Difference Between next() and hasNext() Method in Java Collections

In Java, objects are stored dynamically using objects. Now in order to traverse across these objects is done using a for-each loop, iterators, and comparators. Here will be discussing iterators. The iterator interface allows visiting elements in containers one by one which indirectly signifies retrieval of elements of the collection in forwarding direction only.

This interface compromises of three methods :

1. next()
2. hasNext()
3. remove()

(A) hasNext() Method

hasNext() method is used to check whether there is any element remaining in the List. This method is a boolean type method that returns only true and false as discussed as it is just used for checking purposes. The hasNext() methods of the Iterator and List Iterator returns true if the collection object over which is used to check during traversal whether the pointing element has the next element. If not it simply returns false. So,

Return Value:
        True  - if iteration has more elements 
        False - if iteration has no more elements

Return type: boolean

Example:

Java

// Java program to demonstrate
// the use of hasNext() method

// Importing java input output classes
// Importing all classesfrom
// java.util package
import java.io.*;
import java.util.*;

// Class
class GFG {

    // Main driver method
    public static void main(String[] args)
    {
        // Creating an ArrayList
        // Declaring the ArrayList
        ArrayList<String> list = new ArrayList<String>();

        // Adding (appending) new elements at
        // the end of the List
        // Custom inputs
        list.add("Geeks");
        list.add("for Geeks");

        // Declaring the Iterator
        Iterator<String> iterator = list.iterator();

        // Printing hasNext() values
        // Prints true because iterator has two more values
        System.out.println(iterator.hasNext());

        // Go to next value using next() method
        iterator.next();

        // Prints true because iterator has one more values
        System.out.println(iterator.hasNext());

        // Go to next value using next() method
        iterator.next();

        // Prints false because iterator has no more values
        System.out.println(iterator.hasNext());
    }
}

Output:

true
true
false

(B) next() method

If there is an element after where hasNext() has returned false on which some execution is to be performed then this method is used to display that element on which execution is supposed to be carried on with help of this method. The next() methods of the Iterator and List Iterator return the next element of the collection. And if there is a need to remove this element remove() method is used.

Return type: Same as collection such as ArrayList, Linked List, etc.

Return value: The next element in the iteration.

Exception: Throws NoSuchElementException if the iteration has no more elements.

Example:

Java

// Java program to demonstrate
// the use of next() method

// Importing java input output classes
import java.io.*;
// Importing all classes from
// java.util package
import java.util.*;

// Class
class GFG {

    // Main driver method
    public static void main(String[] args)
    {
        // Creating an ArrayList
        // (Declaring ArrayList of String type)
        ArrayList<String> list = new ArrayList<String>();

        // Adding elements to above List at
        // the end of the list
        // Custom inputs
        list.add("Element1");
        list.add("Element2");
        list.add("Element3");

        // Declaring the Iterator
        Iterator<String> iterator = list.iterator();

        // Printing values showcasing next() method which
        // shows traversal over elements
        // only in forward direction

        // Prints first element traversed
        System.out.println(iterator.next());

        // Prints the succeeding element
        System.out.println(iterator.next());

        // Prints another element succeeding
        // to previous element
        System.out.println(iterator.next());
    }
}

Output:

Element1
Element2
Element3