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What is Actually Passed to a Method? 

In Java, parameters sent to methods are passed-by-value:

Definition clarification:  What is passed "to" a method is referred to as an "argument".  The "type" of data that a method can receive is referred to as a "parameter".  (You may see "arguments" referred to as "actual parameters" and "parameters" referred to as "formal parameters".)

First, notice that if the arguments are variable names, the formal parameters need not be those same names (but must be the same data type).

Pass-by-value means that when a method is called, a copy of the value of each argument is passed to the method.   This copy can be changed  inside the method, but such a change will have NO effect on the argument.

Let's look at some examples:

Ex. 1:  Will the values from main be changed after being sent to this method??

int num = 10;
double decimal = 5.4;

NumberManeuvers(num, decimal);

System.out.println("num = " + num + "and
                                   decimal  = " + decimal);

----------------------------------------------------------------
public static void NumberManeuvers(int i, double j)
{
    if (i == 10)
          j = 6.2;
    i = 12;
}
// ANSWER:  In this case, NO.
The output will be:
num = 10 and decimal = 5.4

Here's what's happening!

Copies of the actual parameter values from main are sent to the method, where they become the values for the formal parameters.  When the method is finished, the copies are discarded.  The actual parameter values remain unchanged.  (Notice that nothing was "returned" from this method.)
Ex. 2:  What will happen to the values in main after being sent to this method??
int a = 2, b = 3, c = 4, answer = 2;
answer = discriminant(a, b, c);
System.out.println("The discriminant is " + answer);
------------------------------------------------------------
public static int discriminant(int x, int y, int z)
{
    int disc;
    disc = y*y - 4*x*z;
    return disc;
}

Here's what's happening!
It is possible for a method to change a value in main by returning and saving the value after the method is complete.  The limitation to this concept is that only ONE  value can be returned from a method. 
In this example, answer was initially 2, but after execution of the method and new assignment to answer, it now has the value -23.
Ex. 3:  Will the happen to the String dog after dog is sent to this method??
String dog = "My dog has fleas."
dog = SpeciesChange(dog);
System.out.println("dog);

public static String SpeciesChange(String pet)
{
    pet = "My cat has fleas.";
    return (pet);
}

Here's what's happening!
As in the examples above, since the method returns a value that is then stored in dog, the value of dog is changed.  The output is:
My cat has fleas.
If no value had been returned, there would have been no change to dog.

 

              
               
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