在文章的開頭，先說下NPE問題，NPE問題就是，我們在開發(fā)中經(jīng)常碰到的NullPointerException.假設我們有兩個類，他們的UML類圖如下圖所示
[圖片上傳失敗...(image-648081-1650026123891)]
在這種情況下，有如下代碼
<pre class="prettyprint hljs less" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">user.getAddress().getProvince();</pre>
這種寫法，在user為null時，是有可能報NullPointerException異常的。為了解決這個問題，于是采用下面的寫法
<pre class="prettyprint hljs vbscript" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">if(user!=null){
Address address = user.getAddress();
if(address!=null){
String province = address.getProvince();
}
}</pre>
這種寫法是比較丑陋的，為了避免上述丑陋的寫法，讓丑陋的設計變得優(yōu)雅。JAVA8提供了Optional類來優(yōu)化這種寫法，接下來的正文部分進行詳細說明
一個連載多年還在繼續(xù)更新的免費教程：http://blog.didispace.com/spring-boot-learning-2x/
API 介紹
先介紹一下API，與其他文章不同的是，本文采取類比的方式來講，同時結(jié)合源碼。而不像其他文章一樣，一個個API羅列出來，讓人找不到重點。
1、Optional(T value),empty(),of(T value),ofNullable(T value)
這四個函數(shù)之間具有相關性，因此放在一組進行記憶。
先說明一下， Optional(T value) ，即構(gòu)造函數(shù)，它是private權(quán)限的，不能由外部調(diào)用的。其余三個函數(shù)是public權(quán)限，供我們所調(diào)用。那么，Optional的本質(zhì)，就是內(nèi)部儲存了一個真實的值，在構(gòu)造的時候，就直接判斷其值是否為空。好吧，這么說還是比較抽象。直接上 Optional(T value) 構(gòu)造函數(shù)的源碼，如下圖所示
[圖片上傳失敗...(image-2a1d33-1650026123891)]
那么，of(T value)的源碼如下
<pre class="prettyprint hljs cs" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public static <T> Optional<T> of(T value) {
return new Optional<>(value);
}</pre>
也就是說of(T value)函數(shù)內(nèi)部調(diào)用了構(gòu)造函數(shù)。根據(jù)構(gòu)造函數(shù)的源碼我們可以得出兩個結(jié)論:
- 通過
of(T value)函數(shù)所構(gòu)造出的Optional對象，當Value值為空時，依然會報NullPointerException。 - 通過
of(T value)函數(shù)所構(gòu)造出的Optional對象，當Value值不為空時，能正常構(gòu)造Optional對象。
除此之外呢，Optional類內(nèi)部還維護一個value為null的對象，大概就是長下面這樣的
<pre class="prettyprint hljs php" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public final class Optional<T> {
//省略....
private static final Optional<?> EMPTY = new Optional<>();
private Optional() {
this.value = null;
}
//省略...
public static<T> Optional<T> empty() {
@SuppressWarnings("unchecked")
Optional<T> t = (Optional<T>) EMPTY;
return t;
}
}</pre>
那么， empty（） 的作用就是返回EMPTY對象。
好了鋪墊了這么多，可以說 ofNullable(T value) 的作用了，上源碼
<pre class="prettyprint hljs cs" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public static <T> Optional<T> ofNullable(T value) {
return value == null ? empty() : of(value);
}</pre>
好吧，大家應該都看得懂什么意思了。相比較 of(T value) 的區(qū)別就是，當value值為null時，of(T value)會報NullPointerException異常； ofNullable(T value) 不會throw Exception， ofNullable(T value) 直接返回一個 EMPTY 對象。那是不是意味著，我們在項目中只用 ofNullable 函數(shù)而不用of函數(shù)呢?不是的，一個東西存在那么自然有存在的價值。當我們在運行過程中，不想隱藏 NullPointerException 。而是要立即報告，這種情況下就用Of函數(shù)。但是不得不承認，這樣的場景真的很少。博主也僅在寫junit測試用例中用到過此函數(shù)。
2、orElse(T other)，orElseGet(Supplier other)和orElseThrow(Supplier exceptionSupplier)
這三個函數(shù)放一組進行記憶，都是在構(gòu)造函數(shù)傳入的value值為null時，進行調(diào)用的。 orElse 和 orElseGet 的用法如下所示，相當于value值為null時，給予一個默認值:
<pre class="prettyprint hljs livescript" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">@Test
public void test() {
User user = null;
user = Optional.ofNullable(user).orElse(createUser());
user = Optional.ofNullable(user).orElseGet(() -> createUser());
}
public User createUser(){
User user = new User();
user.setName("zhangsan");
return user;
}</pre>
這兩個函數(shù)的區(qū)別：當user值不為null時， orElse 函數(shù)依然會執(zhí)行createUser()方法，而 orElseGet 函數(shù)并不會執(zhí)行createUser()方法，大家可自行測試。至于orElseThrow，就是value值為null時,直接拋一個異常出去，用法如下所示
<pre class="prettyprint hljs livescript" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">User user = null;
Optional.ofNullable(user).orElseThrow(()->new Exception("用戶不存在"));</pre>
3、map(Function mapper)和flatMap(Function> mapper)
這兩個函數(shù)放在一組記憶，這兩個函數(shù)做的是轉(zhuǎn)換值的操作。直接上源碼
<pre class="prettyprint hljs kotlin" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public final class Optional<T> {
//省略....
