Java 8 流
Java 8 流
stream是Java 8中引入的一个新的抽象层。使用stream，您可以用类似于SQL语句的声明性方式处理数据。例如，考虑以下SQL语句。
SELECT max(salary), employee_id, employee_name FROM Employee
上述SQL表达式返回最大月薪员工的详细信息，而无需在开发人员端进行任何计算。在Java中使用集合框架，开发人员必须使用循环并进行重复检查。另一个问题是效率；由于多核处理器很容易获得，Java开发人员必须编写易于出错的并行代码处理。
为了解决这些问题，Java 8引入了流的概念，允许开发人员以声明方式处理数据，并利用多核架构，而无需为其编写任何特定代码。
什么是流
stream表示来自支持聚合操作的源的对象序列。以下是流的特征：
序列元素：流以顺序方式提供一组特定类型的元素。流按需获取/计算元素。它从不存储元素。
源：流将集合、阵列或I / O资源作为输入源。
聚合操作：流支持聚合操作，如过滤、映射、限制、减少、查找、匹配等。
流水线操作：大多数流操作返回流本身，以便它们的结果可以被流水线化。这些操作称为中间操作，它们的功能是获取输入，处理它们，并将输出返回给目标。collect ( )方法是一种终端操作，通常出现在流水线操作结束时，用于标记流的结束。
自动迭代：流操作在内部对提供的源元素进行迭代，这与需要显式迭代的集合不同。
生成流
对于Java 8，收集接口有两种方法来生成流。
stream() − Returns a sequential stream considering collection as its source.
parallelStream() − Returns a parallel Stream considering collection as its source.
List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
List<String> filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList());forEach
流提供了一种新的方法“forEach”来迭代stream的每个元素。下面的代码段展示了如何使用forEach打印10个随机数。
Random random = new Random(); random.ints().limit(10).forEach(System.out::println);
map
“map”方法用于将每个元素映射到其相应的结果。下面的代码段使用map打印唯一的数字正方形。
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”方法用于根据标准消除元素。下面的代码段使用过滤器打印空字符串计数。
List<String>strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
//get count of empty string
int count = strings.stream().filter(string -> string.isEmpty()).count();limit
“limit”方法用于减小流的大小。下面的代码段展示了如何使用limit打印10个随机数。
Random random = new Random(); random.ints().limit(10).forEach(System.out::println);
sorted
“sorted”方法用于对流进行排序。下面的代码段展示了如何按排序顺序打印10个随机数。
Random random = new Random(); random.ints().limit(10).sorted().forEach(System.out::println);
Parallel Processing
parallelStream是并行处理的流的替代。看看下面的代码段，它使用parallelStream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();
在顺序流和并行流之间切换非常容易。
Collectors
Collectors用于组合对流元素的处理结果。Collectors可用于返回列表或字符串。
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);Statistics
在Java 8中，引入了统计收集器来计算流处理时的所有统计信息。
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());流的例子
使用任何编辑器创建以下Java程序,例如 C:\> JAVA.
Java8Tester.java
Live Demo
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<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 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();
}
}验证结果
使用javac编译器编译类，如下所示
C:\JAVA>javac Java8Tester.java
现在运行Java8Tester，如下所示：
C:\JAVA>java Java8Tester
它应该产生以下结果：
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