All same hash code string in Java

final String p1 = "AaBB";
final String p2 = "BBAa";
final String p3 = "BBBB";
final String[] pp = new String[] { p1, p2, p3 };
final int ppLength = pp.length;
Random random = new Random();
random.setSeed(System.currentTimeMillis());
final int size = 10;
final int length = size * 4;
final int times = 2_000_000; //n million loops
Map<String, String> map = new HashMap<>(10000);
File file = new File("r:/1.txt");
file.delete();
BufferedWriter bw = new BufferedWriter( //
    new OutputStreamWriter( //
            new FileOutputStream(file, true) // append
            , Charset.forName("UTF-8") // charset
    ) //
    , 1024 // buffer size
);
StringBuffer sb = new StringBuffer(length);
for (int ii = 0; ii < size; ii++) {
    sb.append(pp[random.nextInt(ppLength)]);
}
final int hashCode = sb.toString().hashCode();
sb.setLength(0);
for (int count = 0; count < times; count++) {
    for (int ii = 0; ii < size; ii++) {
        sb.append(pp[random.nextInt(ppLength)]);
    }
    String ss = sb.toString();
    if (!map.containsKey(ss)) {
        if (ss.hashCode() != hashCode) {
            bw.flush();
            bw.close();
            throw new RuntimeException("hashCode not equal");
        }
        map.put(ss, ss);
        bw.append(ss);
        bw.newLine();
    }
    sb.setLength(0);
}
System.out.println(map.size());
bw.flush();
bw.close();

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