tests/gc/Hashcode.java

   1 public class Hashcode {
   2         public static final boolean doPrintHashes = false;
   3         public static final int N = 20;
   4         public static final int N2 = N/2;
   5
   6         public static void compareHashes(int[] a, int[] b) {
   7                 boolean diff = false;
   8
   9                 for (int i=0; i<a.length; i++)
  10                         if (a[i] != b[i]) {
  11                                 System.out.println("\tHash #" + i + " changed!");
  12                                 diff = true;
  13                         }
  14
  15                 if (diff)
  16                         System.out.println("\tHashes are DIFFERENT!");
  17                 else
  18                         System.out.println("\tHashes are ok.");
  19         }
  20
  21         public static void printHashes(int[] a) {
  22                 for (int i=0; i<a.length; i++)
  23                         System.out.println("\t#" + i + ": " + a[i]);
  24         }
  25
  26         public static int[] getHashes(Object[] a) {
  27                 int[] result = new int[a.length];
  28
  29                 for (int i=0; i<a.length; i++)
  30                         result[i] = System.identityHashCode(a[i]);
  31
  32                 return result;
  33         }
  34
  35         public static void main(String[] s) {
  36                 Object[] a = new Object[N];
  37                 int[] hashes1, hashes2;
  38
  39                 System.out.println("Creating objects ...");
  40                 for (int i=0; i<N2; i++)
  41                         a[i] = new Object();
  42                 for (int i=N2+1; i<N; i++)
  43                         a[i] = new String("String-" + i);
  44
  45                 System.out.println("Forgetting some objects ...");
  46                 for (int i=0; i<N; i+=2)
  47                         a[i] = null;
  48
  49                 System.out.println("Getting hashes before GC ...");
  50                 hashes1 = getHashes(a);
  51                 if (doPrintHashes)
  52                         printHashes(hashes1);
  53
  54                 System.out.println("Invoking the GC ...");
  55                 System.gc();
  56                 System.out.println("\tfinished.");
  57
  58                 System.out.println("Getting hashes after GC ...");
  59                 hashes2 = getHashes(a);
  60                 if (doPrintHashes)
  61                         printHashes(hashes2);
  62
  63                 System.out.println("Comparing hashes now ...");
  64                 compareHashes(hashes1, hashes2);
  65         }
  66 }