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Collection interface in java


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 codesjava
 */
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

Download this example.
 
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