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;
                }
                
            });

一定不要把返回0这行代码忽悠掉: