blocks|key|2099691|text|Optional<User>.ifPresent()以一个Consumer<?+super+User>作为参数。您传递给它的是一个类型为空的表达式。所以这不能编译。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2099692|“消费者”将作为lambda表达式实现：|2099693|Optional<User>+user+=+...
user.ifPresent(theUser+->+doSomethingWithUser(theUser));|code-block|syntax|javascript|2099694|甚至更简单，使用方法引用：|2099695|Optional<User>+user+=+...
user.ifPresent(this::doSomethingWithUser);|2099696|这基本上和|2099697|Optional<User>+user+=+...
user.ifPresent(new+Consumer<User>()+{
++++@Override
++++public+void+accept(User+theUser)+{
++++++++doSomethingWithUser(theUser);
++++}
});|2099698|其思想是，只有当用户在场时，doSomethingWithUser()方法调用才会被执行。您的代码直接执行方法调用，并尝试将其空结果传递给ifPresent()。|2099699|entityMap^0|0|Q|T|M|0|0|0|0|0|0|0|E|L|1X|B|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|13|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|15|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|16|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|17|8|@]|D|@]|E|$]]|$1|S|3|T|5|J|7|18|8|@]|D|@]|E|$K|L]]|$1|U|3|V|5|6|7|19|8|@$9|1A|A|1B|B|C]|$9|1C|A|1D|B|C]]|D|@]|E|$]]|$1|W|3|-4|5|6|7|1E|8|@]|D|@]|E|$]]]|X|$]]

<code>Optional&lt;User&gt;.ifPresent()</code> takes a <code>Consumer&lt;? super User&gt;</code> as argument. You're passing it an expression whose type is void. So that doesn't compile. 

A Consumer is intended to be implemented as a lambda expression:

<pre><code>Optional&lt;User&gt; user = ...
user.ifPresent(theUser -&gt; doSomethingWithUser(theUser));
</code></pre>

Or even simpler, using a method reference:

<pre><code>Optional&lt;User&gt; user = ...
user.ifPresent(this::doSomethingWithUser);
</code></pre>

This is basically the same thing as

<pre><code>Optional&lt;User&gt; user = ...
user.ifPresent(new Consumer&lt;User&gt;() {
 @Override
 public void accept(User theUser) {
 doSomethingWithUser(theUser);
 }
});
</code></pre>

The idea is that the <code>doSomethingWithUser()</code> method call will only be executed if the user is present. Your code executes the method call directly, and tries to pass its void result to <code>ifPresent()</code>.

blocks|key|2099729|text|您可以像这样使用方法引用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2099730|user.ifPresent(ClassNameWhereMethodIs::doSomethingWithUser);|code-block|syntax|javascript|2099731|方法ifPresent()获取Consumer对象作为参数，并(来自JavaDoc)：“如果存在值，则使用该值调用指定的使用者。”它是您的变量user。|offset|length|style|CODE|2099732|或者，如果此方法doSomethingWithUser在User类中而不是static，则可以使用以下方法引用：|2099733|user.ifPresent(this::doSomethingWithUser);|2099734|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/8/docs/api/java/util/Optional.html#ifPresent-java.util.function.Consumer-^0|0|0|2|B|F|8|1Z|4|Y|7|0|0|8|J|S|4|11|6|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|10|8|@$I|11|J|12|K|L]|$I|13|J|14|K|L]|$I|15|J|16|K|L]]|9|@$I|17|J|18|1|19]]|A|$]]|$1|M|3|N|5|6|7|1A|8|@$I|1B|J|1C|K|L]|$I|1D|J|1E|K|L]|$I|1F|J|1G|K|L]]|9|@]|A|$]]|$1|O|3|P|5|D|7|1H|8|@]|9|@]|A|$E|F]]|$1|Q|3|-4|5|6|7|1I|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

You can use method reference like this:

<pre><code>user.ifPresent(ClassNameWhereMethodIs::doSomethingWithUser);
</code></pre>

Method <code>ifPresent()</code> get <code>Consumer</code> object as a paremeter and (from <a href="http://docs.oracle.com/javase/8/docs/api/java/util/Optional.html#ifPresent-java.util.function.Consumer-" rel="noreferrer">JavaDoc</a>): "If a value is present, invoke the specified consumer with the value." Value it is your variable <code>user</code>.

