Model and use isEmpty from Apache Collections

Author: owen-mc

java/ql/src/semmle/code/java/dataflow/NullGuards.qll

@@ -145,6 +145,10 @@ predicate nullCheckMethod(Method m, boolean branch, boolean isnull) {
   branch = false and
   isnull = false
   or
+  m instanceof MethodApacheCollectionsIsEmpty and
+  branch = false and
+  isnull = false
+  or
   m instanceof MethodApacheCollectionsIsNotEmpty and
   branch = true and
   isnull = false

java/ql/src/semmle/code/java/frameworks/apache/Collections.qll

@@ -4,6 +4,19 @@ import java
 private import semmle.code.java.dataflow.FlowSteps
 private import semmle.code.java.dataflow.ExternalFlow
 
+/**
+ * The method `isNotEmpty` in either `org.apache.commons.collections.CollectionUtils`
+ * or `org.apache.commons.collections4.CollectionUtils`.
+ */
+class MethodApacheCollectionsIsEmpty extends Method {
+  MethodApacheCollectionsIsEmpty() {
+    this.getDeclaringType()
+        .hasQualifiedName(["org.apache.commons.collections", "org.apache.commons.collections4"],
+          "CollectionUtils") and
+    this.hasName("isEmpty")
+  }
+}
+
 /**
  * The method `isNotEmpty` in either `org.apache.commons.collections.CollectionUtils`
  * or `org.apache.commons.collections4.CollectionUtils`.