List<String> stringList = new ArrayList<>();
stringList.add("a1");
stringList.add("b1");
stringList.add("a2");
stringList.add("b2");
stringList.stream();

/** Implementation for put and putIfAbsent */
    final V putVal(K key, V value, boolean onlyIfAbsent) {
       //省略相关代码
        for (Node<K,V>[] tab = table;;) {
            Node<K,V> f; int n, i, fh;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
                if (casTabAt(tab, i, null,
                             new Node<K,V>(hash, key, value, null)))
                    break;                   // no lock when adding to empty bin
            }
            else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
            else {
                V oldVal = null;
                synchronized (f) {
                    //省略相关代码
                }
                if (binCount != 0) {
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }
        addCount(1L, binCount);
        return null;
    }

static int hash(int h) {
    // This function ensures that hashCodes that differ only by
    // constant multiples at each bit position have a bounded
    // number of collisions (approximately 8 at default load factor).
    h ^= (h >>> 20) ^ (h >>> 12);
    return h ^ (h >>> 7) ^ (h >>> 4);
}

List<String> list = new ArrayList<>();
list.add("a");
list.add("b");
list.add("c");
list.add("a");
Set<String> set =new HashSet<>();
set.addAll(list);

@Configuration
public class WebConfig extends WebMvcConfigurerAdapter {
    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        FastJsonConfig fastJsonConfig = new FastJsonConfig();
        fastJsonConfig.setSerializerFeatures(
                // 输出空置字段
                SerializerFeature.WriteMapNullValue,
                // list字段如果为null，输出为[]，而不是null
                SerializerFeature.WriteNullListAsEmpty,
                // 数值字段如果为null，输出为0，而不是null
                SerializerFeature.WriteNullNumberAsZero,
                // Boolean字段如果为null，输出为false，而不是null
                SerializerFeature.WriteNullBooleanAsFalse,
                // 字符类型字段如果为null，输出为""，而不是null
                SerializerFeature.WriteNullStringAsEmpty);

        fastJsonConfig.setCharset(Charset.forName("UTF-8"));
        fastJsonConfig.setDateFormat("yyyy-MM-dd hh:mm:ss");
        FastJsonHttpMessageConverter Converter = new FastJsonHttpMessageConverter();
        Converter.setFastJsonConfig(fastJsonConfig);
        HttpMessageConverter<?> converter = Converter;
        converters.add(0,converter);
        super.configureMessageConverters(converters);
    }

final Node<K,V> getNode(int hash, Object key) {
        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
                if (first instanceof TreeNode)
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }