Wednesday, September 9, 2009

Groovy Goodness: the as Keyword

We can change the type of objects with the as keyword in Groovy. We can even make maps and closure look like interface implementations when we use as. Furthermore we can use as to create an alias for an import statement.

import java.text.MessageFormat as mf // Alias mf

// Use alias and use as to convert list to Object[]:
assert 'Simple message!' == mf.format('Simple {0}!', ['message'] as Object[])

// Normal Groovy list:
def intList = [1,2,3,4]
assert 'java.util.ArrayList' == intList.class.name
assert 4 == intList.size()

// Coerse to int[]:
def intArray = intList as int[]
assert 'java.util.ArrayList' != intArray.class.name
assert 4 == intArray.length  // Use int[] property.

// Use as to create Date object from list:
def date = [109, 8, 6] as Date
assert 2009 == date[Calendar.YEAR]
assert 8 == date[Calendar.MONTH]
assert 6 == date[Calendar.DATE]

// Use as to treat closure as implementation for
// the Runnable interface:
def t = new Thread({ println 'hello' } as Runnable)
t.start()  // Output: hello

// Use as to treat map as implementation for
// the Runnable interface:
def t2 = new Thread([run: { println 'hello' }] as Runnable)
t2.start()  // Output: hello


Jim Shingler said...

I really like your Groovy Goodness Series, . . . Keep up the GR8 work

Anonymous said...

A closure is ever a Runnable

def c = { "Hello World!" }
assert c instanceof Runnable

mrhaki said...

@Anonymous: thank you, I didn't know that.

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