When declaring a variable in languages such as c java and c?

Answer 1

In C, C++ and Java, all variables must be declared before they can be used. A declaration determine's the variable's type and name, nothing more. The type must be visible to the compiler at the point of declaration in order to determine the amount of memory require by that type. The type naturally determines what type of data may be stored in the variable. The name is user-defined and must be unique to the scope in which the variable is declared. The name provides a reference to the memory address allocated to the variable at runtime.

In C, variables are always uninitialised (they have no value). As such, C programmers typically declare all variables used by a function at the top of the function, initialising them as and when required. This can lead to problems if the programmer forgets to initialise the variable before using it, however the compiler should emit a warning to guard against this. If the programmer ignores the warning, the program will have undefined behaviour.

C++ and Java are similar to C in that respect but, unlike C, variables may be initialised at the point of declaration (assigned a value of the type). This encourages programmers to declare variables at the point they are actually required rather than up front.

Constants are similar to variables except they do not change value once initialised. Constants must always be initialised at the point of declaration except in C++ when declaring a constant at class scope; it must be initialised at global scope (outside the class). In Java, constants are declared with the 'final' keyword before the variable's type. Both C and C++ use the 'const' keyword.

Variables may also be declared volatile which means the variable's value is outwith the control of the process or thread in which it is declared and could change at any time. Declaring a volatile simply ensures that the variable cannot be read during a write (creating a race condition) and that all access must occur in main memory; the variable cannot be cached.