Checking if queue contains object

Question

For a java.util.concurrent.BlockingQueue

By Java specification, for a method contains(Object o)

If I have previously inserted a new object like:

Task task = new Task("taskname", "somevalue"); 
queue.put(task);

on it. And later try to do this:

Task task = new Task("taskname", "somevalue"); 
queue.contains(task);

Since BlockingQueue is just an interface, by Java specification, should this return true or not?

The Task class is Serializable so the comparison will be based on field values right?

Answer 1

The behavior depends on if Task class overrides equals method. Depending on the logic of equals method, these two Tasks may/may not be equal.

From the Java docs of Blocking queue

boolean contains(Object o)

Returns true if this queue contains the specified element. More formally, returns true if and only if this queue contains at least one element e such that o.equals(e).

If the equals method is not overridden, then Java will use the equals method of Object for equality check(which checks if object references are equal).

   public boolean equals(Object obj) {
    return (this == obj);
}

Since these are two distinct objects, so object reference id will be different and hence contains will return false.

Answer 2

Since BlockingQueue is just an interface, by Java specification, should this return true or not?

This is a weird question. As long as queue object is created, it should behave as promised by its interface(BlockingQueue).

Interface is abstract in that it cannot be instantiated by itself, but it's a common contract for all objects created by those classes that implement it.

As for your concreate question, whether queue.contains(task) return true depends on how class Task defines its equals method.