- Queue 中 add() 和 offer() 都是用來向隊列添加一個元素。
- 在容量已滿的情況下，add() 方法會拋出IllegalStateException異常，offer() 方法只會返回 false 。
JDK1.8 源碼中的解釋
/** * 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);