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    <p>Basically, you create a Function interface:</p> <pre><code>public interface Func&lt;In, Out&gt; { public Out apply(In in); } </code></pre> <p>and then pass in an anonymous subclass to your method.</p> <p>Your method could either apply the function to each element in-place:</p> <pre><code>public static &lt;T&gt; void applyToListInPlace(List&lt;T&gt; list, Func&lt;T, T&gt; f) { ListIterator&lt;T&gt; itr = list.listIterator(); while (itr.hasNext()) { T output = f.apply(itr.next()); itr.set(output); } } // ... List&lt;String&gt; myList = ...; applyToListInPlace(myList, new Func&lt;String, String&gt;() { public String apply(String in) { return in.toLowerCase(); } }); </code></pre> <p>or create a new <code>List</code> (basically creating a mapping from the input list to the output list):</p> <pre><code>public static &lt;In, Out&gt; List&lt;Out&gt; map(List&lt;In&gt; in, Func&lt;In, Out&gt; f) { List&lt;Out&gt; out = new ArrayList&lt;Out&gt;(in.size()); for (In inObj : in) { out.add(f.apply(inObj)); } return out; } // ... List&lt;String&gt; myList = ...; List&lt;String&gt; lowerCased = map(myList, new Func&lt;String, String&gt;() { public String apply(String in) { return in.toLowerCase(); } }); </code></pre> <p>Which one is preferable depends on your use case. If your list is extremely large, the in-place solution may be the only viable one; if you wish to apply many different functions to the same original list to make many derivative lists, you will want the <code>map</code> version.</p>
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    1. COBut then you are asking me to put every small function in a different class since they have to have a standard name ('apply' in your case). Right ?
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    2. CONot necessarily; your anonymous class can simply call the small function inside apply(). This is as close as Java gets to function pointers without venturing into the perils of reflection.
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    3. COdoToList is reinventing the wheel. What you have done here is a poor design of what is usually called map. The usual interface is public static <T, U> List<U> map(List<T>, Func<T, U> f); What it does is produce an another list instead of modifying the one in place. If you need to modify the original list without destroying the reference, then just do a .clear() followed by an addAll(). Don't combine all that in one method.
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