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Java List size() Method: A Comprehensive Guide with Examples

Anabelle Justin
By Anabelle Justin
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Java List size() Method: A Comprehensive Guide with Examples

In Java, the List interface is a part of the Java Collections Framework and is commonly used to store ordered collections of elements. One of the most frequently used methods in the List interface is the size() method. This method returns the number of elements present in the list. Knowing the size of a list is important for various operations like traversing, conditional logic, and resizing data structures.

In this article, we’ll explain the size() method, its syntax, and provide examples of how to use it in different scenarios.

What is the size() Method in Java?

The size() method is a part of the java.util.List interface. It returns an int value representing the number of elements in the list. The method does not take any arguments and operates in constant time, making it an efficient way to obtain the length of a list.

Syntax:

java
int size();

Key Points:

  1. The size() method is non-negative and will return a value greater than or equal to 0.
  2. It returns the current number of elements, not the maximum capacity.
  3. The method is typically used to check if a list is empty or to loop through a list by indexing.

Example 1: Using size() to Get the Number of Elements in a List

Here’s an example demonstrating the use of the size() method to get the number of elements in a List.

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

public class SizeMethodExample {
public static void main(String[] args) {
List<String> fruits = new ArrayList<>();

// Adding elements to the list
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Cherry");

// Using size() method to get the number of elements in the list
int numberOfFruits = fruits.size();

// Output the result
System.out.println("Number of fruits in the list: " + numberOfFruits);
}
}

Output:

javascript
Number of fruits in the list: 3

In this example, the size() method returns 3 because three elements have been added to the list.

Example 2: Checking If a List is Empty Using size()

You can use the size() method to check if a list is empty. A list is considered empty if its size is zero.

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

public class EmptyListCheck {
public static void main(String[] args) {
List<String> colors = new ArrayList<>();

// Checking if the list is empty
if (colors.size() == 0) {
System.out.println("The list is empty.");
} else {
System.out.println("The list is not empty.");
}
}
}

Output:

csharp
The list is empty.

In this case, since no elements were added to the colors list, the size() method returns 0, and the program prints that the list is empty.

Example 3: Using size() to Loop Through a List

The size() method is commonly used in loops to iterate over a list. Here’s an example of using size() to loop through a list of integers.

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

public class ListLoopExample {
public static void main(String[] args) {
List<Integer> numbers = new ArrayList<>();

// Adding elements to the list
numbers.add(10);
numbers.add(20);
numbers.add(30);
numbers.add(40);

// Using size() to loop through the list
for (int i = 0; i < numbers.size(); i++) {
System.out.println("Element at index " + i + ": " + numbers.get(i));
}
}
}

Output:

mathematica
Element at index 0: 10
Element at index 1: 20
Element at index 2: 30
Element at index 3: 40

In this example, the size() method is used to determine the loop’s upper bound. The loop iterates through each element of the list by index.

Example 4: Dynamically Changing the List Size

The size of a list can change as elements are added or removed. The size() method reflects these changes in real time. Let’s look at an example of adding and removing elements and how it affects the size of the list.

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

public class DynamicListSize {
public static void main(String[] args) {
List<String> animals = new ArrayList<>();

// Adding elements to the list
animals.add("Lion");
animals.add("Tiger");
animals.add("Elephant");

System.out.println("Size after adding elements: " + animals.size());

// Removing an element from the list
animals.remove("Tiger");

System.out.println("Size after removing an element: " + animals.size());
}
}

Output:

yaml
Size after adding elements: 3
Size after removing an element: 2

In this case, the list initially contains three elements. After removing one element, the size() method reflects the updated size.

The size() method in Java is a simple yet powerful tool for working with lists. It allows you to determine the number of elements in a list, check if it’s empty, and manage iterations. Understanding how to use size() is fundamental when working with the List interface, as it provides valuable insight into the contents of a collection.

Whether you’re managing dynamic collections or performing operations based on the number of elements, the size() method is a key part of Java’s collection framework.

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