what are the different ways to create an Arraylist and Hashmap in groovy

Question

I have created an ArrayList like the following:

def list = new ArrayList()

But the codenarc report it is warning like following.

ArrayList objects are better instantiated using the form "[] as ArrayList"

What are the better ways to instantiate the collections?

Answer 1

You can do:

def list = [] // Default is ArrayList
def list = [] as ArrayList
ArrayList list = []

And again, for HashMap:

HashMap map = [:]
def map = [:] as HashMap

The default in this case is a LinkedHashMap:

def map = [:] 

• srini: no need of putting ':' simply we can write the same as def map = [] as HashMap
• tim_yates: @Oda True, but I wouldn't use that as I couldn't say whether it will always work in future versions

Answer 2

Typical is:

def list = []

other options include

def list = new ArrayList()
def list = new ArrayList<Foo>()
List list = new ArrayList()
def list = [] as ArrayList
List list = [] as ArrayList

and of course:

List<Foo> list = new ArrayList<Foo>();

Similar options exist for HashMap using [:].

• Mahesha999: It should be List list = [] as ArrayList, I mean when using [] as XYZ, parenthesis are not included at the end
• Eric Wilson: @Mahesha999 and four years later, my error is corrected! Thanks, I've edited.

Answer 3

But the codenarc report it is warning like following.

ArrayList objects are better instantiated using the form "[] as ArrayList"

IMO, this is bad advice on Codenarc's part as it assumes that the implementation type of [] is ArrayList. This is true today, but may not always be.

The simplest/best/usual way to create a List implementation is:

def list = []

If possible, I will usually write something like

List<String> list = []

Just to make it a bit more obvious what this list should contain, i.e. for the sake of code readability. If I wanted to create a specific type of list I would instantiate a List "the Java way"

List<String> list = new SomeCustomListImplementation<String>()

But in practice I can't remember ever doing this.

• Eric Wilson: +1 I agree with almost all of what you say, but I don't think that parametrized types List<String> give as much benefit in Groovy as in Java. I feel like either def list = [] or List list = [] fits better with the Groovy mindset.

Answer 4

Why don't you just use a suggestion from codenarc?

def list = [] as ArrayList