Java 8 Stream 閃亮登場！

作者：funnyZpC
出處：cnblogs.com/funnyzpc/p/10382053.html

一. Stream(流)是什么，干什么

Stream是一類用于替代對集合操作的工具類+Lambda式編程，他可以替代現(xiàn)有的遍歷、過濾、求和、求最值、排序、轉(zhuǎn)換等

二. Stream操作方式

• 并行方式parallelStream

• 順序方式Stream

三. Stream優(yōu)勢

• Lambda 可有效減少冗余代碼，減少開發(fā)工作量

• 內(nèi)置對集合List、Map的多種操作方式，含基本數(shù)據(jù)類型處理

• 并行Stream有效率優(yōu)勢(內(nèi)置多線程)

四. Stream(流)的基本使用

• 遍歷forEach

@Test
public?void?stream()?{
????//操作List
????List>?mapList?=?new?ArrayList()?{
????????{
????????????Map?m?=?new?HashMap();
????????????m.put("a",?"1");
????????????Map?m2?=?new?HashMap();
????????????m2.put("b",?"2");
????????????add(m);
????????????add(m2);
????????}
????};
????mapList.stream().forEach(item->?System.out.println(item));

????//操作Map
????Map?mp?=?new?HashMap(){
????????{
????????????put("a","1");
????????????put("b","2");
????????????put("c","3");
????????????put("d","4");
????????}
????};
????mp.keySet().stream().forEachOrdered(item->?System.out.println(mp.get(item)));
}

• 過濾filter

List?mapList?=?new?ArrayList()?{
{
????add(1);
????add(10);
????add(12);
????add(33);
????add(99);
}
};
//mapList.stream().forEach(item->?System.out.println(item));
mapList?=?mapList.stream().filter(item->{
return?item>30;
}).collect(Collectors.toList());
System.out.println(mapList);

• 轉(zhuǎn)換map和極值

@Test
public?void?trans(){
????List?ps?=?new?ArrayList(){
????????{
????????????Person?p1?=?new?Person();
????????????p1.setAge(11);
????????????p1.setName("張強");

????????????Person?p2?=?new?Person();
????????????p2.setAge(17);
????????????p2.setName("李思");

????????????Person?p3?=?new?Person();
????????????p3.setAge(20);
????????????p3.setName("John");

????????????add(p1);
????????????add(p2);
????????????add(p3);
????????}
????};
????
????//取出所有age字段為一個List
????List?sumAge?=?ps.stream().map(Person::getAge).collect(Collectors.toList());
????System.out.println(sumAge);
????
????//取出age最大的那
????Integer?maxAge?=sumAge.stream().max(Integer::compare).get();
????System.out.println(maxAge);
}

class?Person{

????private?String?name;
????private?Integer?age;

????public?String?getName()?{
????????return?name;
????}

????public?void?setName(String?name)?{
????????this.name?=?name;
????}

????public?Integer?getAge()?{
????????return?age;
????}

????public?void?setAge(Integer?age)?{
????????this.age?=?age;
????}
}

五. Stream(流)的效率+ 模擬非耗時簡單業(yè)務(wù)邏輯

class?Person{

????private?String?name;
????private?int?age;
????private?Date?joinDate;
????private?String?label;

????public?String?getName()?{
????????return?name;
????}

????public?void?setName(String?name)?{
????????this.name?=?name;
????}

????public?int?getAge()?{
????????return?age;
????}

????public?void?setAge(int?age)?{
????????this.age?=?age;
????}

????public?Date?getJoinDate()?{
????????return?joinDate;
????}

????public?void?setJoinDate(Date?joinDate)?{
????????this.joinDate?=?joinDate;
????}

????public?String?getLabel()?{
????????return?label;
????}

????public?void?setLabel(String?label)?{
????????this.label?=?label;
????}
}

public?class?DataLoopTest?{

????private?static?final?Logger?LOG=?LoggerFactory.getLogger(DataLoopTest.class);

????private?static?final?List?persons?=?new?ArrayList<>();
????static?{
????????for(int?i=0;i<=1000000;i++){
????????????Person?p?=?new?Person();
????????????p.setAge(i);
????????????p.setName("zhangSan");
????????????p.setJoinDate(new?Date());
????????????persons.add(p);
????????}
????}

????/**
?????*?for?循環(huán)耗時?===>?1.988
?????*?for?循環(huán)耗時?===>?2.198
?????*?for?循環(huán)耗時?===>?1.978
?????*
?????*/
????@Test
????public?void?forTest(){
????????Instant?date_start?=?Instant.now();
????????int?personSize?=?persons.size();
????????for(int?i=0;i????????????persons.get(i).setLabel(persons.get(i).getName().concat("-"+persons.get(i).getAge()).concat("-"+persons.get(i).getJoinDate().getTime()));
????????}
????????Instant?date_end?=?Instant.now();
????????LOG.info("for?循環(huán)耗時?===>?{}",?Duration.between(date_start,date_end).toMillis()/1000.0);
????}

