public static String lowerCase(String str) {
        if (str == null) {
            return null;
        }
        return str.toLowerCase();
    }
<p>Converts a String to lower case as per {@link String#toLowerCase()}.</p>

<p>A {@code null} input String returns {@code null}.</p>

<pre>
StringUtils.lowerCase(null)  = null
StringUtils.lowerCase("")    = ""
StringUtils.lowerCase("aBc") = "abc"
</pre>

<p><strong>Note:</strong> As described in the documentation for {@link String#toLowerCase()},
the result of this method is affected by the current locale.
For platform-independent case transformations, the method {@link #lowerCase(String, Locale)}
should be used with a specific locale (e.g. {@link Locale#ENGLISH}).</p>

@param str  the String to lower case, may be null
@return the lower cased String, {@code null} if null String input
android/android_framework/base/src/main/java/lang3/StringUtils.java