Collection Interface In Java

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Collection Interface

Collection Interface:

The root interface in the collection hierarchy. It represents a group of objects known as its elements.

It the part of java.util package.
The root interface in the collection hierarchy. It represents a group of individual objects as a single entity known as its elements.
The collection interface is a part of the core framework that provides the base for all the collection types. It defined the most common methods which are applicable for any collection object.
It is an interface provided by JAVA along with JDK 1.2 version.

Key Interfaces Of Collection Framework

Collection
List
Set
Sorted Set
Navigable Set
Queue, Dequeue
Navigable Map
Sorted Map
Map

NOTE

Maps are not the part of the Collection Framework because there is no relation between Maps and Collection and Maps not implement the Collection interaface

Methods In Collection Interface

Method	Description
boolean add(E e)	Add the specified element in the collection.
boolean addAll(Collection<? extends E> c)	Adds all of the elements in the specified collection to this collection.
void clear()	Removes all of the elements from this collection.
boolean contains(Object o)	Returns true if this collection contains the specified element.
boolean containsAll(Collection<?> c)	Returns true if this collection contains all of the elements in the specified collection.
boolean equals(Object o)	Compares the specified object with this collection for content equality.
int hashCode()	Returns the hash code value for this collection.
boolean isEmpty()	Returns true if this collection contains no elements.
Iterator<E> iterator()	Returns an iterator over the elements in this collection.
boolean remove(Object o)	Removes a single instance of the specified element from this collection, if it is present.
boolean removeAll(Collection<?> c)	Removes all of this collection's elements that are also contained in the specified collection.
boolean retainAll(Collection<?> c)	Retains only the elements in this collection that are contained in the specified collection. (Intersection between two collection)
int size()	Returns the number of elements in this collection.
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; the runtime type of the returned array is that of the specified array.

boolean equals(Object o)

Compares the specified object with this collection for content equality.

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import java.util.*;​public class Test{    public static void main(String[] args)    {        Collection<String> collection1 = new ArrayList<>();        collection1.add("Hello");        Collection<String> collection2 = new ArrayList<>();        collection2.add("Hello");        boolean isEqual = collection1.equals(collection2);        System.out.println(isEqual); // Output: true    }}

boolean isEmpty()

Returns true if this collection contains no elements.

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import java.util.*;​public class Test{    public static void main(String[] args)    {        Collection<String> collection = new ArrayList<>();        boolean isEmpty = collection.isEmpty();        System.out.println(isEmpty); // Output: true    }}

boolean add(E e)

Add the specified element in the collection.

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import java.util.*;​public class Test{    public static void main(String[] args)    {        Collection<String> collection = new ArrayList<>();        boolean isAdded = collection.add("Hello");        System.out.println(isAdded); // Output: true    }}​

boolean remove(Object o)

Removes a single instance of the specified element from this collection, if it is present.

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import java.util.*;​public class Test{    public static void main(String[] args)    {        Collection<String> collection = new ArrayList<>();        collection.add("Hello");        boolean isRemoved = collection.remove("Hello");        System.out.println(isRemoved); // Output: true        System.out.println(collection); // Output: []​    }}

boolean removeAll(Collection<?> c)

Removes all of this collection's elements that are also contained in the specified collection.

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import java.util.*;​public class Test{    public static void main(String[] args)    {        Collection<String> collection = new ArrayList<>();        collection.add("Hello");        collection.add("World");        Collection<String> anotherCollection = new ArrayList<>();        anotherCollection.add("Hello");        boolean isAllRemoved = collection.removeAll(anotherCollection);        System.out.println(isAllRemoved); // Output: true        System.out.println(collection); // Output: [World]    }}

boolean addAll(Collection<? extends E> c)

Adds all of the elements in the specified collection to this collection.

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import java.util.*;​public class Test{    public static void main(String[] args)    {        Collection<String> collection = new ArrayList<>();        collection.add("Hello");        Collection<String> anotherCollection = new ArrayList<>();        anotherCollection.add("World");        boolean isAllAdded = collection.addAll(anotherCollection);        System.out.println(isAllAdded); // Output: true        System.out.println(collection); // Output: [Hello, World]​    }}

void clear()

Removes all of the elements from this collection.

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import java.util.*;​public class Test{    public static void main(String[] args)    {        Collection<String> collection = new ArrayList<>();        collection.add("Hello");        collection.clear();        System.out.println(collection); // Output: []​    }}

boolean contains(Object o)

Returns true if this collection contains the specified element.

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import java.util.*;​public class Test{    public static void main(String[] args)    {        Collection<String> collection = new ArrayList<>();        collection.add("Hello");        collection.add("World");        boolean contains = collection.contains("Hello");        System.out.println(contains); // Output: true    }}

boolean containsAll(Collection<?> c)

Returns true if this collection contains all of the elements in the specified collection.

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import java.util.*;​public class Test{    public static void main(String[] args)    {        Collection<String> collection = new ArrayList<>();        collection.add("Hello");        collection.add("World");        Collection<String> anotherCollection = new ArrayList<>();        anotherCollection.add("Hello");        anotherCollection.add("World2");        boolean containsAll = collection.containsAll(anotherCollection);        System.out.println(containsAll); // Output: false​    }}

Collection Interface

Methods In Collection Interface

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