# Java8数据流 - Java8教程
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。
* 元素序列 - 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。
* 源- 流使用集合，数组或I/O资源为输入源。
* 聚合操作 - 数据流支持如filter, map, limit, reduced, find, match等聚合操作。
* 管道传输 - 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。
* 自动迭代 - 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。
* stream() -返回顺序流考虑集合作为其源。
* parallelStream() - 返回并行数据流考虑集合作为其源。
```
List strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
List 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 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)。
```
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 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。
```
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);
```
## 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 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.java_
```
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 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 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 = numbers.get(0);
for(int i=1;i< numbers.size();i++){
sum += numbers.get(i).intValue();
}
return sum;
}
private static int getAverage(List 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
```