Suppose you wish to test a class which is dependent on a static call. Is there a way to mock out that call?

Yes, there are two suggested approaches:

Using EMC

Here we are calling Arrays.sort() directly - normally that would be the problematic code within your class under test.

// non-mocked first to prove it works normally
String[] things = [ 'dog', 'ant', 'bee', 'cat' ]
Arrays.sort(things)
println things // => {"ant", "bee", "cat", "dog"}

Arrays.metaClass.'static'.sort = { a ->
    a[0] = 'ewe'
    a[1] = 'fox'
}

things = [ 'dog', 'ant', 'bee', 'cat' ]
Arrays.sort(things)
println things // => {"ewe", "fox", "bee", "cat"}

More details about this approach: ExpandoMetaClass - Adding static methods

Using JMockit

If you are in a position to use Java 6, you should also consider using JMockit.

// require(url:'https://jmockit.dev.java.net', jar='jmockit.jar')
// require(url:'https://jmockit.dev.java.net', jar='jmockit-asm2.jar')
// needs to be run with "-javaagent:jmockit.jar"
// and "-Xbootclasspath/a:jmockit.jar;.;jmockit-asm2.jar"
// The bootclasspath option is only required because we are mocking
// a class from the java.* package (part of the bootclasspath for Java)

import mockit.Mockit

// non-mocked first to show normal case
String[] things = [ 'dog', 'ant', 'bee', 'cat' ]
Arrays.sort(things)
println things // => {"ant", "bee", "cat", "dog"}

Mockit.redefineMethods(Arrays, MockArrays)

things = [ 'dog', 'ant', 'bee', 'cat' ]
Arrays.sort(things)
println things // => {"dog", "elk", "ape", "cat"}

Where MockArrays is the following Java class:

// Java
public class MockArrays {
  public static void sort(Object[] a) {
    a[1] = "elk";
    a[2] = "ape";
  }
}

We use a Java class here because otherwise JMockit tries to replace other GroovyObject methods (e.g. getMetaClass, invokeMethod, ...) and won't find them inside the java.util.Arrays class. Obviously, if your redefining a Groovy class, you can use another Groovy class.

More details: Using JMockit with Groovy.