How to Check if a String is Null or Empty in Java

In Java, dealing with strings is a common task, and it's essential to handle null or empty strings to prevent potential errors in your code. This blog post will guide you through various methods to check if a string is null or empty in Java.

Before we dive into the methods, let's understand the difference between null and empty strings:

Null String: A null string refers to a string object that does not exist in memory. It means the string variable does not point to any instance in the memory heap.

Empty String: An empty string is a string object that exists in memory, but it contains no characters. It has a length of 0.

1. Using equals() Method 

The equals() method can be used to check if a string is null-safe and prevents a NullPointerException. It returns true if the string is not null and contains characters. 

Example:

public class CheckStringExample {
    public static void main(String[] args) {
        String str1 = null;
        String str2 = "";
        String str3 = "Hello";

        System.out.println(str1 == null || str1.equals("")); // Output: false
        System.out.println(str2 == null || str2.equals("")); // Output: true
        System.out.println(str3 == null || str3.equals("")); // Output: false
    }
}

Output:

true
true
false

2. Using isEmpty() Method 

The isEmpty() method is a built-in method in Java that can be used to check if a string is empty (contains no characters). It returns true if the length of the string is 0. 

Example:

public class CheckStringExample {
    public static void main(String[] args) {
        String str1 = null;
        String str2 = "";
        String str3 = "Hello";

        System.out.println(str1 != null && str1.isEmpty()); // Output: false
        System.out.println(str2 != null && str2.isEmpty()); // Output: true
        System.out.println(str3 != null && str3.isEmpty()); // Output: false
    }
}

Output:

false
true
false

3. Using isBlank() Method (Java 11+) 

The isBlank() method is introduced in Java 11, and it not only checks for empty strings but also considers strings containing only white spaces as blank. 

Example:

public class CheckStringExample {
    public static void main(String[] args) {
        String str1 = null;
        String str2 = "";
        String str3 = "   "; // Contains only white spaces

        System.out.println(str1 != null && str1.isBlank()); // Output: false
        System.out.println(str2 != null && str2.isBlank()); // Output: true
        System.out.println(str3 != null && str3.isBlank()); // Output: true
    }
}

Output:

false
true
true

4. Using Objects Class (Java 7+) 

In Java 7 and later versions, the Objects class provides a utility method isNull() that can be used to check for null values. 

Example:

import java.util.Objects;

public class CheckStringExample {
    public static void main(String[] args) {
        String str1 = null;
        String str2 = "";
        String str3 = "Hello";

        System.out.println(Objects.isNull(str1)); // Output: true
        System.out.println(Objects.isNull(str2)); // Output: false
        System.out.println(Objects.isNull(str3)); // Output: false
    }
}

Output:

true
false
false

5. Write Your Own Utility Methods

1. isEmptyOrNull(final CharSequence cs) - Checks if a CharSequence is empty ("") or null.
2. isNotEmptyOrNull(final CharSequence cs) - Checks if a CharSequence is not empty ("") and not null.

1. isEmptyOrNull(final CharSequence cs)

Here is a utility method to check if a CharSequence or String is empty ("") or null:

 public static boolean isEmptyOrNull(final CharSequence cs) {
        return cs == null || cs.length() == 0;
 }

2. isNotEmptyOrNull(final CharSequence cs)

Here is a utility method to check if a CharSequence or String is not empty ("") and not null.

    public static boolean isEmptyOrNull(final CharSequence cs) {
        return cs == null || cs.length() == 0;
    }

    public static boolean isNotEmptyOrNull(final CharSequence cs) {
        return !isEmptyOrNull(cs);
    }

Test Utility methods:

package net.javaguides.lang;

public class StringNullOrEmptyExample {

    public static void main(String[] args) {
        System.out.println(isEmptyOrNull(null));
        System.out.println(isEmptyOrNull(""));
        System.out.println(isEmptyOrNull(" "));
        System.out.println(isEmptyOrNull("bob"));
        System.out.println(isEmptyOrNull("  bob  "));

        System.out.println(isNotEmptyOrNull(null));
        System.out.println(isNotEmptyOrNull(""));
        System.out.println(isNotEmptyOrNull(" "));
        System.out.println(isNotEmptyOrNull("bob"));
        System.out.println(isNotEmptyOrNull("  bob  "));

    }

    public static boolean isEmptyOrNull(final CharSequence cs) {
        return cs == null || cs.length() == 0;
    }

    public static boolean isNotEmptyOrNull(final CharSequence cs) {
        return !isEmptyOrNull(cs);
    }
}

Output:

true
true
false
false
false
false
false
true
true
true

Conclusion 

Checking whether a string is null or empty is a crucial step in Java programming, especially when handling user inputs or external data. You can use any of the methods discussed in this blog post to perform null-safe and empty-safe string checks based on your specific requirements. I would suggest creating the Utility methods for null checks in your Java projects.