
map
map方法用于映射每个元素对应的结果。考虑下面的例子打印唯一的方形数字。
List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5) //get list of unique squares List<Integer> squaresList = numbers.stream().map( i -> i*i).distinct().collect(Collectors.toList())
filter
filter方法用于消除基于标准元素。考虑下面的例子打印空字符串计数(总数)。
List<String>strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl")
//get count of empty string
int count = strings.stream().filter(string -> string.isEmpty()).count()
limit
limit 方法用于减少流的大小。考虑下面的例子打印10个随机数字。
Random random = new Random() random.ints().limit(10).forEach(System.out::println)
sorted
sorted方法用来流排序。考虑下面的例子打印10个随机数字的排序顺序。
Random random = new Random() random.ints().limit(10).sorted().forEach(System.out::println)
并行处理
parallelStream是流进行并行处理的替代方案。考虑下面的例子打印空字符串计数。
List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl")
//get count of empty string
int count = strings.parallelStream().filter(string -> string.isEmpty()).count()
这是很容易在顺序和并行的流之间进行切换。
收集器
收集器是用来处理组合在一个数据流的元素的结果。收集器可用于返回一个列表或一个字符串。
List<String>strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl")
List<String> 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)
统计
使用Java8，统计收集器引入计算所有统计数据时，流处理可以做这些。
List<Integer> 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())
Stream 例子
选择使用任何编辑器创建以下java程序在C:/> JAVA
Java8Tester.javaimport 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<String> 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<String> 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<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5)
//get list of square of distinct numbers
List<Integer> squaresList = getSquares(numbers)
System.out.println("Squares List: " + squaresList)
List<Integer> 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<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 seperator){
StringBuilder stringBuilder = new StringBuilder()
for(String string: strings){
if(!string.isEmpty()){
stringBuilder.append(string)
stringBuilder.append(seperator)
}
}
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=1i< 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=1i< numbers.size()i++){
Integer number = numbers.get(i)
if(number.intValue() < min){
min = number.intValue()
}
}
return min
}
private static int getSum(List<Integer> numbers){
int sum = numbers.get(0)
for(int i=1i< numbers.size()i++){
sum += numbers.get(i).intValue()
}
return sum
}
private static int getAverage(List<Integer> numbers){
return getSum(numbers) / numbers.size()
}
}
验证结果
使用javac编译器编译如下类
C:\JAVA>javac Java8Tester.java
现在运行Java8Tester看到的结果
C:\JAVA>java Java8Tester
看到结果
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