java.lang.IllegalArgumentException: Comparison method violates its general contract!

Collections.sort(list, new Ordering<QtmSysUserListDto>() {
@Override
public int compare(QtmSysUserListDto arg0, QtmSysUserListDto arg1) {
if(arg0.getFlag().length()>arg1.getFlag().length()){
return -1;
}else if(arg0.getFlag().length()==arg1.getFlag().length()){
return 0;
}
return 1;
}

});