Java: Fix FunctionalInterface.

java/ql/lib/semmle/code/java/Type.qll

@@ -874,6 +874,44 @@ class ClassOrInterface extends RefType, @classorinterface {
   }
 }
 
+private string getAPublicObjectMethodSignature() {
+  exists(Method m |
+    m.getDeclaringType() instanceof TypeObject and
+    m.isPublic() and
+    result = m.getSignature()
+  )
+}
+
+private Method getAnAbstractMethod(Interface interface) {
+  interface.inherits(result) and
+  result.isAbstract() and
+  // JLS 9.8, ignore Object methods
+  not result.getSignature() = getAPublicObjectMethodSignature() and
+  // Make sure that there is no other non-abstract method
+  // (e.g. `default`) which overrides the abstract one
+  not exists(Method m |
+    interface.inherits(m) and
+    not m.isAbstract() and
+    m.overrides(result)
+  )
+}
+
+/**
+ * A functional interface is an interface that has just one abstract method
+ * (aside from the methods of Object), and thus represents a single function
+ * contract.
+ *
+ * See JLS 9.8, Functional Interfaces.
+ */
+class FunctionalInterface extends Interface {
+  Method run;
+
+  FunctionalInterface() { run = unique(Method r | r = getAnAbstractMethod(this)) }
+
+  /** Gets the single abstract method of this interface. */
+  Method getRunMethod() { result = run }
+}
+
 /**
  * A primitive type.
  *

java/ql/lib/semmle/code/java/dispatch/WrappedInvocation.qll

@@ -6,37 +6,6 @@
 import java
 import VirtualDispatch
 
-private string getAPublicObjectMethodSignature() {
-  exists(Method m |
-    m.getDeclaringType() instanceof TypeObject and
-    m.isPublic() and
-    result = m.getSignature()
-  )
-}
-
-private Method getAPotentialRunMethod(Interface i) {
-  i.inherits(result) and
-  result.isPublic() and
-  not result.getSignature() = getAPublicObjectMethodSignature()
-}
-
-/**
- * A functional interface is an interface that has just one abstract method
- * (aside from the methods of Object), and thus represents a single function
- * contract.
- *
- * See JLS 9.8, Functional Interfaces.
- */
-class FunctionalInterface extends Interface {
-  FunctionalInterface() {
-    1 = strictcount(getAPotentialRunMethod(this)) and
-    not exists(Method m | this.inherits(m) and m.isDefault())
-  }
-
-  /** Gets the single method of this interface. */
-  Method getRunMethod() { getAPotentialRunMethod(this).getSourceDeclaration() = result }
-}
-
 /**
  * Holds if `m` might invoke `runmethod` through a functional interface on the
  * `n`th parameter.