Introduction to Parameters and Objects - Building Java Programs: A Back to Basics Approach - page 142

The following statement calls doubleNumber again, this time passing it the value
of number . This is an odd situation because the parameter has the same name as the
variable in main , but Java doesn't care. It always creates a new local variable for the
doubleNumber method:

method main method doubleNumber

x 17 number 42 number 42

So, at this point there are two different variables called number , one in each
method. Now it's time to execute the statements of doubleNumber again. It first
reports the value of number ( 42 ), then doubles it:

method main method doubleNumber

x 17 number 42 number 84

Again, notice that doubling number inside doubleNumber has no effect on the
original variable number in main . These are separate variables. The method then
reports the new value of number ( 84 ) and returns to main :

method main

x 17 number 42

The program then reports the value of number and terminates. So, the overall output
of the program is as follows:

Initial value = 17
Final value = 34
x = 17

Initial value = 42
Final value = 84
number = 42

The local manipulations of the parameter do not change these variables outside the
method. The fact that variables are copied is an important aspect of parameters. On
the positive side, we know that the variables are protected from change because the
parameters are copies of the originals. On the negative side, it means that although
parameters will allow us to send values into a method, they will not allow us to get
values back out of a method.