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In how many ways you can retrieve the elements of a collection in Java?
You can retrieve the contents of a collection object in three ways −
Using for each loop
The foreach loop or enhanced for loop, which enables you to traverse the complete collection object sequentially.
Example
import java.util.ArrayList;
public class RetrievingData {
public static void main(String[] args) {
ArrayList <String> list = new ArrayList<String>();
//Instantiating an ArrayList object
list.add("JavaFX");
list.add("Java");
list.add("WebGL");
list.add("OpenCV");
list.add("OpenNLP");
list.add("JOGL");
list.add("Hadoop");
list.add("HBase");
list.add("Flume");
list.add("Mahout");
list.add("Impala");
System.out.println("Contents of the array list: ");
for (String e: list)
System.out.println(e);
}
}Output
Contents of the array list: JavaFX Java WebGL OpenCV OpenNLP JOGL Hadoop HBase Flume Mahout Impala
Using Iterators
Java provides Iterator and ListIterator classes to retrieve the elements of the collection object.
The hasNext() method of these interfaces returns true if the collection object has next element else it returns false.
The next() methods of the Iterator and ListIterator returns the next element of the collection.
Using these two methods you can retrieve the contents from an iterator object.
Similarly, the previous() method of the ListIterator returns the previous element of the collection and hasPrevious() determines whether the current collection object has previous element.
You can get the Iterator or, ListIterator objects of a collection using the Iterator and ListIterator() methods.
Example
import java.util.ArrayList;
import java.util.Iterator;
import java.util.ListIterator;
public class RetrievingData {
public static void main(String[] args) {
ArrayList <String> list = new ArrayList<String>();
//Instantiating an ArrayList object
list.add("JavaFX");
list.add("Java");
list.add("WebGL");
list.add("OpenCV");
list.add("OpenNLP");
list.add("JOGL");
list.add("Hadoop");
list.add("HBase");
list.add("Flume");
list.add("Mahout");
list.add("Impala");
System.out.println("Contents of the array list (first to last): ");
Iterator it = list.iterator();
while(it.hasNext()) {
System.out.println(it.next());
}
System.out.println("Contents of the array list (last to first): ");
ListIterator lit = list.listIterator();
while(lit.hasNext()) {
lit.next();
}
while(lit.hasPrevious()) {
System.out.println(lit.previous());
}
}
}Output
Contents of the array list (first to last): JavaFX Java WebGL OpenCV OpenNLP JOGL Hadoop HBase Flume Mahout Impala Contents of the array list (last to first): Impala Mahout Flume HBase Hadoop JOGL OpenNLP OpenCV WebGL Java JavaFX
Using Enumeration
The Enumeration class contains a method named hasMoreElements() which returns true if the current object contains more elements after the current position (else it returns false).
If you call the nextElement() method of the Enumeration class returns the next element in the current enumeration object.
Using these two methods you can retrieve the contents of a collection object.
Example
import java.util.Enumeration;
import java.util.Vector;
public class EnumerationExample {
public static void main(String args[]) {
//instantiating a Vector
Vector<Integer> vec = new Vector<Integer>( );
//Populating the vector
vec.add(1254);
vec.add(4587);
vec.add(5211);
vec.add(4205);
vec.add(1124);
vec.add(8115);
//Retrieving the elements using the Enumeration
Enumeration<Integer> en = vec.elements();
while(en.hasMoreElements()) {
System.out.println(en.nextElement());
}
}
}Output
1254 4587 5211 4205 1124 8115
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