I would like to go in detail on this.

Firstly Java is a strongly typed language i.e. before using any data we need to tell the complier of what data type this data will be.

In simple words we need to declare the data type of the variable.

Basically there are three type of data types:

1. Primitive data types
2. Derived data types
3. User defined data types

Primitive data types are the general and fundamental data types that we have in Java and those are byte, short, int, long, float, double, char, boolean.

Derived data types are those that are made by using any other data type for example, arrays.

User defined data types are those that user / programmer himself defines. For example, classes, interfaces.

In very-very simple words I can say,

int a

Here a is a variable of int data type.

MyClass obj

Here obj is a variable of data type MyClass and we call them reference variables as they can be used to store the reference to the object of that class.

Of course you have user defined data types in java! Any class you define is a data type for objects that defined by you.