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POJava 8: Mandatory checked exceptions handling in lambda expressions. Why mandatory, not optional?
 

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    <p>I'm playing with the new lambda features in Java 8, and found that the practices offered by Java 8 are really useful. However, I'm wondering is there a <em>good</em> way to make a work-around for the following scenario. Suppose you have an object pool wrapper that requires some kind of a factory to fill the object pool, for example (using <code>java.lang.functions.Factory</code>):</p> <pre class="lang-java prettyprint-override"><code>public class JdbcConnectionPool extends ObjectPool&lt;Connection&gt; { public ConnectionPool(int maxConnections, String url) { super(new Factory&lt;Connection&gt;() { @Override public Connection make() { try { return DriverManager.getConnection(url); } catch ( SQLException ex ) { throw new RuntimeException(ex); } } }, maxConnections); } } </code></pre> <p>After transforming the functional interface into lambda expression, the code above becomes like that:</p> <pre class="lang-java prettyprint-override"><code>public class JdbcConnectionPool extends ObjectPool&lt;Connection&gt; { public ConnectionPool(int maxConnections, String url) { super(() -&gt; { try { return DriverManager.getConnection(url); } catch ( SQLException ex ) { throw new RuntimeException(ex); } }, maxConnections); } } </code></pre> <p>Not so bad indeed, but the checked exception <code>java.sql.SQLException</code> requires a <code>try</code>/<code>catch</code> block inside the lambda. At my company we use two interfaces for long time:</p> <ul> <li><code>IOut&lt;T&gt;</code> that is an equivalent to <code>java.lang.functions.Factory</code>;</li> <li>and a special interface for the cases that usually require checked exceptions propagation: <code>interface IUnsafeOut&lt;T, E extends Throwable&gt; { T out() throws E; }</code>.</li> </ul> <p>Both <code>IOut&lt;T&gt;</code> and <code>IUnsafeOut&lt;T&gt;</code> are supposed to be removed during migration to Java 8, however there is no exact match for <code>IUnsafeOut&lt;T, E&gt;</code>. If the lambda expressions could deal with checked exceptions like they were unchecked, it could be possible to use simply like the following in the constructor above:</p> <pre class="lang-java prettyprint-override"><code>super(() -&gt; DriverManager.getConnection(url), maxConnections); </code></pre> <p>That looks much cleaner. I see that I can rewrite the <code>ObjectPool</code> super class to accept our <code>IUnsafeOut&lt;T&gt;</code>, but as far as I know, Java 8 is not finished yet, so could be there some changes like:</p> <ul> <li>implementing something similar to <code>IUnsafeOut&lt;T, E&gt;</code>? (to be honest, I consider that dirty - the subject must choose what to accept: either <code>Factory</code> or "unsafe factory" that cannot have compatible method signatures)</li> <li>simply ignoring checked exceptions in lambdas, so no need in <code>IUnsafeOut&lt;T, E&gt;</code> surrogates? (why not? e.g. another important change: OpenJDK, that I use, <code>javac</code> now does not require variables and parameters to be declared as <code>final</code> to be captured in an anonymous class [functional interface] or lambda expression)</li> </ul> <p>So the question is generally is: is there a way to bypass checked exceptions in lambdas or is it planned in the future until Java 8 is finally released?</p> <hr> <p>Update 1</p> <p>Hm-m-m, as far as I understand what we currently have, it seems there is no way at the moment, despite the referenced article is dated from 2010: <a href="https://web.archive.org/web/20170313093438/blogs.oracle.com/briangoetz/entry/exception_transparency_in_java" rel="nofollow noreferrer">Brian Goetz explains exception transparency in Java</a>. If nothing changed much in Java 8, this could be considered an answer. Also Brian says that <code>interface ExceptionalCallable&lt;V, E extends Exception&gt;</code> (what I mentioned as <code>IUnsafeOut&lt;T, E extends Throwable&gt;</code> out of our code legacy) is pretty much useless, and I agree with him.</p> <p>Do I still miss something else?</p>
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    1. USLyubomyr Shaydariv
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    1. COFor anyone following the evolution of the lambdas API, it's worth noting that java.util.functions.Factory::make is now java.util.function.Supplier::get. You can see an up-to-date version of the API docs at http://lambdadoc.net, a subsite of the [Lambda FAQ] (http://lambdafaq.org)
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      1. USMaurice Naftalin
    2. CO@MauriceNaftalin, thanks for the comment. I currently use OpenJDK 1.8.0-EA.
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    3. COone approach is given in [this blog](http://java8blog.com/post/37385501926/fixing-checked-exceptions-in-java-8)
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