Fixes builder regression: pop() used to be nullable

CHANGELOG.md

@@ -1,6 +1,10 @@
 Change Log
 ==========
 
+Version 0.12 *(2015-08-13)*
+------
+* Fix: History.Builder#pop is nullable again, and adds History.Builder#isEmpty.
+
 Version 0.11 *(2015-08-11)*
 ------
 * Fix: No longer persists an empty list of states if the filter excludes everything.

flow/src/main/java/flow/History.java

@@ -218,10 +218,19 @@ public Builder addAll(Collection<Object> c) {
     }
 
     public Object peek() {
-      return history.peek().state;
+      final Entry peek = history.peek();
+      return peek == null ? null : peek.state;
     }
 
+    public boolean isEmpty() {
+      return peek() == null;
+    }
+
+    /** @throws IllegalStateException if empty */
     public Object pop() {
+      if (isEmpty()) {
+        throw new IllegalStateException("Cannot pop from an empty builder");
+      }
       Entry entry = history.pop();
       entryMemory.put(entry.state, entry);
       return entry.state;

flow/src/test/java/flow/FlowTest.java

@@ -24,6 +24,7 @@
 import static flow.Flow.TraversalCallback;
 import static java.util.Arrays.asList;
 import static org.fest.assertions.api.Assertions.assertThat;
+import static org.fest.assertions.api.Fail.fail;
 
 public class FlowTest {
   static class Uno {
@@ -287,4 +288,26 @@ public int hashCode() {
 
     assertThat(flow.goBack()).isFalse();
   }
+
+  @Test public void emptyBuilderPeekIsNullable() {
+    assertThat(History.emptyBuilder().peek()).isNull();
+  }
+
+  @Test public void emptyBuilderPopThrows() {
+    try {
+      History.emptyBuilder().pop();
+      fail("Should throw");
+    } catch (IllegalStateException e) {
+      // pass
+    }
+  }
+
+  @Test public void isEmpty() {
+    final History.Builder builder = History.emptyBuilder();
+    assertThat(builder.isEmpty()).isTrue();
+    builder.push("foo");
+    assertThat(builder.isEmpty()).isFalse();
+    builder.pop();
+    assertThat(builder.isEmpty()).isTrue();
+  }
 }