added null checks for views

switcher-library/src/main/java/pl/aprilapps/switcher/Switcher.java

@@ -9,6 +9,8 @@
 import java.util.Iterator;
 import java.util.List;
 
+import static pl.aprilapps.switcher.guava.Preconditions.checkNotNull;
+
 /**
  * Created by Jacek Kwiecień on 14.03.15.
  */
@@ -69,31 +71,37 @@ public Builder(Context context) {
         }
 
         public Builder addContentView(View contentView) {
+            checkNotNull(contentView, "Provided content view is null");
             switcher.addContentView(contentView);
             return this;
         }
 
         public Builder addErrorView(View errorView) {
+            checkNotNull(errorView, "Provided error view is null");
             switcher.addErrorView(errorView);
             return this;
         }
 
         public Builder addEmptyView(View emptyView) {
+            checkNotNull(emptyView, "Provided empty view is null");
             switcher.addEmptyView(emptyView);
             return this;
         }
 
         public Builder addProgressView(View progressView) {
+            checkNotNull(progressView, "Provided progress view is null");
             switcher.addProgressView(progressView);
             return this;
         }
 
         public Builder setProgressLabel(TextView progressLabel) {
+            checkNotNull(progressLabel, "Provided progress label is null");
             switcher.setProgressLabel(progressLabel);
             return this;
         }
 
         public Builder setErrorLabel(TextView errorLabel) {
+            checkNotNull(errorLabel, "Provided error label is null");
             switcher.setErrorLabel(errorLabel);
             return this;
         }

switcher-library/src/main/java/pl/aprilapps/switcher/guava/Preconditions.java

@@ -0,0 +1,26 @@
+package pl.aprilapps.switcher.guava;
+
+/**
+ * Created by Jacek Kwiecień on 08.06.16.
+ */
+public final class Preconditions {
+
+    private Preconditions() {
+    }
+
+
+    /**
+     * Ensures that an object reference passed as a parameter to the calling method is not null.
+     *
+     * @param reference an object reference
+     * @return the non-null reference that was validated
+     * @throws NullPointerException if {@code reference} is null
+     */
+    public static <T> T checkNotNull(T reference, Object errorMessage) {
+        if (reference == null) {
+            throw new NullPointerException(String.valueOf(errorMessage));
+        }
+        return reference;
+    }
+
+}