Adopt public constants.

Author: lukechen116

src/test/java/org/sqids/MinLengthTests.java

@@ -10,7 +10,7 @@
 import org.junit.jupiter.api.Test;
 
 public class MinLengthTests {
-    private final int minLength = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".length();
+    private final int minLength = Sqids.Builder.DEFAULT_ALPHABET.length();
 
     @Test
     public void simple() {