????/**
?????*??forEach?循環(huán)耗時?===>?1.607
?????*??forEach?循環(huán)耗時?===>?2.242
?????*??forEach?循環(huán)耗時?===>?1.875
?????*/
????@Test
????public?void?forEach(){
????????Instant?date_start?=?Instant.now();
????????for(Person?p:persons){
????????????p.setLabel(p.getName().concat("-"+p.getAge()).concat("-"+p.getJoinDate().getTime()));
????????}
????????Instant?date_end?=?Instant.now();
????????LOG.info("forEach?循環(huán)耗時?===>?{}",?Duration.between(date_start,date_end).toMillis()/1000.0);
????}

????/**
?????*??streamForeach?循環(huán)耗時?===>?1.972
?????*??streamForeach?循環(huán)耗時?===>?1.969
?????*??streamForeach?循環(huán)耗時?===>?2.125
?????*/
????@Test
????public?void?streamForeach(){
????????Instant?date_start?=?Instant.now();
????????persons.stream().forEach(p->p.setLabel(p.getName().concat("-"+p.getAge()).concat("-"+p.getJoinDate().getTime())));
????????Instant?date_end?=?Instant.now();
????????LOG.info("streamForeach?循環(huán)耗時?===>?{}",?Duration.between(date_start,date_end).toMillis()/1000.0);
????}

????/**
?????*??parallelStreamForeach?循環(huán)耗時?===>?1.897
?????*??parallelStreamForeach?循環(huán)耗時?===>?1.942
?????*??parallelStreamForeach?循環(huán)耗時?===>?1.642
?????*/
????@Test
????public?void?parallelStreamForeach(){
????????Instant?date_start?=?Instant.now();
????????persons.parallelStream().forEach(p->p.setLabel(p.getName().concat("-"+p.getAge()).concat("-"+p.getJoinDate().getTime())));
????????Instant?date_end?=?Instant.now();
????????LOG.info("parallelStreamForeach?循環(huán)耗時?===>?{}",?Duration.between(date_start,date_end).toMillis()/1000.0);
????}

}

• 模擬耗時簡單業(yè)務(wù)邏輯

public?class?DataLoopBlockTest?{

????private?static?final?Logger?LOG=?LoggerFactory.getLogger(DataLoopTest.class);

????private?static?final?List?persons?=?new?ArrayList<>();
????static?{
????????for(int?i=0;i<=100000;i++){
????????????Person?p?=?new?Person();
????????????p.setAge(i);
????????????p.setName("zhangSan");
????????????p.setJoinDate(new?Date());
????????????persons.add(p);
????????}
????}

????/**
?????*?for?循環(huán)耗時?===>?101.385
?????*?for?循環(huán)耗時?===>?102.161
?????*?for?循環(huán)耗時?===>?101.472
?????*
?????*/
????@Test
????public?void?forTest(){
????????Instant?date_start?=?Instant.now();
????????int?personSize?=?persons.size();
????????for(int?i=0;i????????????try?{
????????????????Thread.sleep(1);
????????????????persons.get(i).setLabel(persons.get(i).getName().concat("-"+persons.get(i).getAge()).concat("-"+persons.get(i).getJoinDate().getTime()));
????????????}catch?(Exception?e){
????????????????e.printStackTrace();
????????????}
????????}
????????Instant?date_end?=?Instant.now();
????????LOG.info("for?循環(huán)耗時?===>?{}",?Duration.between(date_start,date_end).toMillis()/1000.0);
????}

????/**
?????*??forEach?循環(huán)耗時?===>?101.027
?????*??forEach?循環(huán)耗時?===>?102.488
?????*??forEach?循環(huán)耗時?===>?101.608
?????*/
????@Test
????public?void?forEach(){
????????Instant?date_start?=?Instant.now();
????????for(Person?p:persons){
????????????try?{
????????????????Thread.sleep(1);
????????????????p.setLabel(p.getName().concat("-"+p.getAge()).concat("-"+p.getJoinDate().getTime()));
????????????}catch?(Exception?e){
????????????????e.printStackTrace();
????????????}
????????}
????????Instant?date_end?=?Instant.now();
????????LOG.info("forEach?循環(huán)耗時?===>?{}",?Duration.between(date_start,date_end).toMillis()/1000.0);
????}

????/**
?????*??streamForeach?循環(huán)耗時?===>?103.246
?????*??streamForeach?循環(huán)耗時?===>?101.128
?????*??streamForeach?循環(huán)耗時?===>?102.615
?????*/
????@Test
????public?void?streamForeach(){
????????Instant?date_start?=?Instant.now();
????????//persons.stream().forEach(p->p.setLabel(p.getName().concat("-"+p.getAge()).concat("-"+p.getJoinDate().getTime())));
????????persons.stream().forEach(p->{
????????????try?{
????????????????Thread.sleep(1);
????????????????p.setLabel(p.getName().concat("-"+p.getAge()).concat("-"+p.getJoinDate().getTime()));
????????????}catch?(Exception?e){
????????????????e.printStackTrace();
????????????}
????????});
????????Instant?date_end?=?Instant.now();
????????LOG.info("streamForeach?循環(huán)耗時?===>?{}",?Duration.between(date_start,date_end).toMillis()/1000.0);
????}

