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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<title>op4j: bending the Java spoon - About Functions</title>
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<span id="publishDate">Last Published: 19 May 2012</span>
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<strong>· About Functions</strong>
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<div class="section"><h2>Functions<a name="Functions"></a></h2><p>Functions are one of the main components of op4j. They are implementations of the <tt>org.op4j.functions.IFunction<T,R></tt> interface, being <tt>T</tt> the input type and <tt>R</tt> the result type.</p><p>For example: <tt>IFunction<String,Integer></tt> defines a function which takes a <tt>String</tt> as input and returns an <tt>Integer</tt>.</p><p>Inside expressions, functions are executed by the <tt>exec(...)</tt> action or by any of its variants: <tt>map(...)</tt>, <tt>execAsArray(...)</tt>, etc.</p><div class="section"><h3>Predefined functions: Function hub classes<a name="Predefined_functions:_Function_hub_classes"></a></h3><p>op4j provides more than 200 functions out-of-the-box. They are usually obtained by calling static methods in <i>function hub classes</i>. For example:</p><div class="source"><pre>Function<String,String> fn = FnString.toUpperCase();
</pre></div><p><tt>FnString</tt> is one of the predefined <i>function hub classes</i> (most of them start with <tt>Fn</tt>), and the <tt>toUpperCase()</tt> static method returns a function which takes a <tt>String</tt> input and returns a <tt>String</tt> result (<tt>IFunction<String,String></tt>).</p><p>All predefined functions, as well as functions defined with <i>function expressions</i> are <tt>org.op4j.functions.Function</tt> objects, which are an implementation of <tt>IFunction</tt> adding a special <tt>execute(T input)</tt> method for easily calling them <i>even outside op4j expressions</i>.</p><p>Let's see some random examples:</p><div class="source"><pre>Function<String,String> fn = FnString.trim();
Function<String,Calendar> fn = FnString.toCalendar("dd/MM/yyyy");
Function<Number,BigDecimal> fn = FnNumber.toBigDecimal(2, RoundingMode.HALF_UP);
Function<Iterable<Double>,Double> fn = FnDouble().avg();
Function<Iterable<Integer>,Integer> fn = FnInteger().max();
Function<String[],String[]> fn = FnArray.ofString().distinct();
</pre></div><p>As we can see, some function hub classes like <tt>FnArray</tt> are a little bit special because they require specialization (type parameterization), which is what is done with the <tt>ofString()</tt> method.</p><p>Let's see another example of specialization:</p><div class="source"><pre>Type<User> type = ...;
Function<List<User[]>,List<User>> fn = FnList.ofArrayOf(type).flattenArrays();
</pre></div></div><div class="section"><h3>User-defined functions<a name="User-defined_functions"></a></h3><div class="section"><h4>Function expressions<a name="Function_expressions"></a></h4><p>Functions can be created by means of <tt>Fn.on</tt> expressions:</p><div class="source"><pre>Function<List<String>,List<Calendar>> fnParseStringDates =
Fn.onListOf(Types.STRING).map(FnString.toCalendar("dd/MM/yyyy")).get();
</pre></div><p>As functions created with <i>function expressions</i> return <tt>org.op4j.functions.Function</tt> objects they can be easily called <i>even outside op4j expressions</i>:</p><div class="source"><pre>List<Calendar> calendars = fnParseStringDates.execute(stringDates);
</pre></div></div><div class="section"><h4>Custom functions<a name="Custom_functions"></a></h4><p>Creating a custom function without using a function expression is as simple as implementing the one-method <tt>IFunction</tt> interface:</p><div class="source"><pre>public class MyFunction implements IFunction<Integer,String> {
public String execute(Integer input, ExecCtx ctx) throws Exception {
return "The input number is: " + input;
}
}
</pre></div><p>...which can also be done anonymously:</p><div class="source"><pre>String result =
Op.on(value).exec(new IFunction<Integer,String>() {
public String execute(Integer input, ExecCtx ctx) throws Exception {
return "The input number is: " + input;
}
}).get();
</pre></div></div></div></div>
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