Java8 Streams流
流(Stream)是Java 8中引入的一个新的抽象层。使用流,您可以以类似于SQL语句的声明方式处理数据。例如,考虑以下SQL语句。
SELECT max(salary), employee_id, employee_name FROM Employee
上述的SQL表达式会自动返回最高薪水的雇员详细信息,而无需在开发者端进行任何计算。使用Java的集合框架,开发者需要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取,Java开发者需要编写并行处理代码,这可能容易出错。
为了解决这些问题,Java 8引入了流(Stream)的概念,它可以让开发者以声明式的方式处理数据并充分利用多核架构,而无需为此编写任何特定的代码。
什么是流(Stream)
流(Stream)表示从源中获取的对象的序列,可以支持聚合操作。以下是流(Stream)的特点:
- 元素序列 − 流(Stream)以顺序方式提供特定类型的一组元素。流(Stream)按需获取/计算元素,它不会存储元素。
-
源 − 流(Stream)的输入源可以是集合(Collections)、数组(Arrays)或I/O资源。
-
聚合操作 − 流(Stream)支持过滤(filter)、映射(map)、限制(limit)、归约(reduce)、查找(find)、匹配(match)等聚合操作。
-
流水线处理 − 大多数流(Stream)操作返回流(Stream)本身,以便其结果可以连续处理。这些操作被称为中间操作,它们的作用是接受输入,处理它们,并将输出返回给目标。collect()方法是一个终端操作,通常位于流(Stream)流水线操作的末尾,表示流的结束。
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自动迭代 − 流(Stream)操作在内部迭代提供的源元素上进行迭代,与集合(Collections)不同,不需要显式迭代。
生成流(Stream)
在Java 8中,Collection接口有两个方法可以生成流(Stream)。
- stream() − 返回将集合作为其源的顺序流(Stream)。
-
parallelStream() − 返回将集合作为其源的并行流(Stream)。
List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
List<String> filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList());
forEach
Stream提供了一个新的方法‘forEach’,用于迭代流的每个元素。以下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对应的结果。以下代码段使用map打印出数字的唯一平方数。
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());
过滤器
“filter”方法用于根据条件消除元素。下面的代码段使用filter打印空字符串的数量。
List<String>strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
//get count of empty string
int count = strings.stream().filter(string -> string.isEmpty()).count();
限制
‘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方法打印出10个随机数。
Random random = new Random();
random.ints().limit(10).forEach(System.out::println);
按顺序排列
‘sorted’方法用于对流进行排序。下面的代码片段展示了如何以排序的方式打印出10个随机数。
Random random = new Random();
random.ints().limit(10).sorted().forEach(System.out::println);
并行处理
parallelStream是流的替代品,用于并行处理。看下面的代码片段,使用parallelStream打印空字符串的数量。
List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
//get count of empty string
long count = strings.parallelStream().filter(string -> string.isEmpty()).count();
在顺序流和并行流之间切换非常容易。
收集器
收集器被用于将流中元素的处理结果组合起来。收集器可以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);
统计数据
在Java 8中,引入了统计数据收集器,用于在流处理时计算所有统计数据。
List numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
IntSummaryStatistics stats = numbers.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());
流示例
使用您选择的任何编辑器,在C:> JAVA中创建以下Java程序。
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<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
现在按照以下步骤运行Java8Tester:
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