Empty strings, isEmpty

Some strings exist and are not null, but have zero characters. We can test for this case with the isEmpty method.

With isEmpty, we detect when a string has a length of 0. We can also test the length() method for the value 0. And with String equals() we can compare an empty string.

First example

This program uses the isEmpty method. We declare a string called "value" and assign it to an empty string literal.

public class Program {
    public static void main(String[] args) {

        String value = "";
        // See if string is empty.
        if (value.isEmpty()) {
            // Display string length.
            System.out.println("String is empty, length = " +
                value.length());
        }
    }
}
String is empty, length = 0

Equals, empty strings

We can compare a string to see if it is empty with equals(). We cannot use the equals operator == as this will compare the references identities.

public class Program {
    public static void main(String[] args) {

        String value = "";

        // Two equal string literals can be tested for identity.
        if (value == "") {
            System.out.println("Test 1");
        }

        // A new empty string will not have the same identity.
        // ... This test will not succeed.
        String empty = new String("");
        if (value == empty) {
            System.out.println("Test 2"); // Not reached.
        }

        // The equals method tests for character equivalence.
        // ... This test will succeed.
        if (value.equals(empty)) {
            System.out.println("Test 3");
        }
    }
}
Test 1
Test 3

NullPointerException

We cannot use the isEmpty method on a null String reference. We must first test against null if the String reference might be null.

public class Program {
    public static void main(String[] args) {

        String value = null;
        // This program will not work.
        if (value.isEmpty()) {
            System.out.println(0);
        }
    }
}
Exception in thread "main" java.lang.NullPointerException
    at Program.main(Program.java:6)

Performance

I tested the performance of string equals() and isEmpty. The Java runtime is well-optimized. In my simple benchmark, no measurable differences were present.

Empty strings are often present in Java programs. We use them to avoid null strings, which can cause NullPointerExceptions.

I feel isEmpty is the best option, but sometimes testing length() for values is appropriate. It is rare that using string equals() is needed here.