String Null Check in Java (String is Empty or Null) - 5 Ways

Strings are a fundamental data type in Java, used to represent text or character sequences. In many scenarios, it is crucial to check whether a string is empty or null in Java. This is a common operation when working with user inputs, file processing, database interactions, and more.

Understanding how to check for empty or null strings is a fundamental skill for Java developers, as it helps in writing robust and error-free code.

Meaning of Empty and Null Strings:

Before we dive into the Java program, let's clarify what we mean by empty and null strings:

Empty String: An empty string is a string that contains no characters. In Java, an empty string is represented by "" (double quotes with nothing in between). It has a length of 0.
Null String: A null string is a string variable that has not been assigned any value or object reference. In Java, when a string is null, it means it does not refer to any memory location, and trying to access its methods or properties will result in a NullPointerException.

When to Check String is Empty or Null?

The Java programs to check a string is null or empty are used for various purposes, including:

User Input Validation: When designing user interfaces, developers often need to validate user inputs, ensuring that required fields are not empty.
File Handling: While reading from or writing to files, it's essential to check if a read string is empty or null to avoid processing errors.
Database Operations: When interacting with databases, developers need to handle null values that may occur in query results.
API Requests and Responses: In web development, checking for empty or null strings is crucial when parsing data from API responses or sending data to APIs.
Data Cleaning and Transformation: In data processing tasks, cleaning and transforming data often involve dealing with empty or null strings.

In this tutorial, we will explore different methods to check if a string is empty or null in Java. We will provide code examples and explanations to help you become proficient in handling string data effectively and preventing runtime errors in your Java programs. Let's get started!

Concepts to Learn:

Check Null in Java Using isEmpty() Method

Here is a Java program to check string is null or empty using the isEmpty() method:

Code

public class StringCheckExample {
    public static void main(String[] args) {
        String str1 = "";            // An empty string
        String str2 = null;          // A null string
        String str3 = "Hello, GPT!"; // A non-empty string

        // Check if str1 is empty or null
        if (str1 == null || str1.isEmpty()) {
            System.out.println("str1 is either empty or null");
        } else {
            System.out.println("str1 is not empty or null");
        }

        // Check if str2 is empty or null
        if (str2 == null || str2.isEmpty()) {
            System.out.println("str2 is either empty or null");
        } else {
            System.out.println("str2 is not empty or null");
        }

        // Check if str3 is empty or null
        if (str3 == null || str3.isEmpty()) {
            System.out.println("str3 is either empty or null");
        } else {
            System.out.println("str3 is not empty or null");
        }
    }
}

Output

str1 is either empty or null
str2 is either empty or null
str3 is not empty or null

Explanation

In this example, we demonstrate how to use the isEmpty() method to check if a string is empty or null. Here's a breakdown of what's happening:

We declare three string variables: str1, str2, and str3.

str1 is an empty string.
str2 is set to null, indicating a null string.
str3 contains a non-empty string.

We use the isEmpty() method in combination with a null check (== null) to determine if each string is either empty or null.

For str1, the condition (str1 == null || str1.isEmpty()) evaluates to true because it's both empty and null.

For str2, the condition (str2 == null || str2.isEmpty()) also evaluates to true because it's null.

For str3, the condition (str3 == null || str3.isEmpty()) evaluates to false because it's neither empty nor null.

This method provides a straightforward way to check for empty or null strings using the built-in isEmpty() method, which returns true if the string has a length of 0 (empty) and can handle null strings without throwing exceptions.

Check String is Null or Empty Using equals() method

Following is a Java program to check string is empty or null using the equals() method with an empty string:

Code

public class StringCheckExample {
    public static void main(String[] args) {
        String str1 = "";            // An empty string
        String str2 = null;          // A null string
        String str3 = "Hello, GPT!"; // A non-empty string

        // Check if str1 is empty or null
        if (str1 == null || str1.equals("")) {
            System.out.println("str1 is either empty or null");
        } else {
            System.out.println("str1 is not empty or null");
        }

        // Check if str2 is empty or null
        if (str2 == null || str2.equals("")) {
            System.out.println("str2 is either empty or null");
        } else {
            System.out.println("str2 is not empty or null");
        }

        // Check if str3 is empty or null
        if (str3 == null || str3.equals("")) {
            System.out.println("str3 is either empty or null");
        } else {
            System.out.println("str3 is not empty or null");
        }
    }
}

Output

str1 is either empty or null
str2 is either empty or null
str3 is not empty or null

Explanation

We declare three string variables: str1, str2, and str3.

str1 is an empty string.
str2 is set to null, indicating a null string.
str3 contains a non-empty string.

We use the equals() method in combination with a null check (== null) to check if each string is either empty or null.

For str1, the condition (str1 == null || str1.equals("")) evaluates to true because it's both empty and null.
For str2, the condition (str2 == null || str2.equals("")) also evaluates to true because it's null.
For str3, the condition (str3 == null || str3.equals("")) evaluates to false because it's neither empty nor null.

It's worth noting that using equals() may throw a Java NullPointerException if the string is null, so we should always include a null check as shown in the example.

Using null check in Java (!= null)

In this example, we use a null check (!= null) along with the isEmpty() method to check if a string is empty or null.

