diff --git a/applications/test/HashSet/Test-hashSet.C b/applications/test/HashSet/Test-hashSet.C
index 087041db78..1534065a46 100644
--- a/applications/test/HashSet/Test-hashSet.C
+++ b/applications/test/HashSet/Test-hashSet.C
@@ -3,7 +3,7 @@
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
     \\  /    A nd           | Copyright (C) 2011 OpenFOAM Foundation
-     \\/     M anipulation  |
+     \\/     M anipulation  | Copyright (C) 2017 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -29,6 +29,7 @@ Description
 #include "HashSet.H"
 #include "Map.H"
 #include "labelPairHashes.H"
+#include "FlatOutput.H"
 
 using namespace Foam;
 
@@ -66,6 +67,8 @@ int main(int argc, char *argv[])
     tableB.insert("value5", nil());
     tableB.insert("value6", nil());
 
+    Info<< "tableA keys: "; tableA.writeKeys(Info) << endl;
+
     Map<label> mapA
     {
         { 1, 1 },
@@ -149,7 +152,7 @@ int main(int argc, char *argv[])
     labelHashSet setD(1);
     setD.insert({11, 100, 49, 36, 2008});
 
-    Info<< "setD : " << setD << endl;
+    Info<< "setD : " << flatOutput(setD) << endl;
 
     Info<< "setB == setC: " << (setB == setC) << endl;
     Info<< "setC != setD: " << (setC != setD) << endl;
@@ -188,7 +191,7 @@ int main(int argc, char *argv[])
         Info<< "setD has no 11" << endl;
     }
 
-    Info<< "setD : " << setD << endl;
+    Info<< "setD : " << flatOutput(setD) << endl;
 
     // this doesn't work (yet?)
     // setD[12] = true;
@@ -201,7 +204,7 @@ int main(int argc, char *argv[])
 
     label added = setD.set(someLst);
     Info<< "added " << added << " from " << someLst.size() << endl;
-    Info<< "setD : " << setD << endl;
+    Info<< "setD : " << flatOutput(setD) << endl;
 
 
     return 0;
diff --git a/src/OpenFOAM/containers/HashTables/HashSet/HashSet.C b/src/OpenFOAM/containers/HashTables/HashSet/HashSet.C
index c180244f1d..796069c109 100644
--- a/src/OpenFOAM/containers/HashTables/HashSet/HashSet.C
+++ b/src/OpenFOAM/containers/HashTables/HashSet/HashSet.C
@@ -267,6 +267,15 @@ void Foam::HashSet<Key, Hash>::operator-=(const HashSet<Key, Hash>& rhs)
 }
 
 
+// * * * * * * * * * * * * * * * IOstream Operators  * * * * * * * * * * * * //
+
+template<class Key, class Hash>
+Foam::Ostream& Foam::operator<<(Ostream& os, const HashSet<Key, Hash>& tbl)
+{
+    return tbl.writeList(os, 10);  // 10=consistent with UList
+}
+
+
 /* * * * * * * * * * * * * * * * Global operators  * * * * * * * * * * * * * */
 
 template<class Key, class Hash>
diff --git a/src/OpenFOAM/containers/HashTables/HashSet/HashSet.H b/src/OpenFOAM/containers/HashTables/HashSet/HashSet.H
index 38fd2dda8a..c6229c4e3f 100644
--- a/src/OpenFOAM/containers/HashTables/HashSet/HashSet.H
+++ b/src/OpenFOAM/containers/HashTables/HashSet/HashSet.H
@@ -209,6 +209,16 @@ public:
         }
 
 
+    // Writing
+
+        //- Write the unordered keys as a list, with line-breaks if list length
+        //  exceeds shortListLen. Using '0' suppresses line-breaks entirely.
+        Ostream& writeList(Ostream& os, const label shortListLen=0) const
+        {
+            return this->writeKeys(os, shortListLen);
+        }
+
+
     // Member Operators
 
         //- Return true if the entry exists, same as found()
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTable.H b/src/OpenFOAM/containers/HashTables/HashTable/HashTable.H
index 4cf4494da7..f90b0a6a26 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTable.H
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTable.H
@@ -261,9 +261,6 @@ public:
             //- Return the table of contents as a sorted list
             List<Key> sortedToc() const;
 
-            //- Print information
-            Ostream& printInfo(Ostream& os) const;
-
 
         // Edit
 
@@ -572,6 +569,15 @@ public:
         };
 
 
+    // Writing
+
+        //- Print information
+        Ostream& printInfo(Ostream& os) const;
+
+        //- Write the unordered keys as a list, with line-breaks if list length
+        //  exceeds shortListLen. Using '0' suppresses line-breaks entirely.
+        Ostream& writeKeys(Ostream& os, const label shortListLen=0) const;
+
 
     // IOstream Operator
 
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTableIO.C b/src/OpenFOAM/containers/HashTables/HashTable/HashTableIO.C
index 9fb4057735..97193707c1 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTableIO.C
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTableIO.C
@@ -54,8 +54,7 @@ Foam::HashTable<T, Key, Hash>::HashTable(Istream& is, const label size)
 // * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * * //
 
 template<class T, class Key, class Hash>
-Foam::Ostream&
-Foam::HashTable<T, Key, Hash>::printInfo(Ostream& os) const
+Foam::Ostream& Foam::HashTable<T, Key, Hash>::printInfo(Ostream& os) const
 {
     label used = 0;
     label maxChain = 0;
@@ -90,6 +89,53 @@ Foam::HashTable<T, Key, Hash>::printInfo(Ostream& os) const
 }
 
 
+template<class T, class Key, class Hash>
+Foam::Ostream& Foam::HashTable<T, Key, Hash>::writeKeys
+(
+    Ostream& os,
+    const label shortListLen
+) const
+{
+    // Similar to UList::writeList version except the following:
+    // - the keys can never be uniform
+    // - never write in binary
+
+    label i = this->size();
+
+    if (i <= 1 || !shortListLen || (i <= shortListLen))
+    {
+        // Write size and start delimiter
+        os << i << token::BEGIN_LIST;
+
+        i = 0;
+        for (const_iterator iter = this->cbegin(); iter != this->cend(); ++iter)
+        {
+            if (i++) os << token::SPACE;
+            os << iter.key();
+        }
+
+        os << token::END_LIST;  // End delimiter
+    }
+    else
+    {
+        // Write size and start delimiter
+        os << nl << i << nl << token::BEGIN_LIST << nl;
+
+        for (const_iterator iter = this->cbegin(); iter != this->cend(); ++iter)
+        {
+            os << iter.key() << nl;
+        }
+
+        os << token::END_LIST << nl;  // End delimiter
+    }
+
+    // Check state of IOstream
+    os.check("HashSet<Key>::writeList(Ostream&)");
+
+    return os;
+}
+
+
 // * * * * * * * * * * * * * * * IOstream Operators  * * * * * * * * * * * * //
 
 template<class T, class Key, class Hash>