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.
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.
And Two empty strings can be separate in memory. This means they would have difference identities.
However String equals() compares the inner characters of a string. This can correctly tell us if the string is empty.
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.
A summary. Empty strings are often present in Java programs. We use them to avoid null strings, which can cause NullPointerExceptions.
Testing empty strings. 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.
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This page was last updated on Jul 15, 2022 (edit).