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.

Answer 3

Thats depends on what is the logic that you write in the contains() method. When you try to instantiate BlockingQueue<Object> is forces you to implement few methods and contains() is ojne of them.

BlockingQueue<Object> a = new BlockingQueue<Object>()
{

    @Override
    public Object remove()
    {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Object poll()
    {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Object element()
    {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Object peek()
    {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public int size()
    {
        // TODO Auto-generated method stub
        return 0;
    }

    @Override
    public boolean isEmpty()
    {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public Iterator<Object> iterator()
    {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Object[] toArray()
    {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public <T> T[] toArray( T[] a )
    {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public boolean containsAll( Collection< ? > c )
    {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public boolean addAll( Collection< ? extends Object> c )
    {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public boolean removeAll( Collection< ? > c )
    {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public boolean retainAll( Collection< ? > c )
    {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public void clear()
    {
        // TODO Auto-generated method stub

    }

    @Override
    public boolean add( Object e )
    {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public boolean offer( Object e )
    {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public void put( Object e ) throws InterruptedException
    {
        // TODO Auto-generated method stub

    }

    @Override
    public boolean offer( Object e, long timeout, TimeUnit unit ) throws InterruptedException
    {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public Object take() throws InterruptedException
    {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Object poll( long timeout, TimeUnit unit ) throws InterruptedException
    {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public int remainingCapacity()
    {
        // TODO Auto-generated method stub
        return 0;
    }

    @Override
    public boolean remove( Object o )
    {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public boolean contains( Object o )
    {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public int drainTo( Collection< ? super Object> c )
    {
        // TODO Auto-generated method stub
        return 0;
    }

    @Override
    public int drainTo( Collection< ? super Object> c, int maxElements )
    {
        // TODO Auto-generated method stub
        return 0;
    }
};