Or if this method <code>doSomethingWithUser</code> is in the <code>User</code> class and it is not <code>static</code>, you can use method reference like this:

<pre><code>user.ifPresent(this::doSomethingWithUser);
</code></pre>

blocks|key|2099752|text|除了@JBNizet的答案之外，我对ifPresent的一般用例是将.isPresent()和.get()结合起来|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2099753|旧方式：|2099754|Optional+opt+=+getIntOptional();
if(opt.isPresent())+{
++++Integer+value+=+opt.get();
++++//+do+something+with+value
}|code-block|syntax|javascript|2099755|新途径：|2099756|Optional+opt+=+getIntOptional();
opt.ifPresent(value+->+{
++++//+do+something+with+value
})|2099757|对我来说，这更直观。|2099758|entityMap^0|I|9|Y|C|1B|6|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|13|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|15|8|@]|D|@]|E|$]]|$1|S|3|-4|5|6|7|16|8|@]|D|@]|E|$]]]|T|$]]

In addition to @JBNizet's answer, my general use case for <code>ifPresent</code> is to combine <code>.isPresent()</code> and <code>.get()</code>:

Old way:

<pre><code>Optional opt = getIntOptional();
if(opt.isPresent()) {
 Integer value = opt.get();
 // do something with value
}
</code></pre>

New way:

<pre><code>Optional opt = getIntOptional();
opt.ifPresent(value -&gt; {
 // do something with value
})
</code></pre>

This, to me, is more intuitive.

blocks|key|80888|text|使用flatMap。如果存在一个值，则flatMap返回一个只包含该值的顺序流，否则返回一个空流。因此，不需要使用ifPresent()。示例：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|80889|list.stream().map(data+->+data.getSomeValue).map(this::getOptinalValue).flatMap(Optional::stream).collect(Collectors.toList());|code-block|syntax|javascript|80890|entityMap^0|1L|B|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

Use flatMap. If a value is present, flatMap returns a sequential Stream containing only that value, otherwise returns an empty Stream. So there is no need to use <code>ifPresent()</code> . Example:

<pre><code>list.stream().map(data -&gt; data.getSomeValue).map(this::getOptinalValue).flatMap(Optional::stream).collect(Collectors.toList());
</code></pre>

blocks|key|81112|text|既然你可以让代码变得简单，为什么要编写复杂的代码呢？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|81113|实际上，如果您绝对要使用Optional类，那么最简单的代码就是您已经编写的.|offset|length|style|CODE|81114|if+(user.isPresent())
{
++++doSomethingWithUser(user.get());
}|code-block|syntax|javascript|81115|这段代码的优点是|81116|可读性|ordered-list-item|81117|易于调试(断点)|81118|不棘手|81119|仅仅因为Oracle在Java8中添加了Optional类，并不意味着这个类必须在所有情况下都使用。|81120|entityMap^0|0|C|8|0|0|0|0|0|0|K|8|0^^$0|@$1|2|3|4|5|6|7|Z|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|10|8|@$D|11|E|12|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|13|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|14|8|@]|9|@]|A|$]]|$1|O|3|P|5|Q|7|15|8|@]|9|@]|A|$]]|$1|R|3|S|5|Q|7|16|8|@]|9|@]|A|$]]|$1|T|3|U|5|Q|7|17|8|@]|9|@]|A|$]]|$1|V|3|W|5|6|7|18|8|@$D|19|E|1A|F|G]]|9|@]|A|$]]|$1|X|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]]|Y|$]]

Why write complicated code when you could make it simple?

Indeed, if you are absolutely going to use the <code>Optional</code> class, the most simple code is what you have already written ...

<pre><code>if (user.isPresent())
{
 doSomethingWithUser(user.get());
}
</code></pre>

This code has the advantages of being 

<ol>
<li>readable </li>
<li>easy to debug (breakpoint) </li>
<li>not tricky</li>
</ol>

Just because Oracle has added the <code>Optional</code> class in Java 8 doesn't mean that this class must be used in all situation.

I am trying to understand the <code>ifPresent()</code> method of the <code>Optional</code> API in Java 8.

I have simple logic:

<pre><code>Optional&lt;User&gt; user=...
user.ifPresent(doSomethingWithUser(user.get()));
</code></pre>

But this results in a compilation error: 

<pre><code>ifPresent(java.util.functionError:(186, 74) java: 'void' type not allowed here)
</code></pre>

Of course I can do something like this:

<pre><code>if(user.isPresent())
{
 doSomethingWithUser(user.get());
}
</code></pre>

But this is exactly like a cluttered <code>null</code> check.

If I change the code into this:

<pre><code> user.ifPresent(new Consumer&lt;User&gt;() {
 @Override public void accept(User user) {
 doSomethingWithUser(user.get());
 }
 });
</code></pre>

The code is getting dirtier, which makes me think of going back to the old <code>null</code> check.

Any ideas?

Proper usage of Optional.ifPresent()

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我试图理解Java8中的ifPresent() API的Optional方法。我有一个简单的逻辑：Optional<User> user=...user.ifPresent(doSomethingWithUser(user.get()));但这会导致编译错误：ifPresent(java.util.functionEr...

问正确使用Optional.ifPresent()
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问正确使用Optional.ifPresent()EN