what is void in programming?

Void is used when there is no return, for example, You could make a constructor like

public String onColorSwitch(String color) { return color; }

but if you aren't going to return anything, then you could use,

public void onColor() {

}

Void basically means "no type", "no value" or "empty". Void is most commonly used in functions and methods. If you have worked with functions before, this might help you understand better:

functions f = new functions();
String message = "Hello world!";
f.print_func(message);

public void print_func(String var) {
   System.out.println(var);
}

In our main class we make a string with the value "Hello world!" and then call the function print_func with that message as the argument. The function takes the argument and prints it out and returns nothing back to the main class because there is nothing that functions calculated or created -- it just printed a message to the user. This is why we declared our function as a void -- it passes nothing back to it's caller once it's finished running.

An example where void shouldn't be used:

functions f = new functions();
int number = 5;
int new_number = 0;
new_number = f.calc_func(number);
System.out.println(new_number);

public int calc_func(int var) {
   int new_number = var * 2;
   return new_number;
}

In this example, our function takes a number (in this case the number "5") and calculates it's double (5 x 2 = 10 in this example). Once the function has completed this calculation, our main class needs to know what was calculated, so the function needs to return this value back to it's caller. The main class get's the value and stores it in new_number which is then printed out to the user.

So whenever you create a function or method in java, you need to tell java the type that is being returned (int, String, etc). If nothing is returned, then you need to tell this by using void where applicable.

Void can also be used as a pointer for unknown types, but that's a bit advanced.

Hope this helps! :)