Iterate through List in Java

Lists in java allow us to maintain an ordered collection of objects. Duplicate elements as well as null elements can also be stored in a List in Java. The List interface is a part of java.util package and it inherits the Collection interface. It preserves the order of insertion.

There are several ways to iterate over List in Java. They are discussed below:

Methods:

1. Using loops (Naive Approach)
   • For loop
   • For-each loop
   • While loop
2. Using Iterator
3. Using List iterator
4. Using lambda expression
5. Using stream.forEach()

Method 1-A: Simple for loop

Each element can be accessed by iteration using a simple for loop. The index can be accessed using the index as a loop variable.

Syntax:

for (i = 0; i < list_name.size(); i++) 
{
  // code block to be executed
}

Example

Java

// Java Program to iterate over List
// Using simple for loop

// Importing all classes of
// java.util package
import java.util.*;

// CLass
class GFG {

    // Main driver method
    public static void main(String args[])
    {
        // Creating a ArrayList
        List<String> myList = new ArrayList<String>();

        // Adding elements to the list
        // Custom inputs
        myList.add("A");
        myList.add("B");
        myList.add("C");
        myList.add("D");

        // For loop for iterating over the List
        for (int i = 0; i < myList.size(); i++) {

            // Print all elements of List
            System.out.println(myList.get(i));
        }
    }
}

Output

A
B
C
D

Method 1-B: Enhanced for loop

Each element can be accessed by iteration using an enhanced for loop. This loop was introduced in J2SE 5.0. It is an alternative approach to traverse the for a loop. It makes the code more readable.

Syntax:

for(data_type variable : List_name)
{  
 // Body of the loop. 
 // Each element can be accessed using variable.  
}

Java

// Java Program to Iterate over a List
// using enhanced for loop (for-each)

// Importing all classes of
// java.util package
import java.util.*;

// Class
class GFG {

    // Main driver method
    public static void main(String args[])
    {
        // Creating an Arraylist
        List<String> myList = new ArrayList<String>();

        // Adding elements to the List
        // Custom inputs
        myList.add("A");
        myList.add("B");
        myList.add("C");
        myList.add("D");

        // Using enhanced for loop(for-each) for iteration
        for (String i : myList) {

            // Print all elements of ArrayList
            System.out.println(i);
        }
    }
}

Output

A
B
C
D

Method 1-C: Using a while loop

Iterating over a list can also be achieved using a while loop. The block of code inside the loop executes until the condition is true. A loop variable can be used as an index to access each element.

Syntax:

while(variable<list_name.size())
{
   // Block of code to be executed 
}

Java

// Java Program to iterate over a List
// using while loop

// Importing all classes of
// java.util package
import java.util.*;

// Class
class GFG {

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

        // Adding elements to the List
        // Custom inputs
        myList.add("A");
        myList.add("B");
        myList.add("C");
        myList.add("D");

        // Initializing any variable to 0
        int i = 0;

        // If variable value is lesser than
        // value indicating size of List
        while (i < myList.size()) {

            // Print element of list
            System.out.println(myList.get(i));

            // Increase variable count by 1
            i++;
        }
    }
}

Output

A
B
C
D

Method 2: Using iterator

An iterator is an object in Java that allows iterating over elements of a collection. Each element in the list can be accessed using iterator with a while loop.

Syntax:

Iterator<data_type> variable = list_name.iterator();

Example

Java

// Java Program to iterate over the list
// using iterator

// Importing all classes of
// java.util package
import java.util.*;

// Class
class GFG {

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

        // Adding elements to the List
        // Custom inputs
        myList.add("A");
        myList.add("B");
        myList.add("C");
        myList.add("D");

        // Iterator
        Iterator<String> it = myList.iterator();

        // Condition check for elements in List
        // using hasNext() method returning true till
        // there is single element in a List
        while (it.hasNext()) {

            // Print all elements of List
            System.out.println(it.next());
        }
    }
}

Output

A
B
C
D

Method 3: Using List iterator

ListIterator is an iterator is a java which is available since the 1.2 version. It allows us to iterate elements one-by-one from a List implemented object. It is used to iterator over a list using while loop.

Syntax

ListIterator<data_type> variable = list_name.listIterator();

Java

// Java program to iterate over a list
// using ListIterator
import java.util.*;

// Class
class GFG {

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

        // Adding elements to the List
        // Custom inputs
        myList.add("A");
        myList.add("B");
        myList.add("C");
        myList.add("D");

        // List iterator
        ListIterator<String> it = myList.listIterator();

        // Condition check whether there is element in List
        // using hasNext() which holds true till
        // there is single element in List
        while (it.hasNext()) {

            // Print all elements of List
            System.out.println(it.next());
        }
    }
}

Output

A
B
C
D

Method 4: Using Iterable.forEach()

This feature is available since Java 8. It can also be used to iterate over a List. Iteration can be done using a lambda expression.

Syntax:

list_name.forEach(variable->{//block of code})

Java

// Java Program to iterate over a List
// using forEach()

// Importing all classes of
// java.util method
import java.util.*;

// Class
class GFG {

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

        // Adding elements to the List
        // Custom inputs
        myList.add("A");
        myList.add("B");
        myList.add("C");
        myList.add("D");

        // Lambda expression printing all elements in a List
        myList.forEach(
            (temp) -> { System.out.println(temp); });
    }
}

Output

A
B
C
D

Method 5: Using Stream.forEach()

The processing order of stream().forEach() is undefined while in case of forEach(), it is defined. Both can be used to iterate over a List.

Syntax:

list_name.stream.forEach(variable->{//block of code})

Java

// Java Program iterating over a List
// using stream.forEach() method

// Importing all classes of
// java.util method
import java.util.*;

// Class
class GFG {

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

        // Adding elements to the List
        // Custom inputs
        myList.add("A");
        myList.add("B");
        myList.add("C");
        myList.add("D");

        // stream.forEach() method prints
        // all elements inside a List
        myList.stream().forEach(
            (temp) -> System.out.println(temp));
    }
}

Output

A
B
C
D