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copied!<p>Thanks to Java 8 you don't need to do the steps below to pass a function to a method, that's what lambdas are for, see <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html" rel="noreferrer">Oracle's Lambda Expression tutorial</a>. The rest of this post describes what we used to have to do in the bad old days in order to implement this functionality.</p> <p>Typically you declare your method as taking some interface with a single method, then you pass in an object that implements that interface. An example is in commons-collections, where you have interfaces for Closure, Transformer, and Predicate, and methods that you pass implementations of those into. Guava is the new improved commons-collections, you can find equivalent interfaces there.</p> <p>So for instance, commons-collections has org.apache.commons.collections.CollectionUtils, which has lots of static methods that take objects passed in, to pick one at random, there's one called exists with this signature:</p> <pre><code>static boolean exists(java.util.Collection collection, Predicate predicate) </code></pre> <p>It takes an object that implements the interface Predicate, which means it has to have a method on it that takes some Object and returns a boolean.</p> <p>So I can call it like this:</p> <pre><code>CollectionUtils.exists(someCollection, new Predicate() { public boolean evaluate(Object object) { return ("a".equals(object.toString()); } }); </code></pre> <p>and it returns true or false depending on whether <code>someCollection</code> contains an object that the predicate returns true for.</p> <p>Anyway, this is just an example, and commons-collections is outdated. I just forget the equivalent in Guava.</p>

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