Basic Memory Diagrams in Function Calls

This document gives a brief overview of the function call stack and memory heap. It shows a key difference between passing a primitive argument to a function like an int versus passing a reference to a function.

 1  public class Doubler{
 2    public static void doubler1(int x){
 3      x = 2*x;
 4    }
 5    public static void doubler2(int x[]){
 6      x[0] = 2*x[0];
 7    }
 8    public static void main(String args[]){
 9      int r = 10;
10      int s[] = {10};
11  
12      doubler1(r);
13      System.out.println("r: "+r);
14 
15      doubler2(s);
16      System.out.println("s[0]: "+s[0]);
17   }
18 }

Compile/Run
> javac Doubler.java
> java Doubler
r: 10  
s[0]: 20

The central question in the code above is why the function doubler1(int) does not seem to change the value of r while the function doubler2(int []) does change the value of s[0]. This is best understood by examining the memory layout on the stack and heap of the running main method. Observer in the trace below how a "copy" of r is passed to doubler1() which is changed then discarded while a reference to the location of array s is passed to doubler2() enabling the array to actually be changed.