Check if a Collection is Empty or Null in Java

In Java, it's often necessary to check if a collection is either empty or null before performing operations on it to avoid NullPointerException and ensure that the code behaves correctly. 

This guide will cover various approaches to check if a collection is empty or null, explain how they work, and provide examples to demonstrate their functionality. Additionally, we will cover a real-world use case to illustrate its application.

Table of Contents

1. Introduction
2. Checking if a Collection is Null
3. Checking if a Collection is Empty
4. Checking if a Collection is Null or Empty
5. Examples
   • Checking Null and Empty for a List
   • Checking Null and Empty for a Set
   • Checking Null and Empty for a Map
6. Real-World Use Case
7. Conclusion

Introduction

In Java, a collection can be either null or contain no elements. It's essential to handle both cases to prevent runtime errors and ensure that the application functions correctly. This can be achieved using simple conditional checks.

Checking if a Collection is Null

A collection is null if it has not been initialized. You can check if a collection is null using a simple equality check:

if (collection == null) {
    // Collection is null
}

Checking if a Collection is Empty

A collection is empty if it contains no elements. You can check if a collection is empty using the isEmpty() method:

if (collection.isEmpty()) {
    // Collection is empty
}

Checking if a Collection is Null or Empty

To check if a collection is either null or empty, you can combine the two checks:

if (collection == null || collection.isEmpty()) {
    // Collection is null or empty
}

Examples

Checking Null and Empty for a List

The following example demonstrates how to check if a List is null or empty:

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

public class CheckList {
    public static void main(String[] args) {
        List<String> list = null;

        // Check if the list is null or empty
        if (list == null || list.isEmpty()) {
            System.out.println("The list is null or empty.");
        } else {
            System.out.println("The list is not null and not empty.");
        }

        // Initialize the list
        list = new ArrayList<>();

        // Check if the list is null or empty
        if (list == null || list.isEmpty()) {
            System.out.println("The list is null or empty.");
        } else {
            System.out.println("The list is not null and not empty.");
        }

        // Add an element to the list
        list.add("Apple");

        // Check if the list is null or empty
        if (list == null || list.isEmpty()) {
            System.out.println("The list is null or empty.");
        } else {
            System.out.println("The list is not null and not empty.");
        }
    }
}

Output:

The list is null or empty.
The list is null or empty.
The list is not null and not empty.

Checking Null and Empty for a Set

The following example demonstrates how to check if a Set is null or empty:

import java.util.HashSet;
import java.util.Set;

public class CheckSet {
    public static void main(String[] args) {
        Set<String> set = null;

        // Check if the set is null or empty
        if (set == null || set.isEmpty()) {
            System.out.println("The set is null or empty.");
        } else {
            System.out.println("The set is not null and not empty.");
        }

        // Initialize the set
        set = new HashSet<>();

        // Check if the set is null or empty
        if (set == null || set.isEmpty()) {
            System.out.println("The set is null or empty.");
        } else {
            System.out.println("The set is not null and not empty.");
        }

        // Add an element to the set
        set.add("Apple");

        // Check if the set is null or empty
        if (set == null || set.isEmpty()) {
            System.out.println("The set is null or empty.");
        } else {
            System.out.println("The set is not null and not empty.");
        }
    }
}

Output:

The set is null or empty.
The set is null or empty.
The set is not null and not empty.

Checking Null and Empty for a Map

The following example demonstrates how to check if a Map is null or empty:

import java.util.HashMap;
import java.util.Map;

public class CheckMap {
    public static void main(String[] args) {
        Map<String, Integer> map = null;

        // Check if the map is null or empty
        if (map == null || map.isEmpty()) {
            System.out.println("The map is null or empty.");
        } else {
            System.out.println("The map is not null and not empty.");
        }

        // Initialize the map
        map = new HashMap<>();

        // Check if the map is null or empty
        if (map == null || map.isEmpty()) {
            System.out.println("The map is null or empty.");
        } else {
            System.out.println("The map is not null and not empty.");
        }

        // Add an entry to the map
        map.put("Apple", 1);

        // Check if the map is null or empty
        if (map == null || map.isEmpty()) {
            System.out.println("The map is null or empty.");
        } else {
            System.out.println("The map is not null and not empty.");
        }
    }
}

Output:

The map is null or empty.
The map is null or empty.
The map is not null and not empty.

Real-World Use Case

Validating User Input

In a web application, you might need to validate user input, which is often stored in collections. Before processing the input, you can check if the collection is null or empty to ensure that the input is valid.

Example

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

public class UserInputValidation {
    public static void main(String[] args) {
        List<String> userInput = null;

        // Validate user input
        validateInput(userInput);

        // Initialize user input
        userInput = new ArrayList<>();

        // Validate user input
        validateInput(userInput);

        // Add an element to user input
        userInput.add("data");

        // Validate user input
        validateInput(userInput);
    }

    public static void validateInput(List<String> input) {
        if (input == null || input.isEmpty()) {
            System.out.println("Invalid input: The collection is null or empty.");
        } else {
            System.out.println("Valid input: The collection is not null and not empty.");
        }
    }
}

Output:

Invalid input: The collection is null or empty.
Invalid input: The collection is null or empty.
Valid input: The collection is not null and not empty.

Conclusion

Checking if a collection is empty or null in Java is a crucial step in ensuring the robustness of your code. By performing these checks, you can prevent runtime errors and handle collections safely. Whether you are working with lists, sets, or maps, the methods demonstrated in this guide provide a straightforward way to validate collections before performing operations on them. This approach is particularly useful in real-world applications, such as validating user input or processing data collections.