Iterate Over a List in Java

In Java if you try to print a List (or any) object directly using the print statement a hash-code of the object will be printed (default behaviour).

If you want to print each element of your object (such as List, Array, etc.) or perform any operation on them one by one, you need to iterate through it.

Java provides various ways to iterate through objects. Following are different approaches to iterate through a List object:

• Using For Loop
• Using While Loop
• Using Iterator Object

Iterate over a List using For Loop

A for loop is a control-flow statement that is used to execute a block of code repeatedly till the condition is not becomes false.

To iterate over a List, we will run the loop at an initial value 0, and continue until it reaches the last index of the list. You can use the get(index) method to access the List elements.

Example

In the following example, we iterate over a List using a for-loop and use the get(index) method inside the loop to access each element based on the indexed value passed to it:

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

public class iterateOverList {
   public static void main(String[] args) {
   
      //instantiating a List using ArrayList class
      List<Integer> list = new ArrayList<>();
      
      //adding element to it
      list.add(1);
      list.add(2);
      list.add(3);
      list.add(4);
      
      System.out.println("The list elements are: ");
      
      //iterate over a list using for-loop
      for(int i = 0; i<list.size(); i++){
         System.out.println(list.get(i));
      }
   }
}

The above program produces the following output:

The list elements are: 
1
2
3
4

Using while Loop

A while loop is also a control flow statement, which executes a block of code repeatedly until the specified condition is true. Once the condition is false, it will stop executing.

In the while loop, we compare the initial value (i.e., i = 0) with the size() of the list ( i < size) and execute the block of code until the condition does not become false (i.e., i > size).

Example

In the example below, we use the while loop to iterate over a List and use the get(index) method to access each element of the List {"Apple", "Banana", "Orange", "Grapes"}:

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

public class iterateOverList {
   public static void main(String[] args) {
   
      //instantiating a List using ArrayList class
      List<String> fruits = new ArrayList<>();
      
      //adding element to it
      fruits.add("Apple");
      fruits.add("Banana");
      fruits.add("Orange");
      fruits.add("Grapes");
      
      System.out.println("The list elements are: ");
      
      //iterate over a fruits using while-loop
      int i = 0;
      while(i < fruits.size()){
         System.out.println(fruits.get(i));
         i++;
      }
   }
}

Following is the output of the above program:

The list elements are: 
Apple
Banana
Orange
Grapes

Using an Iterator Object

In Java, an Iterator is an object that is used to loop through collections objects, such as ArrayList, HashSet, etc. It provides a method named iterator(), which retrieves an iterator.

Example

In this example, we use an Iterator object to iterate over a List and use the next() method inside a while loop to print each element of the List one by one:

import java.util.ArrayList;
import java.util.Iterator;

public class iterateOverList {
   public static void main(String[] args) {
   
      //instantiating a List using ArrayList class
      ArrayList<String> list = new ArrayList<String>();
      
      //adding element to it
      list.add("JavaFx");
      list.add("Java");
      list.add("WebGL");
      list.add("OpenCV");
      
      System.out.println("The list elements are: ");
      
      //Iterator object
      Iterator iterator = list.iterator();
      while(iterator.hasNext()) {
         System.out.println(iterator.next());
      }
   }
}

Below is the output of the above program:

The list elements are: 
JavaFx
Java
WebGL
OpenCV