General java programming question.

Question

i have trouble knowing when something is referred to in contrast to something being copied.
in this case, the array has changed, but not the character.
public class ReturnArray2
     public static int[] getArray()
          int a[] = new int[3];
          a[0] = 2;
          a[1] = 4;
          a[2] = 6;
          return a;
     public static void mystery(int[] b, char r)
          r = 'Q';
          for(int i = 0; i < b.length; i++)
               b[i] = b[i] + 5;
          System.out.println("array b contains: ");
               for(int i = 0; i < 3; i++)
                    System.out.println(b[i] + " ");
     public static void main(String args[])
          int scores[];
          char x = 'L';
          scores = getArray();
          System.out.println("Array in main.");
          for (int i = 0; i < 3; i++)
               System.out.println(scores[i] + " ");
          mystery(scores, x);
          System.out.println("After return from mystery.");
          System.out.println("character is " + x);
          for(int i = 0; i < 3; i++)
               System.out.println(scores[i] + " ");
Array in main.
2
4
6
array b contains:
7
9
11
After return from mystery.
character is L
7
9
11
*/and in this case, why didn't the array change?
public class ReturnArray2
     public static int[] getArray()
          int a[] = new int[3];
          a[0] = 2;
          a[1] = 4;
          a[2] = 6;
          System.out.println("array in function");
          for(int i = 0; i < 3; i++)
               System.out.println(a[i] + " ");
          return a;
     public static void mystery(int[] b)
          for(int i = 0; i < b.length; i++)
               b[i] = b[i] * 2 + 5;
          int[] c = new int [3];
          c[0] = 55;
          c[1] = 44;
          c[2] = 33;
          System.out.println("array c contains: ");
          for (int i = 0; i < 3; i++)
               System.out.println(c[i] + " ");
          b = c;
          System.out.println("array b contains: ");
               for(int i = 0; i < 3; i++)
                    System.out.println(b[i] + " ");
     public static void main(String args[])
          int scores[];
          scores = getArray();
          System.out.println("Array in main.");
          for (int i = 0; i < 3; i++)
               System.out.println(scores[i] + " ");
          mystery(scores);
          System.out.println("After return from mystery.");
          for(int i = 0; i < 3; i++)
               System.out.println(scores[i] + " ");
array in function
2
4
6
Array in main.
2
4
6
array c contains:
55
44
33
array b contains:
55
44
33
After return from mystery.
9
13
17
*/

Answer 1

i have trouble knowing when something is referred to
in contrast to something being copied.Java has only one passing mode: pass-by-value. The value, whether it be of an int or a reference, is always copied.
in this case, the array has changed, but not the
character.A reference has a value, that value is either null or the address of an Object. The value of scores is the address of the array returned by getArray(). When you pass scores to mystery() the value of scores is copied to a new reference called b on the local stack frame. The value of b is the same as the value of scores, but they are two different references. However, since the value is the same they both contain the same address which is the address of the array. When you modify the array you are modifying the array at the address that b has as it's value. Since scores has that same address as it's value, in other words they both point to the same array, you will see it modified in both places. When you passed the char the value (L) was copied to the local stack frame. Your copy r now has the value L. Modifying r modifies the value at r, which is independent of the value at x.
So, with the char you changed the value at the address. With the reference you changed the value at the address the reference contained as it's value.
and in this case, why didn't the array change?You didn't modify the array, you simply printed it's contents concatenated with an empty String.