The Java collection framework was introduced in the Java 1.2 version.
A Collection Framework provides an architecture to store and manipulate a group of objects as a single unit. The collection framework provides many interfaces and classes for implementing various data structures and algorithms.
In the Java collection framework, the java.util.Collection interface and the java.util.Map interface are the two main "root" interfaces of Java collection classes.
In the collections framework hierarchy, the Collection interface is the root interface, which was introduced in the Java 1.2 version and placed in the java.util package.
All the collection framework classes and interfaces directly or indirectly implement the Collection interface.
A Collection Interface provides many methods to perform the basic operations such as add, remove, clear, etc., which are used by all the child classes of the collection interface.
Java Collection Interface Declaration
public interface Collection<E> extends Iterable<E>
Java Collection Interface Methods
Methods & Descriptions
boolean add(E e) Inserts the specified element to the collection.
boolean addAll(Collection<? extends E> c) Adds all the elements of a specified collection to the collection.
void clear() Removes all of the elements from the collection.
boolean contains(Object o) Returns true if this collection contains the specified element else false.
boolean containsAll(Collection<?> c) Returns true if this collection contains all of the elements in the specified collection else false.
boolean equals(Object o) Compares the specified object with this collection for equality.
int hashCode() Returns the hash code value for the collection.
boolean isEmpty() Returns true if this collection contains no elements else false.
Iterator<E> iterator() Returns an iterator over the elements in this collection.
default Stream<E> parallelStream() Returns a possibly parallel Stream with this collection as its source.
boolean remove(Object o) Removes a single instance of the specified element from the collection. If specified element is more than one, then this method removes the first occurrence of the object.
boolean removeAll(Collection<?> c) Removes all of the collection's elements that are also contained in the specified collection.
default boolean removeIf(Predicate<? super E> filter) Removes all of the elements of this collection that satisfy the given predicate.
boolean retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection.
int size() Returns the number of elements in the collection.
default Spliterator<E> spliterator() This method is used to create a Spliterator over the elements in this collection.
default Stream stream() Returns a sequential Stream with this collection as its source.
Object[] toArray() Returns an array containing all of the elements in this collection.
<T> T[] toArray(T[] a) Returns an array containing all of the elements in this collection.
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Collection Framework - Interview Questions and Answers
What are the advantages of the Java Collection Framework?
Ans. The Java collection framework provides the following benefits:
Reduces programming effort by providing data structures and algorithms.
Increases performance by providing high-performance implementations of data structures and algorithms.
Provides interoperability between unrelated APIs.
Reduces the effort required to design and implement APIs.
Fosters software reuse by providing a standard interface for collections and algorithms with which to manipulate them.
What is the difference between Iterator and Enumeration in Java?
Ans. Though both Iterator and Enumeration allow us to traverse over elements of Collections in Java, there are some significant differences between them:
Enumeration
Iterator
Enumeration is a legacy interface that was introduced in JDK 1.0.
Iterator is not a legacy interface; it was introduced in JDK 1.2.
Enumeration does not allow us to remove elements from a collection during traversal.
Iterator also allow us to remove elements from collection during traversal. It has a remove() method.
Enumeration supports only legacy classes like Vector, Stack, Hashtable, Properties, Dictionary.
Iterator supports all the Collection framework classes.
public interface Enumeration<E>
public interface Iterator<E>
hasMoreElement() nextElement()
hasNext() next() remove()
In summary, both Enumeration and Iterator will give successive elements, but Iterator is a new and improved version where method names are shorter and it has a new method called remove.
What is the difference between an Iterator and a ListIterator in Java?
Ans. Iterator and ListIterator are both used for traversing purposes in Java, but there are many differences between them that we will see in the given below table.
Iterator
ListIterator
It can traverse only in the forward direction using an iterator.
We can use ListIterator to traverse List only.
We can traverse in only forward direction using Iterator.
Using ListIterator, we can traverse a List in both directions (forward and backward).
We cannot obtain indexes while using an Iterator.
We can obtain indexes at any point of time while traversing a list using ListIterator. The methods nextIndex() and previousIndex() are used for this purpose.
We cannot add elements to a collection while traversing it using an iterator. It throws a ConcurrentModificationException when we try to do it.
We can add an element at any point of time while traversing a list using ListIterator.
We cannot replace the existing element value when using an Iterator.
By using the set(E e) method of ListIterator, we can replace the last element returned by the next() or previous() methods.
public interface Iterator<E>
public interface ListIterator<E> extends Iterator<E>
