How to Reliably Check for Non-Null and Non-Empty Strings in Java?

Assessing Non-Null and Non-Empty Strings

In programming, ensuring the validity of string inputs is crucial. A common task involves verifying whether a string is neither null nor empty. Let's explore various approaches to handle this scenario.

Considering the code snippet provided:

public void doStuff(String str)
{
    if (str != null && str != "")
    {
        /* handle empty string */
    }
    /* ... */
}

While this seems intuitive, it introduces a potential error if str is null. In Java, comparing a string to an empty string using == evaluates to false if str is null. This can lead to an unintended null pointer exception when checking the emptiness of str.

Using isEmpty() Method

A safer approach involves utilizing Java's isEmpty() method available since Java SE 1.6:

if(str != null && !str.isEmpty())

This expression ensures that str is not null and has at least one non-whitespace character. Using the && operator, it short-circuits the evaluation, avoiding a null pointer exception if str is null.

Considering Whitespace

If it's necessary to check for strings that do not contain any whitespace characters, the trim() method can be employed:

if(str != null && !str.trim().isEmpty())

This expression removes leading and trailing whitespace characters and then checks for emptiness. Alternatively, Java 11 introduces the isBlank() method, which accomplishes the same functionality:

if(str != null && !str.isBlank())

Concise Function Approach

For a concise solution, a utility function can be defined:

public static boolean empty( final String s ) {
  // Null-safe, short-circuit evaluation.
  return s == null || s.trim().isEmpty();
}

This function can be utilized as follows:

if( !empty( str ) )

By employing these methods, developers can effectively ascertain whether a string is not null and contains non-whitespace characters, enhancing the robustness and accuracy of their code.

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