In how many ways you can retrieve the elements of a collection in Java?

In Java, the collections framework provides various classes like ArrayList, HashSet, and LinkedList to store groups of elements. But once data is stored, how do you access or retrieve it?

Java provides multiple ways to retrieve elements from collections, depending on whether you're reading values, modifying them during traversal, or iterating forward or backward. You can retrieve the elements of a collection in four ways-

• Using For-Loop

• Using For-Each Loop

• Uisng Iterator and ListIterator

• Using Enumeration

Using For-Loop

A for loop retrieves elements by iterating over index values and accessing each element using the get(index) method of the collection.

import java.util.ArrayList;
import java.util.List;

public class ForLoopExample {
   public static void main(String[] args) {
      List < String > names = new ArrayList < > ();
      names.add("Manisha");
      names.add("Amit");
      names.add("Priya");

      for (int i = 0; i < names.size(); i++) {
         System.out.println(names.get(i));
      }
   }
}

Let us compile and run the above program, this will give the following result -

Manisha
Amit
Priya

Using For-Each Loop

The foreach loop or enhanced for loop enables you to traverse the complete collection object sequentially.

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);
      }
}

Let us compile and run the above program, this will give the following result -

Contents of the array list:
JavaFX
Java
WebGL
OpenCV
OpenNLP
JOGL
Hadoop
HBase
Flume
Mahout
Impala

Using Iterator

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 a next element, else it returns false.

• The next() method 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 a previous element.

You can get the Iterator or ListIterator objects of a collection using the Iterator and ListIterator() methods.

import java.util.ArrayList;
import java.util.Iterator;
import java.util.ListIterator;
public class RetrievingData {
   public static void main(String[] args) {
      ArrayList  list = new ArrayList  ();
      //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());
      }
   }
}

Let us compile and run the above program, this will give the following result -

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.

import java.util.Enumeration;
import java.util.Vector;
public class EnumerationExample {
   public static void main(String args[]) {
      //instantiating a Vector
      Vector  vec = new Vector  ();
      //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  en = vec.elements();
      while (en.hasMoreElements()) {
         System.out.println(en.nextElement());
      }
   }
}

Let us compile and run the above program, this will give the following result -

1254
4587
5211
4205
1124
8115