dangertree techblog

One thing I found myself doing more than I wanted with java was collection comparison. Many times, you might have a large list of all available items, and a smaller list of selected items. Commonly, you might want to find all the items that exist in the large collection, but have not been ’selected’ in the smaller collection. Invariably in java, this means nested loops.

Not so much with groovy.

def smallList = [1,3,5]
def bigList = [1,1,1,2,3,4,4,4,5,6,7,7,8,9]

def result = bigList - smallList
println result.class
println result

The result is:

class java.util.ArrayList
[2, 4, 4, 4, 6, 7, 7, 8, 9]

Groovy correctly made the assumption that I was making a List in my snippet above. And it removed any element within the first list where equals() was true for an element in the first list. That means it took away three 1’s and one 3. Keep in mind the original collections are untouched.

But what if I want to compare a List and a Set?

Set mySet = [1,3,5]

List myList = [0,1,2,3,4,0,1,2,3,4,5,6,7,8,9]

def addition = myList + mySet
println addition.class
println addition

addition = mySet + myList
println addition.class
println addition

Now we’re adding a List and a Set together, and the result is different depending on which is the first operand:

class java.util.ArrayList
[0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 3, 5]
class java.util.HashSet
[2, 4, 9, 8, 6, 1, 3, 7, 5, 0]

If the List is the first operand, groovy assumes you want to treat the following operand as a List as well, and it just tacks the values onto the end. But when the Set is first, groovy first converts the second argument into a Set as well before doing its addition. This removes all but unique elements from it before performing the addition.

Set mySet = [1,3,5]

List myList = [0,1,2,3,4,0,1,2,3,4,5,6,7,8,9]

def subtraction = myList - mySet
println subtraction.class
println subtraction

subtraction = mySet - myList
println subtraction.class
println subtraction

We should again see the automatic type conversion as in the above examples depending on which type is the first operand. In the results below, you can see that all elements within the set were removed from the list (there are no 1’s, 3’s, or 5’s). The second subtraction results in 0 elements, because the List (which is converted to a Set) being subtracted contains 1, 3, and 5, so all are removed from the original Set.

class java.util.ArrayList
[0, 2, 4, 0, 2, 4, 6, 7, 8, 9]
class java.util.HashSet
[]

Good stuff. This is just another example of how groovy does away with a lot of the annoying things you have to do with java.

This entry was posted on Wednesday, March 26th, 2008 at 9:58 pm and is filed under groovy. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.