Assignment and initialization

Once a variable is declared, it must be explicitly initialized using an assignment operator, since a variable that has not been assigned a value cannot be used. To assign a previously declared variable a specific value, you need to indicate its name on the left, put an equal sign ( =), and write some expression in Java that specifies the required value on the right. Eg:

int daysInMay; // объявление целочисленной переменной
daysInMay = 31; // оператор присваивания значения этой переменной

Here is an example of assigning a value to a symbolic variable:

char noChar;
noChar = 'N';

The Java language has the convenient ability to combine variable declaration and initialization on the same line.

int daysInMay = 31; // Пример объявления и инициализации переменной в одной строке

Also in Java, a variable declaration can be placed anywhere in the code, for example, the code below works:

int daysInJune = 30;
System.out.println(daysInJune);
int daysInMay = 31;

It is clear that in one scope it is impossible to declare two variables with the same names. The C and C++ languages ​​differ in the declaration and definition of a variable. Here is an example of a variable definition:

int i = 100;

А вот пример ее объявления:extern int i;

In Java, variable declarations and definitions are the same. Link to source: Assignment and initialization