public<U> Optional<U> map(Function<? super T, ? extends U> mapper) {
Objects.requireNonNull(mapper);
if (!isPresent())
return empty();
else {
return Optional.ofNullable(mapper.apply(value));
}
}
//省略...
public<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) {
Objects.requireNonNull(mapper);
if (!isPresent())
return empty();
else {
return Objects.requireNonNull(mapper.apply(value));
}
}
}</pre>
這兩個函數(shù)，在函數(shù)體上沒什么區(qū)別。唯一區(qū)別的就是入?yún)?，map函數(shù)所接受的入?yún)㈩愋蜑? Function<? super T, ? extends U> ，而flapMap的入?yún)㈩愋蜑? Function<? super T, Optional<U>> 。在具體用法上，對于map而言：如果User結(jié)構(gòu)是下面這樣的
<pre class="prettyprint hljs kotlin" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class User {
private String name;
public String getName() {
return name;
}
}</pre>
這時候取name的寫法如下所示
<pre class="hljs vbnet" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 0.75em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">String city = Optional.ofNullable(user).map(u-> u.getName()).get();</pre>
對于flatMap而言:
如果User結(jié)構(gòu)是下面這樣的
<pre class="prettyprint hljs kotlin" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class User {
private String name;
public Optional<String> getName() {
return Optional.ofNullable(name);
}
}</pre>
這時候取name的寫法如下所示
<pre class="prettyprint hljs vbnet" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">String city = Optional.ofNullable(user).flatMap(u-> u.getName()).get();</pre>
4、isPresent()和ifPresent(Consumer consumer)
這兩個函數(shù)放在一起記憶， isPresent 即判斷value值是否為空，而 ifPresent 就是在value值不為空時，做一些操作。這兩個函數(shù)的源碼如下
<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public final class Optional<T> {
//省略....
public boolean isPresent() {
return value != null;
}
//省略...
public void ifPresent(Consumer<? super T> consumer) {
if (value != null)
consumer.accept(value);
}
}</pre>
需要額外說明的是，大家千萬不要把
<pre class="prettyprint hljs ruby" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">if (user != null){
// TODO: do something
}</pre>
給寫成
<pre class="prettyprint hljs ruby" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">User user = Optional.ofNullable(user);
if (Optional.isPresent()){
// TODO: do something
}</pre>
因為這樣寫，代碼結(jié)構(gòu)依然丑陋。博主會在后面給出正確寫法至于 ifPresent(Consumer<? super T> consumer) ，用法也很簡單，如下所示
<pre class="prettyprint hljs less" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">Optional.ofNullable(user).ifPresent(u->{
// TODO: do something
});</pre>
5、filter(Predicate predicate)
不多說，直接上源碼
<pre class="prettyprint hljs kotlin" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public final class Optional<T> {
//省略....
Objects.requireNonNull(predicate);
if (!isPresent())
return this;
else
return predicate.test(value) ? this : empty();
}</pre>
filter 方法接受一個 Predicate 來對 Optional 中包含的值進行過濾，如果包含的值滿足條件，那么還是返回這個 Optional；否則返回 Optional.empty 。用法如下
<pre class="prettyprint hljs dockerfile" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">Optional<User> user1 = Optional.ofNullable(user).filter(u -> u.getName().length()<6);</pre>
如上所示，如果user的name的長度是小于6的，則返回。如果是大于6的，則返回一個EMPTY對象。一個連載多年還在繼續(xù)更新的免費教程：http://blog.didispace.com/spring-boot-learning-2x/
實戰(zhàn)使用
例一
在函數(shù)方法中以前寫法
<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public String getCity(User user) throws Exception{
if(user!=null){
if(user.getAddress()!=null){
Address address = user.getAddress();
if(address.getCity()!=null){
return address.getCity();
}
}
}
throw new Excpetion("取值錯誤");
}</pre>
JAVA8寫法
<pre class="prettyprint hljs cpp" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public String getCity(User user) throws Exception{
return Optional.ofNullable(user)
.map(u-> u.getAddress())
.map(a->a.getCity())
.orElseThrow(()->new Exception("取指錯誤"));
}</pre>
例二
比如，在主程序中
以前寫法
<pre class="prettyprint hljs less" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">if(user!=null){
dosomething(user);
}</pre>
JAVA8寫法
<pre class="prettyprint hljs less" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">Optional.ofNullable(user)
.ifPresent(u->{
dosomething(u);
});</pre>
例三
以前寫法
<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public User getUser(User user) throws Exception{
if(user!=null){
String name = user.getName();
if("zhangsan".equals(name)){
return user;
}
}else{
user = new User();
user.setName("zhangsan");
return user;
}
}</pre>
java8寫法
<pre class="prettyprint hljs livescript" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public User getUser(User user) {
return Optional.ofNullable(user)
.filter(u->"zhangsan".equals(u.getName()))
.orElseGet(()-> {
User user1 = new User();
user1.setName("zhangsan");
return user1;
});
}</pre>
其他的例子，不一一列舉了。
