• Queue 中 add() 和 offer()都是用来向队列添加一个元素。
• 在容量已满的情况下，add() 方法会抛出IllegalStateException异常，offer() 方法只会返回 false 。

JDK1.8 源码中的解释

/**
 * Inserts the specified element into this queue if it is possible to do so
 * immediately without violating capacity restrictions, returning
 * {@code true} upon success and throwing an {@code IllegalStateException}
 * if no space is currently available.
 *
 * @param e the element to add
 * @return {@code true} (as specified by {@link Collection#add})
 * @throws IllegalStateException if the element cannot be added at this
 *         time due to capacity restrictions
 * @throws ClassCastException if the class of the specified element
 *         prevents it from being added to this queue
 * @throws NullPointerException if the specified element is null and
 *         this queue does not permit null elements
 * @throws IllegalArgumentException if some property of this element
 *         prevents it from being added to this queue
 */
boolean add(E e);
 
/**
 * Inserts the specified element into this queue if it is possible to do
 * so immediately without violating capacity restrictions.
 * When using a capacity-restricted queue, this method is generally
 * preferable to {@link #add}, which can fail to insert an element only
 * by throwing an exception.
 *
 * @param e the element to add
 * @return {@code true} if the element was added to this queue, else
 *         {@code false}
 * @throws ClassCastException if the class of the specified element
 *         prevents it from being added to this queue
 * @throws NullPointerException if the specified element is null and
 *         this queue does not permit null elements
 * @throws IllegalArgumentException if some property of this element
 *         prevents it from being added to this queue
 */
boolean offer(E e);