为什么Java闭包不能通过返回值之外的方式向外传递值？

String a; ........(()->a="a"); return a; 为什么不行，是存在技术问题么？ 其它语言可以么？c#可以么？莫非从此我要Java一生黑？ 至于我为什么问这个问题嘛，就是一个方法有可能有返回值，有可能没有返回值，这就要写两次，还不能同名，写成void和Object两个简直太不优雅。 这个方法其实就是jdbc事务，有查询有不查询，我把事务回滚，异常，日志等写在了一个函数里，这个函数调用这个函数式接口，以后就只要写事务，不用再写回滚，记录日志等操作了。 现在我写了两个方法，感觉很不爽:-(

回复内容：

题主要变通…

Java 8语言上的lambda表达式只实现了capture-by-value，也就是说它捕获的局部变量都会拷贝一份到lambda表达式的实体里，然后在lambda表达式里要变也只能变自己的那份拷贝而无法影响外部原本的变量；但是Java语言的设计者又要挂牌坊不明说自己是capture-by-value，为了以后语言能进一步扩展成支持capture-by-reference留下后路，所以现在干脆不允许向捕获的变量赋值，而且可以捕获的也只有“效果上不可变”（effectively final）的参数/局部变量。
关于Java闭包的讨论可以参考我之前的另一个回答：JVM的规范中允许编程语言语义中创建闭包(closure)吗？ - RednaxelaFX 的回答

但是Java只是不允许改变被lambda表达式捕获的变量，并没有限制这些变量所指向的对象的状态能不能变。要从lambda表达式向外传值的常见workaround之一就是用长度为1的数组：

<code class="language-text">String[] a = new String[1];
... ( () -> a[0] = "a" );
return a[0];
</code>

<code class="language-java"><span class="cm">/* 你可以用AtomicReference将你所需要的值包装起来 */</span>
<span class="kd">private</span> <span class="kd">static</span> <span class="kt">int</span> <span class="nf">sum</span><span class="o">(</span><span class="kt">int</span> <span class="n">low</span><span class="o">,</span> <span class="kt">int</span> <span class="n">high</span><span class="o">)</span> <span class="o">{</span>
    <span class="n">AtomicReference</span><span class="o"><span class="n">Integer</span><span class="o">></span> <span class="n">sum</span> <span class="o">=</span> <span class="k">new</span> <span class="n">AtomicReference</span><span class="o">(</span><span class="mi">0</span><span class="o">);</span>
    <span class="n">IntStream</span><span class="o">.</span><span class="na">rangeClosed</span><span class="o">(</span><span class="n">low</span><span class="o">,</span> <span class="n">high</span><span class="o">).</span><span class="na">forEach</span><span class="o">(</span><span class="n">i</span> <span class="o">-></span> <span class="n">sum</span><span class="o">.</span><span class="na">set</span><span class="o">(</span><span class="n">sum</span><span class="o">.</span><span class="na">get</span><span class="o">()</span> <span class="o">+</span> <span class="n">i</span><span class="o">));</span>
    <span class="k">return</span> <span class="n">sum</span><span class="o">.</span><span class="na">get</span><span class="o">();</span>
<span class="o">}</span>

<span class="cm">/* 对于基本类型有专用的包装类更方便使用（且是原子化操作） */</span>
<span class="kd">private</span> <span class="kd">static</span> <span class="kt">int</span> <span class="nf">sum2</span><span class="o">(</span><span class="kt">int</span> <span class="n">low</span><span class="o">,</span> <span class="kt">int</span> <span class="n">high</span><span class="o">)</span> <span class="o">{</span>
    <span class="n">AtomicInteger</span> <span class="n">sum</span> <span class="o">=</span> <span class="k">new</span> <span class="n">AtomicInteger</span><span class="o">(</span><span class="mi">0</span><span class="o">);</span>
    <span class="n">IntStream</span><span class="o">.</span><span class="na">rangeClosed</span><span class="o">(</span><span class="n">low</span><span class="o">,</span> <span class="n">high</span><span class="o">).</span><span class="na">forEach</span><span class="o">(</span><span class="n">sum</span><span class="o">::</span><span class="n">addAndGet</span><span class="o">);</span>
    <span class="k">return</span> <span class="n">sum</span><span class="o">.</span><span class="na">get</span><span class="o">();</span>
<span class="o">}</span>

<span class="cm">/* 其实很多时候，我们并不需要改变外部状态不是吗？ */</span>
<span class="kd">private</span> <span class="kd">static</span> <span class="kt">int</span> <span class="nf">sum3</span><span class="o">(</span><span class="kt">int</span> <span class="n">low</span><span class="o">,</span> <span class="kt">int</span> <span class="n">high</span><span class="o">)</span> <span class="o">{</span>
    <span class="k">return</span> <span class="n">IntStream</span><span class="o">.</span><span class="na">rangeClosed</span><span class="o">(</span><span class="n">low</span><span class="o">,</span> <span class="n">high</span><span class="o">).</span><span class="na">sum</span><span class="o">();</span>
<span class="o">}</span>
</span></code>

C++：可以

C#：可以

F#：可以

VB：可以

补充 @vczh
JavaScript：也可以

欢迎加入Java黑的大家庭。

<code class="language-java"><span class="kt">void</span> <span class="n">SomeObject</span> <span class="nf">someMethod</span><span class="o">(</span><span class="n">Supplier</span><span class="o"><span class="n">Optional</span><span class="o"><span class="n">Object</span><span class="o">>></span> <span class="n">foo</span><span class="o">)</span> <span class="o">{</span>
    <span class="k">return</span> <span class="n">foo</span><span class="o">.</span><span class="na">get</span><span class="o">().</span><span class="na">map</span><span class="o">(</span><span class="n">a</span> <span class="o">-></span> <span class="o">((</span><span class="n">Object</span> <span class="n">a</span><span class="o">)</span> <span class="o">-></span> <span class="o">...)).</span><span class="na">orElseGet</span><span class="o">(()</span> <span class="o">-></span> <span class="o">...);</span>
<span class="o">}</span>
</span></span></code>

java的函数无法进行引用传递，闭包的捕获也一样。so sad。 什么叫"不能向外传递值"?

改一个field叫不叫传递值? 调用另外的方法叫不叫传递值? Java：如果我有指针或者引用。
指针：怪我了？
引用：…… 你的需求就是多种返回值，那么为什么不考虑使用异常？比修改外部变量来的高明。在开发过程中，其实异常是可以作为逻辑的一部分的。