difference between int and Integer type in groovy

Question

I have just started learning groovy and I am reading "Groovy in Action". In this book I came across a statement that it doesn’t matter whether you declare or cast a variable to be of type int or Integer.Groovy uses the reference type ( Integer ) either way.

So I tried to assign null value to a variable with type int

int a = null

But it is giving me below exception

org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object 'null' with class 'null' to class 'int'. Try 'java.lang.Integer' instead at Script1.run(Script1.groovy:2)

Then I tried to assign null value to a variable with type Integer

Integer a = null

and it is working just fine.

Can anyone help me understand how groovy behaves such way or the reason behind it?

Representational

Answer 1

The core problem is that primitives can’t be null. Groovy fakes that out with autoboxing.

If you store a null value in a number, you can’t store that in a int/long/etc field. It’s not correct to convert a null number to 0, since this might be valid values. Null means that no value or choice has been made yet.

int is a primitive type and it is not considered as an object. Only objects can have a null value while int value can't be null because It's a value type rather than a reference type

For primitive types, we have fixed memory size i.e for int we have 4 bytes and null is used only for objects because there memory size is not fixed.

So by default we can use :-

int a = 0

Chess

Answer 2

Groovy uses wrapper types in all time when you call primitives

int a = 100
assert Integer == a.class

groovy takes int and wrap it into Integer before using it value But groovy cannot set int value to null, because variable is int(primitive type) but not Integer.

int a = 100
int b = 200
a + b not int + int, but Integer + Integer

Chlori

Answer 3

As I understand it, Groovy uses the wrapper types if you use literals. For example:

def a = 11 // equivalent to Object a = 11. 11 is an java.lang.Integer

or

assert ['java.lang.Integer'] == [ 11, 22, 33 ]*.class*.name.unique()

Although if you use a specific type in your definition, the compiler has to perform casting.

You can do:

def a = 11
a = 'ssss'

but

int a = 11 // or int a
a = 'ssss'

gives GroovyCastException.

This what you see in your case

Longshore

Sorry but I am still not able to understand the reason. – Representational