Added bunch of javadocs and restricted some visibility.

No functional changes

src/main/java/org/scribe/utils/Preconditions.java

@@ -4,27 +4,60 @@ import java.util.regex.Pattern;
 
 import org.scribe.model.OAuthConstants;
 
+/**
+ * Utils for checking preconditions and invariants
+ * 
+ * @author Pablo Fernandez
+ */
 public class Preconditions
 {
   private static final String DEFAULT_MESSAGE = "Received an invalid parameter";
   private static final Pattern URL_PATTERN = Pattern.compile("[a-zA-Z_-]+://\\S+");
 
+  /**
+   * Checks that an object is not null.
+   * 
+   * @param object any object
+   * @param errorMsg error message
+   * 
+   * @throws IllegalArgumentException if the object is null
+   */
   public static void checkNotNull(Object object, String errorMsg)
   {
     check(object != null, errorMsg);
   }
 
+  /**
+   * Checks that a string is not null or empty
+   * 
+   * @param string any string
+   * @param errorMsg error message
+   * 
+   * @throws IllegalArgumentException if the string is null or empty
+   */
   public static void checkEmptyString(String string, String errorMsg)
   {
     check(string != null && !string.trim().equals(""), errorMsg);
   }
 
+  /**
+   * Checks that a URL is valid
+   * 
+   * @param url any string
+   * @param errorMsg error message
+   */
   public static void checkValidUrl(String url, String errorMsg)
   {
     checkEmptyString(url, errorMsg);
     check(isUrl(url), errorMsg);
   }
   
+  /**
+   * Checks that a URL is a valid OAuth callback
+   *  
+   * @param url any string
+   * @param errorMsg error message
+   */
   public static void checkValidOAuthCallback(String url, String errorMsg)
   {
     checkEmptyString(url, errorMsg);
@@ -34,7 +67,7 @@ public class Preconditions
     }
   }
   
-  static boolean isUrl(String url)
+  private static boolean isUrl(String url)
   {
     return URL_PATTERN.matcher(url).matches();
   }