流/Stream是在JAVA8中引入的一個抽象，可以處理類似SQL語句宣告資料。 例如，考慮下面的SQL語句。
SELECT max(salary),employee_id,employee_name FROM Employee
上面的SQL表示式會自動返回最大薪水員工的細節，沒有對開發者的最終做任何計算。在Java中使用集合框架，開發人員必須使用迴圈，使檢查反復。另一個值得關注的是效率，多核處理器可放心，Java開發人員必須編寫的並行程式碼處理，但是非常容易出錯。
為了解決這樣的問題，JAVA8引入了流的概念，它允許開發者通過宣告處理資料，並可以leverate多核架構，而不需要編寫任何特定的程式碼。
流代表從支援聚合操作源的序列的物件。以下是資料流的特點。
元素序列 - 流提供了一組特定型別的以順序方式元素。流獲取/計算需求的元素。它不儲存元素。
源- 流使用集合，陣列或I/O資源為輸入源。
聚合操作 - 資料流支援如filter, map, limit, reduced, find, match等聚合操作。
管道傳輸 - 大多數流操作的返回流本身使他們的結果可以被管道傳輸。這些操作被稱為中間操作以及它們的功能是利用輸入，處理輸入和輸出返回到目標。collect()方法是終端操作，這是通常出現在管道傳輸操作結束標記流的結束。
自動迭代 - 流操作內部做了反復對比，其中明確疊代需要集合提供源元素。
使用Java8，Collection 介面有兩個方法來生成流。
stream() -返回順序流考慮集合作為其源。
parallelStream() - 返回並行資料流考慮集合作為其源。
List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
List<String> filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList());
資料流提供了新的forEach方法遍歷該流中的每個元素。考慮下面的例子列印10個亂數字。
Random random = new Random(); random.ints().limit(10).forEach(System.out::println);
map方法用於對映每個元素對應的結果。考慮下面的例子列印唯一的方形數位。
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方法用於消除基於標準元素。考慮下面的例子列印空字串計數(總數)。
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 方法用於減少流的大小。考慮下面的例子列印10個亂數字。
Random random = new Random(); random.ints().limit(10).forEach(System.out::println);
sorted方法用來流排序。考慮下面的例子列印10個亂數字的排序順序。
Random random = new Random(); random.ints().limit(10).sorted().forEach(System.out::println);
parallelStream是流進行併行處理的替代方案。考慮下面的例子列印空字串計數。
List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
//get count of empty string
int count = strings.parallelStream().filter(string -> string.isEmpty()).count();
這是很容易在順序和並行的流之間進行切換。
收集器是用來處理組合在一個資料流的元素的結果。收集器可用於返回一個列表或一個字串。
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);
使用Java8，統計收集器引入計算所有統計資料時，流處理可以做這些。
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());
選擇使用任何編輯器建立以下java程式在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<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=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<Integer> 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<Integer> numbers){
return getSum(numbers) / numbers.size();
}
}
使用javac編譯器編譯如下類
C:\JAVA>javac Java8Tester.java
現在執行Java8Tester看到的結果
C:\JAVA>java Java8Tester
看到結果
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