Given a user-defined data type such as Dog, we would like to create an instance of Dog and use it subsequently in a program. Doing so requires both variable definition and assignment operations. A data definition operation specifies a data type and a variable name, and optionally, an initial value:

Data Definition

<data-type> <variable>;

<data-type> <variable-1>, <variable-2>, ..., <variable-n>;

<data-type> <variable> = <data-value>;

The data type may be a primitive, or built-in, type or a user-defined type such as Dog. The value may be a literal value or an instance of a user-defined type such as Dog. Primitive data types are discussed in Data types.

Several examples of data definitions follow:

Data Definition Examples

int x;

int x = 9;

boolean terminate = false;

Dog dog = new Dog();

The new operator is described in the next panel (Creating class instances).

An assignment operation can occur in the following contexts:

Assignment Operation

<data-type> <variable> = <data-value>;

<data-type> <variable>; <other-statements>... <variable> = <data-value>;

The data value to the right of the assignment operator can be a literal value, or any operation that produces a scalar value. Several examples follow:

Assignment Example	Comment
int x = 4;	Data definition with assignment
x = 9;	Assumes prior definition of x
temperature = 21.4;	Assumes prior definition of temperature
dog = new Dog();	Assumes prior definition of dog