Difference between instantiating, declaring, and initializing

The terms instantiate, declare, and initialize are easy to stumble upon while programming. But what exactly do these terms mean?

Declaring :

Declaring a variable means the first “mention” of the variable, which tells the compiler “Hello, I am here and can be used”. In a statically typed language like Java, this also means that the declared type of the variable is determined. The value itself is not determined during the declaration.

String name;
int nbr;

Declaration of the variable “name” of type String and the variable “nbr” of type int.

Initializing :

The term initialization usually means the first assignment of a value to a variable.

String name = "Thomas";
int nbr = 5;

The variables “name” and “nbr” were declared in the first example, but not yet initialized. Now the values of the variables were “initially assigned with values”, thus initialized.

Note: The fact that the variables have not yet been initialized is not entirely true (see here), but relying on automatic initialization is such a bad programming style 🙂

Instantiating :

The term instantiation actually has nothing to do with assigning a value to a variable, even if a new object is sometimes instantiated when a variable is initialized.

The term simply means the creation of a new object, i.e. an instance, from a class.

String name = new String("Thomas");

In Java, the instantiation of an object is always accompanied by a call to the constructor.
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