java.lang.IllegalArgumentException: Comparison method violates its general contract! at java.util.TimSort.mergeHi(TimSort.java:868) at java.util.TimSort.mergeAt(TimSort.java:485) at java.util.TimSort.mergeCollapse(TimSort.java:408) at java.util.TimSort.sort(TimSort.java:214) at java.util.TimSort.sort(TimSort.java:173) at java.util.Arrays.sort(Arrays.java:659) at java.util.Collections.sort(Collections.java:217) ...

public class Test implements Externalizable { private String s; @Override public void writeExternal(ObjectOutput out) throws IOException { if (s == null) { out.writeShort(-1); } else { byte[] bb = s.getBytes("utf-8"); out.writeShort(bb.length); out.write(bb); } } @Override public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException { int len = in.readShort(); if (len < 0) return null; byte[] bb = new byte[len]; in.read(bb); return new String(bb, "utf-8"); } }