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?

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