Collection Interface is the Root interface in the collection hierarchy.

Important properties Collection interface

1. JDK does not provide any direct implementations of this interface: it provides implementations of more specific subinterfaces like Set and List etc.

2. All child classes (direct or indirect) should provide minimum two constructors: a no argument constructor, which creates an empty collection, and a constructor with a single argument of type Collection, which creates a new collection with the same elements as its argument.

3. Overriden methods should throw UnsupportedOperationException if this collection does not support the operation.  

   • may, but are not required to, throw an UnsupportedOperationException if the invocation would have no effect on the collection.

     • For example, invoking the addAll(Collection) method on an unmodifiable collection may, but is not required to, throw the exception if the collection to be added is empty.

Method Summary

• add(E e)

  • Add element e

• addAll(Collection<? extends E> c)

  • Add all from specified collection

• clear()

  • clear collection

• contains(Object o)

  • if o is present

• containsAll(Collection<?> c)

  • if this collection contains all of the elements in the specified collection.

• equals(Object o)

  • Check if meaningfully equal

• hashCode()

  • Return hash code integer

• isEmpty()

  • Check if empty

• iterator()

  • Return an iterator

• remove(Object o)

  • Remove o

• removeAll(Collection<?> c)

  • Remove all elements present in the specified collection

• retainAll(Collection<?> c)

  • Retains only the elements in this collection that are contained in the specified collection

• size()

  • size of the collection

• toArray()

  • convert to an array

• toArray(T[] a)

  • runtime type of the returned array is that of the specified array.

Following methods from the above list are marked as optional: 

• add(E e)

• addAll(Collection<? extends E> c)

• clear()

• remove(Object o)

• removeAll(Collection<?> c)

• retainAll(Collection<?> c)