????/**
?????*??parallelStreamForeach?循環(huán)耗時?===>?51.391
?????*??parallelStreamForeach?循環(huán)耗時?===>?53.509
?????*??parallelStreamForeach?循環(huán)耗時?===>?50.831
?????*/
????@Test
????public?void?parallelStreamForeach(){
????????Instant?date_start?=?Instant.now();
????????//persons.parallelStream().forEach(p->p.setLabel(p.getName().concat("-"+p.getAge()).concat("-"+p.getJoinDate().getTime())));
????????persons.parallelStream().forEach(p->{
????????????try?{
????????????????Thread.sleep(1);
????????????????p.setLabel(p.getName().concat("-"+p.getAge()).concat("-"+p.getJoinDate().getTime()));
????????????}catch?(Exception?e){
????????????????e.printStackTrace();
????????????}
????????});
????????Instant?date_end?=?Instant.now();
????????LOG.info("parallelStreamForeach?循環(huán)耗時?===>?{}",?Duration.between(date_start,date_end).toMillis()/1000.0);
????????//LOG.info("\r\n===>?{}",JSON.toJSONString(persons.get(10000)));
????}

}

可以看到在百萬數(shù)據(jù)下做簡單數(shù)據(jù)循環(huán)處理，對于普通for(for\foreach)循環(huán)或Stream(并行、非并行)下，幾者的效率差異并不明顯。

注意: 在百萬數(shù)據(jù)下，普通for、foreach循環(huán)處理可能比Stream的方式快許多，對于這點效率的損耗，其實lambda表達式對代碼的簡化更大！

另外,在并行流的循環(huán)下速度提升了一倍之多，當(dāng)單個循環(huán)耗時較多時，會拉大與前幾者的循環(huán)效率 ?(以上測試僅對于循環(huán)而言，其他類型業(yè)務(wù)處理,比如排序、求和、最大值等未做測試，個人猜測與以上測試結(jié)果相似)

六. Stream(流)注意項

• 并行Stream不是線程安全的，當(dāng)對循壞外部統(tǒng)一對象進行讀寫時候會造成意想不到的錯誤，這需要留意
• 因Stream總是惰性的，原對象是不可以被修改的，在集合處理完成后需要將處理結(jié)果放入一個新的集合容器內(nèi)
• 普通循環(huán)與Stream(非并行)循環(huán)，在處理處理數(shù)據(jù)量比較大的時候效率是一致的，推薦使用Stream的形式
• 對于List刪除操作，目前只提供了removeIf方法來實現(xiàn)，并不能使用并行方式
• 對于lambda表達式的寫法

• 當(dāng)表達式內(nèi)只有一個返回boolean類型的語句時候語句是可以簡寫的，例如：

persons.parallelStream().forEach(p->p.setLabel(p.getName().concat("-"+p.getAge()).concat("-"+p.getJoinDate().getTime())));

• 當(dāng)表達式內(nèi)會有一些復(fù)雜處理邏輯時需要加上大括號，這與初始化List參數(shù)方式大致一致

try?{
????????Thread.sleep(1);
????????p.setLabel(p.getName().concat("-"+p.getAge()).concat("-"+p.getJoinDate().getTime()));
????}catch?(Exception?e){
????????e.printStackTrace();
????}
});

七. stream&Lambda表達式常用api方法

• 流到流之間的轉(zhuǎn)換類

• filter(過濾)
• map(映射轉(zhuǎn)換)
• mapTo[Int|Long|Double] (到基本類型流的轉(zhuǎn)換)
• flatMap(流展開合并)
• flatMapTo[Int|Long|Double]
• sorted(排序)
• distinct(不重復(fù)值)
• peek(執(zhí)行某種操作，流不變，可用于調(diào)試)
• limit(限制到指定元素數(shù)量)
• skip(跳過若干元素)

• 流到終值的轉(zhuǎn)換類

• toArray（轉(zhuǎn)為數(shù)組）
• reduce（推導(dǎo)結(jié)果）
• collect（聚合結(jié)果）
• min(最小值)
• max(最大值)
• count (元素個數(shù))
• anyMatch (任一匹配)
• allMatch(所有都匹配)
• noneMatch(一個都不匹配)
• findFirst（選擇首元素）
• findAny(任選一元素)

• 直接遍歷類

• forEach (不保證順序遍歷，比如并行流)
• forEachOrdered（順序遍歷)

• 構(gòu)造流類

• empty (構(gòu)造空流)
• of (單個元素的流及多元素順序流)
• iterate (無限長度的有序順序流)
• generate (將數(shù)據(jù)提供器轉(zhuǎn)換成無限非有序的順序流)
• concat (流的連接)
• Builder (用于構(gòu)造流的Builder對象)