put和putIfAbsent的区别

1.使用 put 方法添加键值对，如果 map 集合中没有该 key 对应的值，则直接添加，并返回 value；如果已经存在对应的值，则会覆盖旧值，value 为新的值，返回值为 value。

2.使用 putIfAbsent 方法添加键值对，如果 map 集合中没有该 key 对应的值，则直接添加，并返回 null，如果已经存在对应的值，则依旧为原来的值，返回值为 value（旧的值）。

    /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

    /**
     * If the specified key is not already associated with a value (or is mapped
     * to {@code null}) associates it with the given value and returns
     * {@code null}, else returns the current value.
     *
     * @implSpec
     * The default implementation is equivalent to, for this {@code
     * map}:
     *
     * <pre> {@code
     * V v = map.get(key);
     * if (v == null)
     *     v = map.put(key, value);
     *
     * return v;
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with the specified key, or
     *         {@code null} if there was no mapping for the key.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with the key,
     *         if the implementation supports null values.)
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the key or value is of an inappropriate
     *         type for this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key or value is null,
     *         and this map does not permit null keys or values
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V putIfAbsent(K key, V value) {
        V v = get(key);
        if (v == null) {
            v = put(key, value);
        }

        return v;
    }