Since Groovy 2.1 we have a couple of extra methods available for objects that implement the java.lang.Appendable interface. A lot of Writer objects implement this interface. The documentation of the Appendable interface mentions: An object to which char sequences and values can be appended.

First the leftShift() method is added. This means we can use the leftShift operator (<<) to append a value. Then we also can use the withFormatter() method. We can pass a closure to this method or a java.util.Locale object and a closure. The closure has a single parameter which is a java.util.Formatter instance. We can use this method to add printf-style formatted strings to an Appendable object. If we pass a Locale object to the method the Formatter is created with the specified Locale to created localized printf-sytle formatted strings.

// Create object that implements the Appendable interface.
final Appendable appendable = new StringWriter()

assert appendable in Appendable


// Use leftShift operator to add to Appendable implementation.
appendable << 'Groovy is Gr8!' << newLine


// Use withFormatter() method.
// Formatter object 
// is passed to closure as parameter.
appendable.withFormatter { formatter ->
    // Simple formatter pattern to reorder the arguments.
    formatter.format(/m r %3$1s %2$1s %1$1s %4$1s%n/, 'k', 'a', 'h', 'i')
}


// Use withFormatter() method and use Locale object
// as extra argument. The Locale is passed on
// to create the Formatter object.
appendable.withFormatter(Locale.US) { formatter ->
    formatter.format("US: " + datePattern, date)
}
// Use different Locale.
appendable.withFormatter(new Locale('nl')) { formatter ->
    formatter.format("Dutch: $datePattern", date)
}


// Check result is as expected:
assert appendable.toString() == '''Groovy is Gr8!
m r h a k i
US: January 27, 2013
Dutch: januari 27, 2013
'''


// Helper getters:

String getNewLine() {
    System.getProperty('line.separator')
}

Date getDate() {
    // Simple date to use in withFormatter() methods.
    Date.parse('yyyy-MM-dd', '2013-01-27')
}

String getDatePattern() {
    // Date pattern for Formatter.
    '%1$tB %1$te, %1$tY%n'
}

Code written with Groovy 2.1