Code

public class StringCheckExample {
    public static void main(String[] args) {
        String str1 = "";            // An empty string
        String str2 = null;          // A null string
        String str3 = "Hello, GPT!"; // A non-empty string

        // Check if str1 is empty or null
        if (str1 == null || str1.isEmpty()) {
            System.out.println("str1 is either empty or null");
        } else {
            System.out.println("str1 is not empty or null");
        }

        // Check if str2 is empty or null
        if (str2 == null || str2.isEmpty()) {
            System.out.println("str2 is either empty or null");
        } else {
            System.out.println("str2 is not empty or null");
        }

        // Check if str3 is empty or null
        if (str3 == null || str3.isEmpty()) {
            System.out.println("str3 is either empty or null");
        } else {
            System.out.println("str3 is not empty or null");
        }
    }
}

Output

str1 is either empty or null
str2 is either empty or null
str3 is not empty or null

Explanation

We declare three string variables: str1, str2, and str3.

str1 is an empty string.
str2 is set to null, indicating a null string.
str3 contains a non-empty string.

We use a null check (== null) to first determine if each string is null.

If a string is null, the condition (str == null) evaluates to true, and the code inside the corresponding if block is executed.

If a string is not null, we proceed to check if it is empty using the isEmpty() method.

For str1, the null check (str1 == null) is false, but the isEmpty() check evaluates to true because it's empty.
For str2, the null check (str2 == null) is true, so it doesn't go to the isEmpty() check. The string is considered both empty and null.
For str3, the null check (str3 == null) is false, and the isEmpty() check evaluates to false because it's neither empty nor null.

This method combines a null check with the isEmpty() method to accurately determine if a string is either empty or null. It ensures that null strings are handled without causing a NullPointerException.

Using StringUtils class from Apache Commons Lang library

To use this method, you need to include the Apache Commons Lang library in your Java project. You can download the library from the Apache Commons website (https://commons.apache.org/proper/commons-lang/) and add it to your project's classpath.

Here's an example of how to check if a string is empty or null using the StringUtils class.

Code

import org.apache.commons.lang3.StringUtils;

public class StringCheckExample {
    public static void main(String[] args) {
        String str1 = "";            // An empty string
        String str2 = null;          // A null string
        String str3 = "Hello, GPT!"; // A non-empty string

        // Check if str1 is empty or null using StringUtils
        if (StringUtils.isEmpty(str1)) {
            System.out.println("str1 is either empty or null");
        } else {
            System.out.println("str1 is not empty or null");
        }

        // Check if str2 is empty or null using StringUtils
        if (StringUtils.isEmpty(str2)) {
            System.out.println("str2 is either empty or null");
        } else {
            System.out.println("str2 is not empty or null");
        }

        // Check if str3 is empty or null using StringUtils
        if (StringUtils.isEmpty(str3)) {
            System.out.println("str3 is either empty or null");
        } else {
            System.out.println("str3 is not empty or null");
        }
    }
}

Output

str1 is either empty or null
str2 is either empty or null
str3 is not empty or null

Explanation

We declare three string variables: str1, str2, and str3.

str1 is an empty string.
str2 is set to null, indicating a null string.
str3 contains a non-empty string.

We import the StringUtils class from the Apache Commons Lang library.

We use the StringUtils.isEmpty() method to check if each string is either empty or null.

For str1, the StringUtils.isEmpty(str1) method call returns true because it's empty.
For str2, the StringUtils.isEmpty(str2) method call also returns true because it's null.
For str3, the StringUtils.isEmpty(str3) method call returns false because it's neither empty nor null.

Using the StringUtils class from Apache Commons Lang is a convenient and widely used way to check for empty or null strings, especially when you're working with larger Java applications or need to perform more complex string operations. It helps handle null strings without causing a NullPointerException.

Using Java 11 isBlank() method

In Java 11 and later versions, you can use the isBlank() method introduced in the java.lang.String class to check if a string is empty or consists only of whitespace characters.

Here's an example of how to use the isBlank() method:

Code

public class StringCheckExample {
    public static void main(String[] args) {
        String str1 = "";            // An empty string
        String str2 = null;          // A null string
        String str3 = "Hello, GPT!"; // A non-empty string

        // Check if str1 is empty or null using isBlank()
        if (str1 == null || str1.isBlank()) {
            System.out.println("str1 is either empty or null");
        } else {
            System.out.println("str1 is not empty or null");
        }

        // Check if str2 is empty or null using isBlank()
        if (str2 == null || str2.isBlank()) {
            System.out.println("str2 is either empty or null");
        } else {
            System.out.println("str2 is not empty or null");
        }

        // Check if str3 is empty or null using isBlank()
        if (str3 == null || str3.isBlank()) {
            System.out.println("str3 is either empty or null");
        } else {
            System.out.println("str3 is not empty or null");
        }
    }
}

Output

str1 is either empty or null
str2 is either empty or null
str3 is not empty or null

Explanation

We declare three string variables: str1, str2, and str3.

str1 is an empty string.
str2 is set to null, indicating a null string.
str3 contains a non-empty string.

We use a null check (== null) to first check if each string is null.

If a string is null, the condition (str == null) evaluates to true, and the code inside the corresponding if block is executed.

If a string is not null, we proceed to check if it is blank using the isBlank() method.

For str1, the null check (str1 == null) is false, but the isBlank() check evaluates to true because it's empty.
For str2, the null check (str2 == null) is true, so it doesn't go to the isBlank() check. The string is considered both empty and null.
For str3, the null check (str3 == null) is false, and the isBlank() check evaluates to false because it contains non-whitespace characters.

Using the isBlank() method is a concise and efficient way to check for empty or null strings, and it can handle whitespace-only strings as well. It's a convenient addition to Java for working with string data.

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Examples

String Null Check in Java (String is Empty or Null) - 5 Ways

Meaning of Empty and Null Strings:

When to Check String is Empty or Null?

Check Null in Java Using isEmpty() Method

Code

Output

Explanation

Check String is Null or Empty Using equals() method

Code

Output

Explanation

Using null check in Java (!= null)

Code

Output

Explanation

Using StringUtils class from Apache Commons Lang library

Code

Output

Explanation

Using Java 11 isBlank() method

Code

Output

Explanation