为什么方法Queue＃add（）有布尔返回值？

Question

I revised knowledge about Queue . I watch to the Queue interface.

method add has following declaration :

boolean add(E e);

In java doc writes following:

 * @return <tt>true</tt> (as specified by {@link Collection#add})

Hence this method can return only true!!! Why does not this method return value declared as void ?

This method declaration is enough confusing for me.

Answer 1

If you look at the docs for Collection#add , it becomes clear:

true if this collection changed as a result of the call

Since the queue always changes as a result of the call, Queue#add always has to return true . (And it has to have a return value, in order to implement the interface correctly.)

Compare with Set , which also implements Collection , which will only add the element if it's not already in the set, and so might return false from add .

Answer 2

Queue implements Collection which is a more generic interface. In java, you can't implement or extend an interface without including all its methods as they are initially declared and thus, sometimes, you get classes with stub methods as add in this case.

Collection add method is declared as:

boolean add(E e)

So it has to be declared at Queue . In a list its return value will always be true given the nature of the queue data structure but not for others.

Imagine you are developing a new data structure implementing Collection interface which internally uses an array to store its elements. In that case you may find useful to return false when adding new elements: You could already used all the array positions.

Answer 3

The method java.util.Queue.add(E) overrides the method java.util.Collection.add(E). So, it has to follow the rules of method overriding.

One of this rule is that the return type of the method java.util.Queue.add(E) has to be a sub-type of the method java.util.Collection.add(E). Because the method java.util.Collection.add(E) returns a boolean, the method java.util.Queue.add(E) must also return a boolean.

The method java.util.Queue.add(E) is boolean true, because the queue is changed everytime we add new object into it.