static String toHex(int n) {
    int yu = 0;
    Deque<String> result = new LinkedList<>();
    while(n!=0){
        yu = n%16;
        result.push(yu<10? ""+yu : yu==10? "A" : yu==11? "B" : yu==12? "C" : yu==13? "D" : yu==14? "E" : "F");
        n /= 16;
    }
    return String.join("",result);
}

本来是foreach遍历Deque的，看到评论里用了join才想起来。