In this tutorial, we will see how to iterate LinkedList in Java. There are multiple ways to iterate such as loops, using Iterator or using Java 8 syntax. Let’s see these ways to iterate over LinkedList in Java.
Introduction
The LinkedList class is present in the java.util package.
LinkedList internally uses a doubly LinkedList for storing the variables. That’s why LinkedList is preferred when our frequent operation is insertion and deletion.
There are multiple ways to iterate over the LinkedList. We will see the following ways
- Iterate the LinkedList using basic for loop
- Iterate the LinkedList using Enhanced For Loop i.e. For-each loop
- Iterate the LinkedList using the while loop
- Iterate the LinkedList using Iterator
- Iterate the LinkedList using Java 8 the forEach() Method
So let’s see all the options one by one.
1. Iterate the LinkedList using basic for-loop
We can write a simple for loop for the LinkedList and access the elements of the LinkedList using the get(int index) method.
Please refer to the following Java example to iterate over LinkedList using the basic for loop.
import java.util.LinkedList;
import java.util.List;
/**
* A Java program to iterate over LinkedList
* using the basic for loop.
*
* @author coderolls.com
*
*/
public class LinkedListForLoop {
public static void main(String[] args) {
List<String> linkedList = new LinkedList<String>();
linkedList.add("Mango");
linkedList.add("Banana");
linkedList.add("Grapes");
linkedList.add("Watermelon");
linkedList.add("Apple");
linkedList.add("Avocado");
System.out.println("Iterating through LinkedList using the basic for loop!\n");
int len = linkedList.size();
// iterate over linkedList using basic for loop
for (int i = 0; i < len; i++) {
// get String object from linkedList at index i
String str = linkedList.get(i);
System.out.println("The object from the LinkedList at " + i + " is " + str);
}
}
}
Output:
Iterating through LinkedList using the basic for loop.......
The object from the LinkedList at 0 is Mango
The object from the LinkedList at 1 is Banana
The object from the LinkedList at 2 is Grapes
The object from the LinkedList at 3 is Watermelon
The object from the LinkedList at 4 is Apple
The object from the LinkedList at 5 is Avocado
2. Iterate the LinkedList using Enhanced For Loop i.e. For-each loop
Here we will use the foreach loop i.e. Enhanced For Loop to iterate through ArrayList.
Let us understand the syntax for the Enhanced For Loop.
for(DataType item:Array | Collection){
// operate on the item here
}
Please refer to the following Java program to iterate over LinkedList using the foreach loop.
import java.util.LinkedList;
import java.util.List;
/**
* A Java program to iterate over LinkedList
* using the foreach loop.
*
* @author coderolls.com
*
*/
public class LinkedListForeachLoop {
public static void main(String[] args) {
List<String> linkedList = new LinkedList<String>();
linkedList.add("Mango");
linkedList.add("Banana");
linkedList.add("Grapes");
linkedList.add("Watermelon");
linkedList.add("Apple");
linkedList.add("Avocado");
System.out.println("Iterating through LinkedList using the foreach loop!\n");
// iterate over linkedList using for-each loop
for (String str: linkedList) {
System.out.println("The object from the LinkedList is "+str);
}
}
}
Output:
Iterating through LinkedList using the for-each loop!
The object from the LinkedList is Mango
The object from the LinkedList is Banana
The object from the LinkedList is Grapes
The object from the LinkedList is Watermelon
The object from the LinkedList is Apple
The object from the LinkedList is Avocado
3. Iterate the LinkedList using the while loop
Similar to the for and for-each loop, we can also use the while loop to iterate over LinkedList.
I have given a Java program below to iterate over LinkedList using the while loop.
/**
* A Java program to iterate over LinkedList
* using the while loop.
*
* @author coderolls.com
*
*/
public class LinkedListWhileLoop {
public static void main(String[] args) {
List<String> linkedList = new LinkedList<String>();
linkedList.add("Mango");
linkedList.add("Banana");
linkedList.add("Grapes");
linkedList.add("Watermelon");
linkedList.add("Apple");
linkedList.add("Avocado");
System.out.println("Iterating through LinkedList using the while loop!\n");
int len = linkedList.size();
// iterate over linkedList using the while loop
int i =0;
while(i<len) {
// get String object from linkedList at index i
String str = linkedList.get(i);
System.out.println("Object form the LinkedList is "+str);
//increment index after getting the object
i++;
}
}
}
Output:
Iterating through LinkedList using the while loop!
The object from the LinkedList is Mango
The object from the LinkedList is Banana
The object from the LinkedList is Grapes
The object from the LinkedList is Watermelon
The object from the LinkedList is Apple
The object from the LinkedList is Avocado
4. Iterate the LinkedList using Iterator
Iterator is an interface in Java. We can use an Iterator to iterate over a collection.
