Groovy is a very powerful language on a JVM platform and with this great power comes great responsibility. There are many language features that are not intuitive for many people that start using Groovy. One of these features is dynamic coercion and type promotion which may cause you a headache if you use it carelessly.

The good parts

Dynamic type coercion and promotion may sound strange to you. If you work with statically compiled language like Java you know that expression:

Listing 1. Compilation error thrown because of incompatible types
String str = 123; // Error:(4, 18) java: incompatible types: int cannot be converted to java.lang.String

does not compile. Groovy took a different approach and as long as you skip static compilation you are allowed to coerce a left side type to a right side expression. In this case expression like:

Listing 2. Type coercion from Number to String in Groovy
String str = 234
println str.dump() // <java.lang.String@c213 value=234 hash=49683>

compiles and converts 234 numeric value to its String representation.


We just saw that Groovy allows us to convert numeric value to its String representation without any issue. Now let’s take a quick look at some of the other examples where type coercion and promotion makes more or less sense, yet still can make developers life easier.

1. Type to Boolean coercion

Listing 3. Examples of types to Boolean coercions
Boolean numberToFalse = 0 // false
Boolean numberToTrue = -10 // true
Boolean stringToFalse = '' // false
Boolean stringToTrue = 'false' // true
Boolean listToFalse = [] // false
Boolean listToTrue = [false, false] // true

The interesting part of this coercion is that you can take advantage of it when calling an if statement. For instance you can do something like this:

Listing 4. Simplified if-statement
if (number) {
    // ....

and it will coerce numeric value to false if number is equal to 0 and true otherwise.

2. String to Enum coercion

Listing 5. String to Enum coercion example
enum Type {

Type basic = 'BASIC'
Type advanced = 'ADVANCED'

println basic.dump() // prints <Type@10e92f8f name=BASIC ordinal=0>
println advanced.dump() // prints <Type@1d119efb name=ADVANCED ordinal=1>

As you can see this is a short version of Enum.valueOf(String value) call.

3. String to Class coercion

Listing 6. String to Class coercion example
Class clazz = 'java.lang.String'
println clazz.dump() // prints <java.lang.Class@5ef04b5 cachedConstructor=null newInstanceCallerCache=null name=java.lang.String reflectionData=java.lang.ref.SoftReference@bef2d72 classRedefinedCount=0 genericInfo=sun.reflect.generics.repository.ClassRepository@69b2283a enumConstants=null enumConstantDirectory=null annotationData=java.lang.Class$AnnotationData@22a637e7 annotationType=null classValueMap=null>

4. Closure to a functional interface coercion

Listing 7. Closure to a functional interface coercion example
interface Worker<T,U> {
  U work(T)

Worker<String, Integer> worker = { it.length() }
println'abc') // prints 3

5. List to a custom type coercion

Listing 8. Passing implicitely list elements to a class constructor
import groovy.transform.Immutable

final class Point {
  final int x
  final int y

Point point = [10,23]
println point.dump() // <Point@1de0c x=10 y=23 $hash$code=122380>

In this example we define constructor parameters as a list of elements. This type of coercion works only if number (and types) of list elements matches the number of constructor parameters. For instance, if we pass a list of size 3 we would get GroovyCastException while trying to initialize the object:

Listing 9. Passing incorrect number of constructor parameters throws an exception
Point point = [10,23,43] // Throws org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object '[10, 23, 43]' with class 'java.util.ArrayList' to class 'Point' due to: groovy.lang.GroovyRuntimeException: Could not find matching constructor for: Point(java.lang.Integer, java.lang.Integer, java.lang.Integer)

6. Map to a custom type coercion

Listing 10. Passing map to a class constructor method
import groovy.transform.Immutable

final class Point {
  final int x
  final int y

Point point = [x: 4, y: -32]
println point.dump() // <Point@1dd1b x=4 y=-32 $hash$code=122139>

This use case is similar to the previous one. In this case map keys have to match constructor parameters - number of map entries has to match number of constructor parameters and keys names have to match class properties names.

The bad parts

You may find some of these dynamic coercions useful, however there are use cases where dynamic coercion and promotion causes more problems. There was one pretty interesting question on Stack Overflow which inspired me to write this blog post. Let’s consider following example.

Listing 11. Collection coercion to Set type
Set<Integer> integers = [1,2,3,4,3,2,1].asCollection()

println integers // prints [1, 2, 3, 4]

This kind of assignment is not possible in Java - if you try casting Collection to Set you would get ClassCastException:

Exception in thread "main" java.lang.ClassCastException: java.util.Collections$UnmodifiableCollection cannot be cast to java.util.Set

Groovy calls DefaultTypeTransformation.continueCastOnCollection(Object object, Class type) method in this case and allows promoting Collection to a Set (LinkedHashSet in this case).

Well, what’s the problem with that? If you get familiar with Groovy’s source code then such conversions are pretty straightforward to you, right? That is true, however there are use case that confuse people even more. Take a look at following example:

Listing 12. Casting unmodifiable collection to Set example
Set<Integer> integers = Collections.unmodifiableCollection([1,2,3,4,3,2,1].asCollection())
println integers

Now, do you think this code compiles? Or what println integers prints to the console? If you read the source code carefully you already know the answer. It compiles and it prints [1, 2, 3, 4, 10]. Why? Because unmodifiable collection does not get promoted to a unmodifiable set, but LinkedHashSet instead. If we only be more careful and stop relying on dynamic type coercion than the code like:

Listing 13. Adding an element to unmodifiable set
Set<Integer> integers = Collections.unmodifiableSet([1,2,3,4,3,2,1] as Set)
println integers

would produce a compile time error that saves a lot of our time:

Caught: java.lang.UnsupportedOperationException
	at java_util_Set$ Source)

How to disable dynamic type coercion?

It’s simple - enable static compilation and all dynamic coercions are turned off.


I really like all different features of Groovy programming language, however exaggerating dynamic features usage may cause you a lot of problems when you are not careful enough. I always tend to be as explicit as possible when writing Groovy code - I don’t overuse dynamic type coercions and only use them when they are very straightforward and don’t add any level of complication to my code.

How does it look like on your side? What are the use cases that work for you if it comes to dynamic type coercion? Please share your story in the comments section below.

Szymon Stepniak

Upwork's Top Rated freelancer, Toruń Java User Group founder, open source contributor, Stack Overflow active user, bedroom guitar player.