Dynamically adding elements to ArrayList in Groovy

Question

I am new to Groovy and, despite reading many articles and questions about this, I am still not clear of what is going on. From what I understood so far, when you create a new array in Groovy, the underlying type is a Java ArrayList. This means that it should be resizable, you should be able to initialize it as empty and then dynamically add elements through the add method, like so:

MyType[] list = []
list.add(new MyType(...))

This compiles, however it fails at runtime: No signature of method: [LMyType;.add() is applicable for argument types: (MyType) values: [MyType@383bfa16]

What is the proper way or the proper type to do this?

Solution 1

The Groovy way to do this is

def list = []
list << new MyType(...)

which creates a list and uses the overloaded leftShift operator to append an item

See the Groovy docs on Lists for lots of examples.

Solution 2

What you actually created with:

MyType[] list = []

Was fixed size array (not list) with size of 0. You can create fixed size array of size for example 4 with:

MyType[] array = new MyType[4]

But there's no add method of course.

If you create list with def it's something like creating this instance with Object (You can read more about def here). And [] creates empty ArrayList in this case.

So using def list = [] you can then append new items with add() method of ArrayList

list.add(new MyType())

Or more groovy way with overloaded left shift operator:

list << new MyType() 

Comments

[Captain Franz]

Ok this works. Actually also list.add(new MyType(...)) works if you define the list like this. But I have a comment and a question. Comment: so there is no way of specifying what type you are going to put in the list when you create it? Question: why does it work in this way and it doesn't if I specify a type? It looks as if it were using a different underlying type then ArrayList in that case.

[Captain Franz]

Thanks. So is there no way of creating a dynamic list with a specific type?

[tim_yates]

You can use List<MyList> list = [], but if you're not using CompileStatic, it will basically be ignored

[Paweł Piecyk]

The question is already answered by tim_yates in a comment to the second answer :)

[Kasper Ziemianek]

You can also def list = [new MyType(...)]