From 30eb9314a4076c6adc06b348556650297b2b9283 Mon Sep 17 00:00:00 2001
From: Andy Seaborne <andy@apache.org>
Date: Fri, 26 Jan 2018 13:53:05 +0000
Subject: [PATCH] JENA-1468: Writing vars when table empty

---
 .../org/apache/jena/sparql/algebra/table/TableBase.java  | 2 ++
 .../org/apache/jena/sparql/algebra/table/TableEmpty.java | 4 ++++
 .../org/apache/jena/sparql/algebra/table/TableUnit.java  | 9 ++++-----
 .../org/apache/jena/sparql/sse/writers/WriterOp.java     | 9 ++++++---
 4 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/jena-arq/src/main/java/org/apache/jena/sparql/algebra/table/TableBase.java b/jena-arq/src/main/java/org/apache/jena/sparql/algebra/table/TableBase.java
index 1dfdc29e388..739fa957ae6 100644
--- a/jena-arq/src/main/java/org/apache/jena/sparql/algebra/table/TableBase.java
+++ b/jena-arq/src/main/java/org/apache/jena/sparql/algebra/table/TableBase.java
@@ -106,6 +106,8 @@ public boolean equals(Object other) {
         Table table = (Table)other ;
         if ( table.size() != this.size() )
             return false ;
+        if ( !table.getVars().equals(getVars()) )
+            return false ;
         QueryIterator qIter1 = iterator(null) ;
         QueryIterator qIter2 = table.iterator(null) ;
         try {
diff --git a/jena-arq/src/main/java/org/apache/jena/sparql/algebra/table/TableEmpty.java b/jena-arq/src/main/java/org/apache/jena/sparql/algebra/table/TableEmpty.java
index eaf6336e585..2640db0f3d9 100644
--- a/jena-arq/src/main/java/org/apache/jena/sparql/algebra/table/TableEmpty.java
+++ b/jena-arq/src/main/java/org/apache/jena/sparql/algebra/table/TableEmpty.java
@@ -23,6 +23,7 @@
 import java.util.List ;
 
 import org.apache.jena.atlas.iterator.Iter ;
+import org.apache.jena.sparql.algebra.Table;
 import org.apache.jena.sparql.core.Var ;
 import org.apache.jena.sparql.engine.ExecutionContext ;
 import org.apache.jena.sparql.engine.QueryIterator ;
@@ -31,6 +32,9 @@
 
 public class TableEmpty extends TableBase
 {
+    static public boolean isTableEmpty(Table table)
+    { return (table instanceof TableEmpty) ; } 
+
     public TableEmpty()
     { }
     
diff --git a/jena-arq/src/main/java/org/apache/jena/sparql/algebra/table/TableUnit.java b/jena-arq/src/main/java/org/apache/jena/sparql/algebra/table/TableUnit.java
index 5eb139d8691..1eb6f370878 100644
--- a/jena-arq/src/main/java/org/apache/jena/sparql/algebra/table/TableUnit.java
+++ b/jena-arq/src/main/java/org/apache/jena/sparql/algebra/table/TableUnit.java
@@ -18,7 +18,7 @@
 
 package org.apache.jena.sparql.algebra.table;
 
-import java.util.ArrayList ;
+import java.util.Collections;
 import java.util.Iterator ;
 import java.util.List ;
 
@@ -35,7 +35,7 @@ public class TableUnit extends TableBase
 {
     static public boolean isTableUnit(Table table)
     { return (table instanceof TableUnit) ; } 
-    
+ 
     public TableUnit() {}
     
     @Override
@@ -61,10 +61,9 @@ public void closeTable()    { }
     public boolean isEmpty()    { return false ; }
 
     @Override
-    public List<String> getVarNames()   { return new ArrayList<>() ; }
-
+    public List<String> getVarNames()   { return Collections.emptyList(); }
     @Override
-    public List<Var> getVars()       { return new ArrayList<>() ; }
+    public List<Var> getVars()          { return Collections.emptyList(); }
     
     @Override
     public String toString()    { return "TableUnit" ; }
diff --git a/jena-arq/src/main/java/org/apache/jena/sparql/sse/writers/WriterOp.java b/jena-arq/src/main/java/org/apache/jena/sparql/sse/writers/WriterOp.java
index f78f2776175..c8ff3087b32 100644
--- a/jena-arq/src/main/java/org/apache/jena/sparql/sse/writers/WriterOp.java
+++ b/jena-arq/src/main/java/org/apache/jena/sparql/sse/writers/WriterOp.java
@@ -32,6 +32,7 @@
 import org.apache.jena.sparql.algebra.OpPrefixesUsed ;
 import org.apache.jena.sparql.algebra.OpVisitor ;
 import org.apache.jena.sparql.algebra.op.* ;
+import org.apache.jena.sparql.algebra.table.TableEmpty;
 import org.apache.jena.sparql.algebra.table.TableUnit ;
 import org.apache.jena.sparql.core.* ;
 import org.apache.jena.sparql.expr.Expr ;
@@ -349,7 +350,7 @@ public void visit(OpTable opTable) {
                 return ;
             }
 
-            if ( opTable.getTable().isEmpty() ) {
+            if ( TableEmpty.isTableEmpty(opTable.getTable()) ) {
                 start(opTable, NoNL) ;
                 out.print("empty") ;
                 finish(opTable) ;
@@ -358,8 +359,10 @@ public void visit(OpTable opTable) {
 
             start(opTable, NoNL) ;
             WriterNode.outputVars(out, opTable.getTable().getVars(), sContext) ;
-            out.println() ;
-            WriterTable.outputPlain(out, opTable.getTable(), sContext) ;
+            if ( ! opTable.getTable().isEmpty() ) {
+                out.println();
+                WriterTable.outputPlain(out, opTable.getTable(), sContext) ;
+            }
             finish(opTable) ;
         }