It is present in the java.util package.
If we want to iterate over a LinkedList using an iterator, we should invoke the iterator() method on the LinkedList object to get the iterator reference as given below
// linkedList is a LinkedList object
Iterator iterator = linkedList.iterator();
Iterator has three important methods. They are as given below.
hasNext() - This method checks if the collection has any object available next, if found it will return true else false
next() - This method returns the next object from the LinkedList.
remove() - This method removes the object from the LinkedList
I have given a Java program below to iterate over a LinkedList using the iterator interface. We will be using the hasNext() and next() methods in the program.
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
/**
* A Java program to iterate through LinkedList using the Iterator interface.
*
* @author coderolls.com
*
*/
public class LinkedListIterator {
public static void main(String[] args) {
List<String> linkedList = new LinkedList<String>();
linkedList.add("Mango");
linkedList.add("Banana");
linkedList.add("Grapes");
linkedList.add("Watermelon");
linkedList.add("Apple");
linkedList.add("Avocado");
System.out.println("Iterating through LinkedList using Iterator interface........\n");
// getting an iterator
Iterator<String> iterator = linkedList.iterator();
// hasNext() returns true only if the object is available at the next call
while (iterator.hasNext()) {
// next() returns the object, we can cast it to the required type
String str = iterator.next();
System.out.println("The object from the LinkedList is " + str);
}
}
}
Output:
Iterating through LinkedList using Iterator interface........
The object from the LinkedList is Mango
The object from the LinkedList is Banana
The object from the LinkedList is Grapes
The object from the LinkedList is Watermelon
The object from the LinkedList is Apple
The object from the LinkedList is Avocado
Note: It is always recommended to use the Iterator.remove() method for removing an element from a list while iterating on it. Also, we have the remove() method in the LinkedList class, if you use the LinkedList.remove() method to remove an element while iterating over the list, you will get the ConcurrentModificationException.
5. Iterate the LinkedList using ListIterator
I have written a detailed article on the ListIterator method of the LinkedList class. Read LinkedList listIterator() method in Java
6. Iterate the LinkedList using Java 8 forEach() Method
In Java 8, we got the new forEach() method to iterator over collections.
Inside the forEach() method, we can write the lambda expression for the Consumer functional interface.
I have given a sample Java program below to iterate over the LinkedList using the forEach() method.
import java.util.LinkedList;
import java.util.List;
/**
* A Java program to iterate over LinkedList
* using the java 8 forEach() method.
*
* @author coderolls.com
*/
public class LinkedListForEachMethod {
public static void main(String[] args) {
List<String> linkedList = new LinkedList<String>();
linkedList.add("Mango");
linkedList.add("Banana");
linkedList.add("Grapes");
linkedList.add("Watermelon");
linkedList.add("Apple");
linkedList.add("Avocado");
System.out.println("Iterating through LinkedList using forEach method and lamda expression........\n");
//Add lambda expression in the forEach() method
linkedList.forEach(s -> System.out.println("The object from the LinkedList is " + s));
}
}
Output:
Iterating through LinkedList using forEach method and lambda expression........
The object from the LinkedList is Mango
The object from the LinkedList is Banana
The object from the LinkedList is Grapes
The object from the LinkedList is Watermelon
The object from the LinkedList is Apple
The object from the LinkedList is Avocado
Conclusion
We can iterate over ArrayList using the following ways.
-
Iterating the ArrayList using basic for loop
int len = linkedList.size(); for (int i = 0; i < len; i++) { String str = linkedList.get(i); System.out.println("Object form the LinkedList at " + i + " is " + str); } -
Iterating the ArrayList using Enhanced For Loop i.e. For-each loop
for (String str: linkedList) { System.out.println("Object form the LinkedList is "+str); } -
Iterating the ArrayList using a while loop
int len = linkedList.size(); int i =0; while(i<len) { String str = linkedList.get(i); System.out.println("Object form the LinkedList is "+str); i++; } -
Iterating the ArrayList using Iterator
Iterator<String> iterator = linkedList.iterator(); while (iterator.hasNext()) { String str = iterator.next(); System.out.println("Object form the LinkedList is " + str); } -
Iterating the ArrayList using ListIterator- I have written a detailed article on the ListIterator method of the LinkedList class. Read LinkedList listIterator() method in Java
-
Iterating the ArrayList using Java 8 forEach() Method
linkedList.forEach(s -> System.out.println("Object form the LinkedList is " + s));
The example Java programs given in the above tutorial can be found at this GitHub Repository.
Let me know your thoughts or suggestions on the topic discussed above in the comment section below.