Collection interface:
Collection interface is the root interface in the collection hierarchy. It declares the core methods for all collections.
Commonly used methods of Collection interface:
1. add(Object obj): Add object to the calling collection. It returns true if collection changed after this call otherwise returns false.
Syntax: public boolean add(Object obj)
2. addAll(Collection c): Add all objects to the calling collection. It returns true if collection changed after this call otherwise returns false.
Syntax: public boolean addAll(Collection c)
3. clear(): Remove all elements from the calling collection.
Syntax: public void clear()
4. contains(Object obj): It returns true if this collection contains the specified object otherwise returns false.
Syntax: public boolean contains(Object obj)
5. containsAll(Collection c):It returns true if this collection contains all elements of the specified collection otherwise returns false.
Syntax: public boolean containsAll(Collection c)
6. equals(Object obj): It returns true if this collection equals to the specified object otherwise returns false.
Syntax: public boolean equals(Object obj)
7. hashCode(): It returns the hash code of the calling collection.
Syntax: public int hashCode()
8. isEmpty(): It returns true if the calling collection is empty otherwise returns false.
Syntax: public boolean isEmpty()
9. iterator(): It returns an iterator instance for the calling collection.
Syntax: public Iterator iterator()
10. remove(Object obj): It remove the specified object from the calling collection. It returns true if collection changed after this call otherwise returns false.
Syntax: public boolean remove(Object obj)
11. removeAll(Collection c): It remove all elements of the specified collection from the calling collection. It returns true if collection changed after this call otherwise returns false.
Syntax: public boolean removeAll(Collection c)
12. retainAll(Collection c): It remove all elements except those of the specified collection from the calling collection. It returns true if collection changed after this call otherwise returns false.
Syntax: public boolean retainAll(Collection c)
13. size(): It returns the number of elements in the calling collection.
Syntax: public int size()
14. toArray( ): It returns an array of containing all elements of the calling collection.
Syntax: public Object[ ] toArray()
A simple example of HashSet class to explain few methods of Collection interface.
HashSetTest.java
import java.util.HashSet; import java.util.Iterator; import java.util.Set; /** * This class is used to show the HashSet functionality. * @author w3spoint */ public class HashSetTest { public static void main(String args[]){ //Create HashSet object. Set hashSet = new HashSet(); //Add objects to the HashSet. hashSet.add("Roxy"); hashSet.add("Sunil"); hashSet.add("Sandy"); hashSet.add("Munish"); hashSet.add("Pardeep"); //Print the HashSet object. System.out.println("HashSet elements:"); System.out.println(hashSet); //Print the HashSet elements using iterator. Iterator iterator=hashSet.iterator(); System.out.println("HashSet elements using iterator:"); while(iterator.hasNext()){ System.out.println(iterator.next()); } } } |
Output
HashSet elements: [Sandy, Pardeep, Munish, Sunil, Roxy] HashSet elements using iterator: Sandy Pardeep Munish Sunil Roxy |
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