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    copied!<p>If you are looking for a library, my first thought was to use <a href="http://guava-libraries.googlecode.com/" rel="nofollow">Google Guava Libraries</a> as follows:</p> <pre><code>public &lt;T, U extends T&gt; List&lt;U&gt; homogenize(List&lt;T&gt; list, final Class&lt;U&gt; subclass) { Predicate&lt;T&gt; pred = new Predicate&lt;T&gt;() { @Override public boolean apply(T input) { return input.getClass().isAssignableFrom(subclass); } }; return Iterables.all(list, pred) ? (List&lt;U&gt;)list : null; } </code></pre> <p>I didn't try it out yet to make sure the kinks are out. However, I looked at it, and decided it was pretty butt-ugly. SLightly better Guava approach is:</p> <pre><code>public &lt;T, U extends T&gt; List&lt;U&gt; homogenize(List&lt;T&gt; list, Class&lt;U&gt; subclass) { Iterable&lt;U&gt; ret = Iterables.filter(list, subclass); if (list.size() != Lists.newArrayList(ret).size()) return null; return (List&lt;U&gt;)list; } </code></pre> <p>However, it's still a bit ugly. And it uses an internal copy of the collection. It does still return a cast view of the original. After all is said and done, the cleanest approach seems to use regular Java:</p> <pre><code>public &lt;T, U extends T&gt; List&lt;U&gt; homogenize(List&lt;T&gt; list, Class&lt;U&gt; subclass) { for( T t : list) { if (!t.getClass().isAssignableFrom(subclass)) return null; } return (List&lt;U&gt;)list; } </code></pre> <p>Depending on your aversion to typecast warnings, you could even drop the cast operators in all three options.</p> <p><strong>EDITS PER COMMENTS</strong> The following changes/improvements were suggested in the comments.</p> <p>Option one improved:</p> <pre><code>public &lt;T, U extends T&gt; List&lt;U&gt; homogenize(List&lt;T&gt; list, final Class&lt;U&gt; subclass) { return Iterables.all(list, Predicates.instanceOf(subclass)) ? (List&lt;U&gt;)list : null; } </code></pre> <p>Option two improved:</p> <pre><code>public &lt;T, U extends T&gt; List&lt;U&gt; homogenize(List&lt;T&gt; list, Class&lt;U&gt; subclass) { Iterable&lt;U&gt; ret = Iterables.filter(list, subclass); return (list.size() != Iterables.size(ret)) ? null : (List&lt;U&gt;)list; } </code></pre> <p>Option three improved:</p> <pre><code>public &lt;T, U extends T&gt; List&lt;U&gt; homogenize(List&lt;T&gt; list, Class&lt;U&gt; subclass) { for( T t : list) { if (!subclass.isInstance(t.getClass())) return null; } return (List&lt;U&gt;)list; } </code></pre> <p>With these improvements the 1st Guava example shines quite a bit. If you don't mind static imports, both Guava examples become extremely readable.</p>
 

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